I'm always surprised by how wobbly things are in slow motion. The sponsor of the video is Brilliant: Visit www.brilliant.org/stevemould to try everything they have to offer for free for a full 30 days. You’ll also get 20% off an annual premium subscription.
Does this mean that if you threw the ball under the table, then lowered a hinge extension for the table, can you make the ball reverse direction multiple times?
I think people would enjoy knowing how tires work, how can they produce so much grip. it is one of those mind blowing realizations that tires use static friction: the contact patch does not move relative to the ground. The tire has as much grip when the car is stationary than when it is moving (excluding some minor things). This means that the force that you need to turn the tires on the ground to make them slide, is the maximum force that can turn the car. If you push it from any direction and the tires start to slide: that is the maximum force at any speed you can use to control the car. All of the turning, accelerating etc. is done by rubber stretching and squishing, and it wanting to return back to the original shape. So, the force that moves the car forward comes from deformation of rubber, like million rubber bands stretching. The engine rotates the axle faster than the tire is rotating and this stretches the tire, rubber wants to retain its shape as it is between the ground and the wheel, and we accelerate. Slip angle as a concept.. man that has been one of the most rewarding things to learn, it was just constant "ahaa!" feeling, massive amount of rewarding chemicals floating around. It is such a simple thing that you just don't think about but... the part of the tire that contacts the ground does not move even if you are going 500kmh.
I suspect that the tennis ball continues spinning in the same direction because its friction with the surface is very low. It's fuzzy, so a very small area is in contact with the surface during the bounce. That lets it simply slide along.
@@nokbeen3654 Ahh, 👍 . I thought it might be green screen, but wondered why would it be green screened, and there’s shadows that show up… but without knowing about an ottoman it was messing with my brain. That helps, thanks!
I think if said mathematicians had had to apply these formulas they might still be at it, and their pool playing opponents would have conceded the game because they were all being kicked because it was closing time already.
When I was a little kid I once saw my dad throw a hula hoop he'd put backspin on, and when it hit the lawn edge-on the hula hoop rolled back to him, and I realized right then and there that, despite what everyone had told me, magic was real, and my dad was a wizard. It was the only explanation. 🤣
When I was a kid, my dad told me that if I could salt a bird's tail then I could catch it. I got excited and took a fish net and a salt shaker in hand and went stalking birds. Never did catch anything. It was only years later that I realized that if I was sneaky enough to get close enough to salt the tail then I could indeed catch the bird, but the salt had nothing to do with it. He might as well have said, "If you can catch a bird then you can catch a bird."
@@mailleweaveryeah, but where is the fun? the best phrases are those that seem absurd, but still carry the truth underneath. your dad got it well to push you to keep going
Tangentially related; In water polo you can bounce the ball on the surface of the water by giving it the right combination of backspin and angle of attack (otherwise it just stops dead in the water). Generally more backspin, steeper angle, more power, lower ball pressure, less textured grip on the ball increase the bounce height, but sometimes a grippy ball thrown at a low angle with a bit of sidespin can pop up just as high. I can do it instinctively but I've never quite been able to grasp how it works...
_@NKuijlaars_ Cannot grasp the concept, you say? It is identical to the phenomenon of _skipping stones._ Plenty of blather on that topic, accessible from a Google search.
@@brianhiles8164Firstly, by grasp I meant being able to control it based on my understanding of the physics, and vice versa, but that just doesn't really work here. When skipping stones the side spin is only for stability (?) and there is no backspin, but with a ball stability isn't really relevant (?) and the ball *cannot* bounce without backspin. The ball is also capable of bouncing out of the water with a steeper angle than it was thrown which makes me think it's more of a restitution problem. My apologies for daring to think there might be an interesting little nugget of physics in the mechanism here.
@@NKuijlaars Obviously, you have a “sufficient“ technical background, that you are both _able_ to be fascinated by the nuances and intricacies of the physics involved, and just as importantly have a _discrimination_ of when you have indeed “wrapped your mind around“ an understanding of the problem. This is key. On more than one occasion, I have purposefully _not_ remanded my understanding to an expert, that I have forced myself to figure out a given mystery myself. As an aside, though, in the matter of _restitution_ being the prevalent phenomenon of a Newtonian liquid: If I understand the context of your observation, angle-of-attack is predominantly a function of the forward momentum of the rock inducing an inclined plane of the “sticky“ water surface to then be “launching“ it upwards, converting some of the forward momentum to an upward trajectory -- thus the “skipping“. However, in the matter of providing the “real world“ model of rock skipping: In general, I understand now that you are advanced beyond its intended purpose.
I would think it would be quite similar to the bouncing bomb used in WW2, as that was a cylindrical shape given backspin and forward speed to make it skip over the surface of the water. Iirc, they also tested spherical bombs
@@SteveMouldI imagine that was the bit that got replaced with "well actually 0*inf _is_ defined in this case because it's a limit but we're really getting into the weeds here". Probably a good call to cut out the rigorous definitions, especially since the video was really about how we can *avoid* having to work with delta functions directly.
You use take a limit as t (time) approaches zero. (I had to derive these physics myself (without taking spin into account) for a high school senior capstone project I was writing a Pool game in C.
The "steel ball dropped on an anvil" experiment demonstrates just how short collision times can be in a fun way --- the ball bounces many times, each one a little lower (and thus taking a bit less time) than the one before, until it goes "tap tap tap taptap taptaptap bzzzthwip!" meaning that at the end it's bouncing thousands of times per second, and the time the two surfaces are in contact must be a fraction of a millisecond.
As someone who's gotten really into table tennis, one of the interesting facets of the sport is that the players effectively get to choose their coefficient of horizontal restitution via their equipment choice. We can't change the ball, but generally attacking-style players tend to choose gippy rubbers and thick, springy sponges on their paddles to maximize thier spin (aka maximize their coefficient), while defensive players tend to choose less grippy pips-out rubbers and a thin sponge to minimize the impact of the opponent's spin on their bounce angle (aka, minimize their coefficient), while actually preserving the spin of their opponent. Thanks for this video, it has given me a lot to think about with my game and technique as well.
In table tennis, if you have a spinny rubber, as most modern players do, and your opponent uses a rubber that has no spin, you own spin comes back to you inverted. If you hit with topspin, then the return has backspin. I no longer play competitively but when I did I always asked to see my opponent's bat before the match (that is your right). The rule about the two sides of the bat having different colours (red and black mandatory) was introduced so that you can clearly see which rubber your opponent is using when hitting the ball.
Years ago I noticed that 'power-balls' (a very popular 1970s toy) a high density plastic ball with a very high coefficient of restitution would jump backward and forward - reversing direction and spin on each bounce because their elasticity worked rotationally as well as vertically, hence the 'can't bounce a ball off the bottom of a table without it coming back' paradox in the opening few seconds of the video. Fascinating stuff physics.
The superballs from Wham-O newer than 2002 use a "new recipe" of Zectron that drop that direction change feature as well as lose 5-10% of vertical cof of restitution. I bought a couple old ones for $25+ and they work like a charm. Their path is so bizarre- for 3-4 bounces they look like a genuine "W" when spinning tangent to their direction of momentum.
@@unwaryquerier Yea, I was thinking about those balls also. I once knocked over a tray of them by accident (at a birthday party and they fell off the 10 ft high deck onto the driveway) and they bounced all over like grass hoppers where half the bounces were back towards me instead of all bouncing away as I had expected. After picking up the tray and collecting all the balls, I tossed it down again a few more times just to make sure I had seen what I thought I had seen.
Some of my favorite undergrad physics problems involved the physics of collisions. Here's a neat one: A solid ball sits on a frictionless plane, subject to a standard gravitational field. It is struck with an impulse 'I' from a direction horizontal to the plane and in line with the center of the ball. Where does the impulse need to be directed so that the ball rolls with zero slippage? The answer, it turns out, is 3/5D (3/5 the way up on the ball.) I use this fact when breaking a racked set of pool balls. It really makes for a solid break.
Applied physics is so much easier to grasp. Thank you for sharing this, I enjoyed the problem and the lesson feels intuitive when applied to a pool ball 😊
If the impulse is horizontal to the plane, and also hit the ball in line with its center, how could it strike the ball anywhere except the center? To hit the center of the ball 3/5 of the way up the ball it needs to be angled down towards the table.
You have to integrate, and you're screwed? Too bad you don't know another TH-cam creator who specializes in Maths. Someone like that could probably help you out (unless you were trying to come up with a magic square for some reason).
Would love to see how a golf ball interacts with a club face. There’s so much compression of both the ball and face, with such a soft cover on higher-end balls.
I wonder if manufacturers of clubs can limit the horizontal COR while maintaining vertical/neutral COR. Low-spin (but non-zero) on a driver is typically ideal, but is typically modified through a difference in swing path and face angle, since you normally hit a stationary target in golf. All bets are off if it’s a links course in Scotland, you may need to hit a moving ball. 💨
@@dannymac6368 I'm more interested in the compression of irons and wedges. Longer woods you're getting most of the compression of the ball on the clubface and the elasticity of the face itself. With an iron (particularly some of the newer hollow distance irons) you'll have similar face and ball compression (just less), but also the compression of the ball against the ground. And those two compressive forces are in different directions.
Smarter every day has a video with slow mo footage of golf balls colliding with things! I don't recall if a golf club was used, but they definitely have some cool footage!
10:49 - 12:12 perfectly sums up why billiards games are so tricky. Kick which is when the chalk that was transferred to the cue ball is perfectly positioned so that it's at the contact point with another ball, changing the variables. Billiards and curling keep physicist up at night.
As an enjoyer of pool, I did not understand that sentence. Kick is normally discussed in terms of coming off rails, kicking off the rail. Chalk is "transferred to the cue ball" on a good hit, where they make firm contact (coef. of restitution = 0, in the parlance of this video (sorry, I keep using that word today)), but it's not the chalk being transferred that's the good thing, it's the English or spin you put on the ball that dictates if the cue goes where you need it to after you make your shot. Idk what you mean about contact points and other balls and changing variables. You are right that billiards has a LOT of physics in it, a lot more than most people give it credit for. You can't use much math or physics, really, it's all feel and practice.
Great. Imma ask him if gravity is a fundamental force that is carried by bosons, or a downstream effect that arises from more basic elements of the universe.
@@BudewFan_ if it's a fundamental force, it is conveyed by a boson... the way the electromagnetic force is carried by photons, and the strong nuclear force is carried by gluons
Screw pips. I want to see a super slow mo of the chinese. I want to see what they are doing, how long the ball stays really on the rubber and how spin is created.
This actually explains a weird thing I noticed as a kid when I played with bouncy balls and tossed them with some back spin, where it would be spinning slower after it bounced and sometimes even slowly spun in the opposite direction from before. If I remember correctly it usually happened on surfaces with high friction like concrete, and now it makes sense where that force reversing the spin direction comes from.
Ever play around with a superball on a gymnasium floor? The way spin interacts with the bounces will really surprise you at first. Back when I was a kid, you could get superballs in various sizes. The ones that were about the size of a tennis ball, those were the best. Easily could make one bounce over a two-story house. (and put someone's windows out, maybe that's why they are hard to find now...)
I remember having one of those! My brother became a licensed hot-air balloon pilot in that same "era" and the first time he was able to take me up was the last day I saw that ball. Worth it, but now I miss that crazy thing!!! (The ball, of course. lol)
We had an egg-shaped superball which provided much dangerous fun on a squash court. Give it a good belt, cover your head and duck and wait for the pain. (Extra points if you hit the duck).
One small but important thing I would change about the way you talk about this: "friction always acts in the opposite direction to the direction of motion." I *would not* use the word motion here. The motion of the ball is downwards, but friction doesn't point upwards. Friction doesn't oppose motion, friction opposes *sliding.* Using the word motion here makes it hard for people to understand how friction can actually make things move (like how we walk or cars drive, friction actually pushes us forwards because our legs/wheels are trying to slide backwards).
…” Friction doesn’t oppose motion, friction opposes sliding “… So, you are saying that “sliding” is not “motion”, or that “motion” is not “sliding”.. ??? And here I always thought that ‘sliding’ involved something ‘moving’… Well slap my hide and snap my suspenders.. I learned something new today…
@@ernestgalvan9037 I am saying, quite clearly despite your apparent inability to understand, that motion is a broader category and sliding is a particular type. Friction is concerned only with sliding. When you walk, you are moving forward but your foot would be sliding backwards along the ground, so friction pushes you forwards because friction doesn't give a shit about how your body is moving, only how your foot would slide
Friction occurs with air as well, an that is against the direction of motion. Unless you are only talking about things moving over a surface you're technically wrong.
Regarding your correction at the end: momentum and angular momentum are only conserved in a closed system. For the ball-in-a-tube to be a closed system you would have to also be accounting the momentum and angular momentum of the tube. If you treat the tube as static, you are giving it infinite inertia which I believe grants it the power to create or destroy momentum. Or alternatively you analyze the ball as a system with the tube being the outside environment, in which case momentum is not conserved, as it can enter and exit the system (of the ball) from the outside environment (of the tube).
When you have a negative vertical coefficient of restitution, that would probably suggest that the projectile is passing through the target. Bullets have a negative vertical coefficient of restitution against soft materials.
I don't think it works like that. The coefficient of restitution is based on an assumption, and in that assumption the surface against which the projectile bounces is unbreakable and immovable. If a bullet collided with such an object, it would probably bounce but absorb most of the energy, having a really low coefficient of restitution.
Though it is the case that linear momentum and angular momentum are conserved independently of eachother, I think your correction may need to be qualified a bit. Linear and angular momentum are conserved, but that's because the earth takes the ball's linear momentum and gives the ball some of its angular momentum, but the ball doesn't gain energy from the earth. We see linear momentum "converted" to angular momentum when a non-spinning ball hits a wall at an angle. The linear kinetic energy of the ball must decrease because the rotating kinetic energy of the ball increased and the only source of energy is the ball. In this case "converted to" means something more like "allowed for the formation of" which is a common understanding of converted. e.g. "plants convert light into food" when really the food is not made of the light, but the decrease of light made energy available for the food to form.
Feelz good knowing this video exists (in part) from some points I made =p *and knowing it it wasn't me, us liking it forces him to look at the irony the mistake ****Mr Steve I've been watching you for a LONG LONG LONG time.. I'm clueless how I was not subscribed... perhaps TH-cam was being funny however I assure you, I love you stuff and sub
Good video, but minor quip. Elastic refers to conservation of kinetic energy, NOT momentum. Momentum is always conserved (in some system) however energy is not always conserved. Conservation of momentum and conservation of kinetic energy are NOT the same thing.
Not really related, but I couldn't stop thinking about how ABS work. One would think that the highest stopping power comes from blocking the tires completely, but in fact, due to static vs dynamic frictions and the deformation of the tyres, the most stopping power is achieved at a wheel slip of around 20%.
Could you use your Newtonian water particles and 'visualize' with a pump how water would flow in a black hole? Both "spin suction" and with 'vertical flow' lines. Wouldn't that be fun?
Your final conclusion is what I came to after a brief think, that each form of momentum is its own dimension, since objects can move with multiple degrees of freedom. It was also very use to reduce the values to 1-0 or 1-0-1. That reminded me of Planck's use of arbitrary values for things like speed, temperature, etc.
Iirc the coefficient of restitution measure the _energy_ retained, not the speed. A ball bouncing with a coefficient of 0.5 would retain about 71% of its speed (and bounce to 50% of its previous height).
Hey there really love your videos and make awesome content Just had a request for you man Recently I was trying to make a hidden storage for a PS5 like a pop up drawer which comes out with a press of a button but was not able to find any reliable info Would be great if you could show a step by step video about it keeping it in mind that it cost the bare minimum 🙏🙏
Hey there really love your videos and make awesome content Just had a request for you man Recently I was trying to make a hidden storage for a PS5 like a pop up drawer which comes out with a press of a button but was not able to find any reliable info Would be great if you could show a step by step video about it keeping it in mind that it cost the bare minimum 🙏🙏
3:45 The coefficient of restitution is essential to answering the age old "unstoppable force vs. immovable object" question, which can be modeled as a collision between two objects of infinite mass. If the coefficient if restitution in this scenario is not 1, there is a tendency for the collision to be quite destructive.
Hi @SteveMould, I'm not a scientist by any mention of the subject, I'm actually an illustration student. but whenever i picture this i imagine the ball as bendy lines or sticks coming from the centre of the ball (like a hedgehog or a sea urchin) and whenever the ball is spining and catches the table, the the point of contact is where the sticks stop moving but everything else keeps moving until it snaps back (like a mouse trap or a bow or something along those lines) idk it might not make any sense to anyone else but it does to me. anyways, love the videos, you always make my day a bit more interesting 😁
Why is it, that when you apply spin while kicking a leather football (the spherical kind, not the lemon-shaped kind), it will veer off in a certain direction, but if you were to replace that football with one of those plastic toy footballs and do the exact same thing, it will veer off in the opposite direction compared to the leather ball?
If the coefficient of horizontal restitution isn't constant when changing the initial conditions, even between the same two objects, then what's the point? You've just moved the complexity somewhere else and made no progress. I might as well say I've "solved" the problem by defining the Mouldian coefficient to be the horizontal velocity of the ball after the collision? What's the Mouldian coefficient you ask? Well it changes with every collision and there's no known method to calculate it, but it always give the exact right answer!
If you're interested in the whole static/dynamic friction relationship, it's worth exploring the exception to the rule - rubber! Tyres specifically, experience a greater dynamic friction than static friction. It's referred to as the slip ratio or slip angle depending on the circumstance, and it's the core principle behind why cars drive the way they do. For anybody that works in vehicle dynamics or any kind of racing, it's the whole reason the career exists!
3:20 I'm not a physicist, but I thought momentum was always conserved, and that elasticity is whether the kinetic energy is conserved or transferred into alternate forms of energy?
yeah, that was awkward. The thing is, this is not a closed system: the table or other bouncing surface can take up momentum, and since energy is p^2/2M, when you plug in M=mass of earth, the energy is irrelevant. Now if you were a physicist, you would know momentum is only conserved in systems that are invariant under translation, and a spinning ball bouncing on a table is not. (angular momentum is only conserved if the system is invariant under rotations--gravity breaks these symmetries, and with the spin coupling horizontal translation to rotations, that's broken too).
3:45 One of my favorite things to do in a physics simulator as a kid (can you tell I'm a nerd) was to set the restitution value greater than 1. Pretty much no matter what else you did, everything would eventually implode, how much greater than 1 the value was would just determine how long it took.
Fascinating and insightful video, thank you! One small comment, though: I don't agree with the statement that the difference between kinetic and static friction is responsible or necessary for the collisions to often give a horizontal restitution coefficient near zero. I think this is rather due to the fact that the tangential force is always dissipative (in particular always antiparallel to the direction of motion). So given enough contact time, the slip velocity is bound to approach zero.
Bravo! Nice analysis of what happens when a rotating object contacts a surface. Also nice would be the effects of top spin or back spin on aerodynamic lift.
Can you please make a part 2 of this where you mess around with some Ping Pong or even just spinning a basketball or bouncy ball as fast as you can in weird ways to see all the unique behavior you can get. It's fun to see all the crazy angles and behavior you can get to happen
Can you please do a video about spin in table tennis? It's cool to see in a bouncy ball, but I'd love to get a much more detailed 2d visual of how the rubber pips deform in a table tennis sponge and why that may affect the angle the ball returns at or the amount of spin in the returned ball vs the total returned kinetic energy, etc.
A fun thing is to drop a basketball vertically with it spinning on its vertical axis. As it presses against the ground during the bounce, the rubber grips and the spinning energy gets converted into an elastic twist. As the ball bounces back the stored elastic energy is converted back into a spin in the opposite direction.
I feel as if the rate at which comments appear here is somehow related to the length of the video and the total length of slow motion footage. I completely forget what I wanted to comment when slow motion footage appears.
Since I was a kid, I always imagined a situation where a very sticky ball (like your black one) spinning at an ungodly speed, could convert it's vertical motion almost entirely into horizontal motion after contact with the surface. I don't know if your air cannon would allow for stupid high spin rates, but it would be awesome to see that black ball contacting the surface at around 10,000 rpm (if it can hold itself together at that rotation, lol)
Steve out here flexing his physics crown on people. Be careful as some might see that as a challenge :). Physicists seem to like "hiding" information in equations and "constants". But imo asking what is happening at these friction and contact points is the more fun question than to be given the "solution" and be told "don't worry about the details". But hey that's just me.
If you really want to go into the weeds, look up car tire physics. All that stuff gets exponentially more complex, but is also super important to what makes a tire, a tire, in short. Car racing simulators, these days, even do some simplified, real-time finite element analysis to get all the little subtleties. Then once you've done that, look up racing bike physics.
I thought backspins were pretty self explanatory. You have a surface and ball with low friction, you hit the ball the right way to cause it to move forwards while it spins in the reverse direction, the ball has too much forward momentum for the spin to gain any traction until it slows down enough that the ball can grip the surface. At which point, the backwards spinning takes over and causes the ball to move in the reverse direction. You don't really need a high speed camera to work it out. But, I'll see if I'll eat these words after I actually watch the video. =P
Can you do a practical science video demonstrating the chemistry math of burning fossil fuels? The weight of one liter of petrol (gasoline) and the CO2 produced by its combustion can be understood with a few calculations: 1. **Weight of One Liter of Petrol:** - Petrol has a density of approximately 0.74 to 0.76 kg/liter. For calculation purposes, let's use an average value of 0.75 kg/liter. 2. **Combustion and CO2 Production:** - The combustion of petrol is a chemical reaction primarily involving octane (C8H18) with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). - The balanced chemical equation for the combustion of octane is: \[ 2C_8H_{18} + 25O_2 ightarrow 16CO_2 + 18H_2O \] - From this equation, we see that 2 moles of octane produce 16 moles of CO2. 3. **Molecular Weights:** - Molecular weight of octane (C8H18): \( 8 \times 12 (carbon) + 18 \times 1 (hydrogen) = 114 \) g/mol. - Molecular weight of CO2: \( 1 \times 12 (carbon) + 2 \times 16 (oxygen) = 44 \) g/mol. 4. **Calculation:** - For 1 liter of petrol, which weighs about 0.75 kg (750 grams): \[ \text{Moles of octane} = \frac{750 \text{ grams}}{114 \text{ grams/mol}} \approx 6.58 \text{ moles} \] - From the balanced equation, 2 moles of octane produce 16 moles of CO2, so 6.58 moles of octane will produce: \[ \text{Moles of CO2} = 6.58 \times \frac{16}{2} = 52.64 \text{ moles} \] - The weight of CO2 produced: \[ 52.64 \text{ moles} \times 44 \text{ grams/mol} \approx 2316.16 \text{ grams} \] Therefore, the combustion of one liter of petrol, which weighs about 0.75 kg, produces approximately 2.316 kg of CO2.
0 times infinity is not undefined. It is indeterminate. There is an important difference there. Indeterminate basically means there isn’t enough information, and the answer could be anything.
What a pleasant surprise. I was fidgeting with a bouncy ball at work a few days ago and started wondering this exact thing. I had a rudimentary understanding of what was happening, but these visuals help a lot. Thanks for yet another great video!
Dude... you're so good at this I can understand the concepts you are conveying. This is helpful because I'm not very smart. Some people are SO smart I can't understand a fucking word they're saying. Great video, well done.
Angular Momentum is a pseudo vector. The very definition of Angular Momentum is constructed from Linear Momentum. L = r x (mv) At the lowest level Angular Momentum only requires Linear Momentum and a choice of Reference Origin. So there is only Linear Momentum. en.wikipedia.org/wiki/Angular_momentum minutephysics ‘What IS Angular Momentum?’ th-cam.com/video/iWSu6U0Ujs8/w-d-xo.htmlsi=dk2Ns-BifknB6Qxr
Great video! I have been wondering for ages why, in tennis, a GROUNDSTROKE topspin ball bounces higher than a non-spinning ball, whereas in the case of a topspin LOB, the ball accelerates horizontally after the bounce!? Thanks to you I've just understood (at last) that's because the trajectory of the latter is much more vertical (incidence angle closer to 0) and thus the static friction has more time to kick in. Same phenomenon with a backspin OFFENSIVE low ball (e.g. Federer's slice backhands, especially on slippery surfaces like GreenSet or grass) which bounces lower, versus a backspin DROP SHOT which can stop its horizontal momentum after bouncing (or even bounce backwards). Thank you for this aha moment!
Is no difference between static and kinetic friction. Pull cloth under the table trick Ff=b*m*a, b>0 - dimensionless friction coefficient between table and cloth. a - acceleration a=v/t^2, if you pull cloth from under the table with objects with mass m on it very fast, let say per 0.001s, acceleration will be a=v/(0.001)^2=v*10^6 m/s^2. Object will remain still if you pull cloth very slowly 10s. Object fall on the ground from table, because acceleration equal to v/10^2=0.01*v. Feynman as quantum physics theorist can be very controversial, but as classical mechanics explainer he is the best educator ever lived. quote about static and kinetic friction > Pull cloth under the table trick is independent from velocity v but is is very sensitive to pull time squared 1/t^2. a∝1/t^2. No static no kinetic frictions is just friction described by b>0 - dimensionless friction coefficient. Static friction can be described described direct through Ff - friction through experiment. Angular velocity independent from kinetic or potential? Is depended if you right energy conservation condition for example ball rolling from ramp they can be equal. depends from physics experiment conditions.
TH-cam commenter: "I wanna know what's happening in that collision." Classical physicist: Wouldn't we all. You can get ever closer to modelling how the collision works, but you'll need quantum electrodynamics to describe the actual surface-to-surface interactions (not to mention the internal interactions that create elasticity, or even just what keeps the molecules of the ball from flying apart due to centripetal forces). But this is a Steve Mould video, not a Angela Collier or PBS Space Time video. This is an almost exclusively classical physics channel. This is spherical-cow-in-a-vacuum-on-a-frictionless-infinite-plane territory rather than this-is-a-37-minute-video-explaining-why-it-isn't-possible-to-explain-quantum-chromodynamics-in-a-ten-part-series-of-hour-long-videos territory.
Another great video about a surprisingly complicated physics problem. I need to watch it again when I am not sleepy. The one thing that I missed in the video was the interaction between the spinning ball and the air, but I understand that is a completely different story that is better to ignore for the bouncing ball problem.
What, no fancy editing to show the frame of reference for the surface, where the ball looks to have no spin or drop and the surface flies all around the ball? Pffff, 0/10, worst how backspin ACTUALLY works - in super slow motion video of the year. 😂 Joking, of course, but I'm trying to figure out if the change in direction would look "intuitive", or completely wrong. I might just have to try that on my own.
I'm always surprised by how wobbly things are in slow motion.
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The opening music almost made we shout "Twelve!"
Does this mean that if you threw the ball under the table, then lowered a hinge extension for the table, can you make the ball reverse direction multiple times?
You made this video because you watched Matt Parker and Grant Sanderson's recent billiards/pool video, didn't you?
I think people would enjoy knowing how tires work, how can they produce so much grip. it is one of those mind blowing realizations that tires use static friction: the contact patch does not move relative to the ground. The tire has as much grip when the car is stationary than when it is moving (excluding some minor things). This means that the force that you need to turn the tires on the ground to make them slide, is the maximum force that can turn the car. If you push it from any direction and the tires start to slide: that is the maximum force at any speed you can use to control the car. All of the turning, accelerating etc. is done by rubber stretching and squishing, and it wanting to return back to the original shape. So, the force that moves the car forward comes from deformation of rubber, like million rubber bands stretching. The engine rotates the axle faster than the tire is rotating and this stretches the tire, rubber wants to retain its shape as it is between the ground and the wheel, and we accelerate.
Slip angle as a concept.. man that has been one of the most rewarding things to learn, it was just constant "ahaa!" feeling, massive amount of rewarding chemicals floating around. It is such a simple thing that you just don't think about but... the part of the tire that contacts the ground does not move even if you are going 500kmh.
I suspect that the tennis ball continues spinning in the same direction because its friction with the surface is very low. It's fuzzy, so a very small area is in contact with the surface during the bounce. That lets it simply slide along.
Are you actually sitting in the yellow chair?
I just wondered the same, after reading your comment😂🤣
In Sweden the chair sits on you.
🤣
It’s an ikea chair that also has a matching footstool. I think he’s sitting on that.
@@nokbeen3654 Ahh, 👍 . I thought it might be green screen, but wondered why would it be green screened, and there’s shadows that show up… but without knowing about an ottoman it was messing with my brain. That helps, thanks!
'Tis a bit too far. My guess it's a chaise long 😉
I thought Ur observation concerned atoms' stuff: we never touch anything 😅👌
This video is secretly about two idiots playing pool using math, and forgetting to account for friction in their calculations 😆😆😆
classic mathematicians
I was thinking the same thing!
I was so hoping someone would have said, "oi, go grab that Parker triangle and rack 'em, Matt!"
I think if said mathematicians had had to apply these formulas they might still be at it, and their pool playing opponents would have conceded the game because they were all being kicked because it was closing time already.
He probably just sent Matt the video and said nothing else
When I was a little kid I once saw my dad throw a hula hoop he'd put backspin on, and when it hit the lawn edge-on the hula hoop rolled back to him, and I realized right then and there that, despite what everyone had told me, magic was real, and my dad was a wizard. It was the only explanation. 🤣
When I was a kid, my dad told me that if I could salt a bird's tail then I could catch it. I got excited and took a fish net and a salt shaker in hand and went stalking birds. Never did catch anything. It was only years later that I realized that if I was sneaky enough to get close enough to salt the tail then I could indeed catch the bird, but the salt had nothing to do with it. He might as well have said, "If you can catch a bird then you can catch a bird."
@@mailleweaveryeah, but where is the fun?
the best phrases are those that seem absurd, but still carry the truth underneath.
your dad got it well to push you to keep going
Gee, my dad told me that toilet paper was getting really expensive, so we had to start conserving it by using both sides.
Just fold it
@@mailleweaver Up for a Snipe hunt, anyone?
Tangentially related;
In water polo you can bounce the ball on the surface of the water by giving it the right combination of backspin and angle of attack (otherwise it just stops dead in the water).
Generally more backspin, steeper angle, more power, lower ball pressure, less textured grip on the ball increase the bounce height, but sometimes a grippy ball thrown at a low angle with a bit of sidespin can pop up just as high.
I can do it instinctively but I've never quite been able to grasp how it works...
_@NKuijlaars_ Cannot grasp the concept, you say? It is identical to the phenomenon of _skipping stones._
Plenty of blather on that topic, accessible from a Google search.
@@brianhiles8164Firstly, by grasp I meant being able to control it based on my understanding of the physics, and vice versa, but that just doesn't really work here. When skipping stones the side spin is only for stability (?) and there is no backspin, but with a ball stability isn't really relevant (?) and the ball *cannot* bounce without backspin. The ball is also capable of bouncing out of the water with a steeper angle than it was thrown which makes me think it's more of a restitution problem.
My apologies for daring to think there might be an interesting little nugget of physics in the mechanism here.
@@NKuijlaars Obviously, you have a “sufficient“ technical background, that you are both _able_ to be fascinated by the nuances and intricacies of the physics involved, and just as importantly have a _discrimination_ of when you have indeed “wrapped your mind around“ an understanding of the problem. This is key.
On more than one occasion, I have purposefully _not_ remanded my understanding to an expert, that I have forced myself to figure out a given mystery myself.
As an aside, though, in the matter of _restitution_ being the prevalent phenomenon of a Newtonian liquid: If I understand the context of your observation, angle-of-attack is predominantly a function of the forward momentum of the rock inducing an inclined plane of the “sticky“ water surface to then be “launching“ it upwards, converting some of the forward momentum to an upward trajectory -- thus the “skipping“.
However, in the matter of providing the “real world“ model of rock skipping: In general, I understand now that you are advanced beyond its intended purpose.
I would think it would be quite similar to the bouncing bomb used in WW2, as that was a cylindrical shape given backspin and forward speed to make it skip over the surface of the water. Iirc, they also tested spherical bombs
As a polo goalie the bounce shot is my worst nightmare, impossible to predict
“How could you work out the area under graph that’s infinitely thin and infinitely tall?”
Laughs in Dirac Delta
The original script had that it but it got cut!
Thats 1 way of describing an area
@@SteveMouldI imagine that was the bit that got replaced with "well actually 0*inf _is_ defined in this case because it's a limit but we're really getting into the weeds here". Probably a good call to cut out the rigorous definitions, especially since the video was really about how we can *avoid* having to work with delta functions directly.
@@SteveMould Was it cut? Or did it become an infinitely informative segment with zero duration?
You use take a limit as t (time) approaches zero. (I had to derive these physics myself (without taking spin into account) for a high school senior capstone project I was writing a Pool game in C.
The "steel ball dropped on an anvil" experiment demonstrates just how short collision times can be in a fun way --- the ball bounces many times, each one a little lower (and thus taking a bit less time) than the one before, until it goes "tap tap tap taptap taptaptap bzzzthwip!" meaning that at the end it's bouncing thousands of times per second, and the time the two surfaces are in contact must be a fraction of a millisecond.
"bzzzthwip" is spot on. I knew there had to be a word for it.
IMAGINE GETTING 69 LIKES.
As someone who's gotten really into table tennis, one of the interesting facets of the sport is that the players effectively get to choose their coefficient of horizontal restitution via their equipment choice. We can't change the ball, but generally attacking-style players tend to choose gippy rubbers and thick, springy sponges on their paddles to maximize thier spin (aka maximize their coefficient), while defensive players tend to choose less grippy pips-out rubbers and a thin sponge to minimize the impact of the opponent's spin on their bounce angle (aka, minimize their coefficient), while actually preserving the spin of their opponent.
Thanks for this video, it has given me a lot to think about with my game and technique as well.
hello (i play too!)
In table tennis, if you have a spinny rubber, as most modern players do, and your opponent uses a rubber that has no spin, you own spin comes back to you inverted. If you hit with topspin, then the return has backspin. I no longer play competitively but when I did I always asked to see my opponent's bat before the match (that is your right). The rule about the two sides of the bat having different colours (red and black mandatory) was introduced so that you can clearly see which rubber your opponent is using when hitting the ball.
@@trapkat8213oh huh TIL the colours on ping pong paddles are used to actually convey information among the pros. Which one is which if I may ask?
Years ago I noticed that 'power-balls' (a very popular 1970s toy) a high density plastic ball with a very high coefficient of restitution would jump backward and forward - reversing direction and spin on each bounce because their elasticity worked rotationally as well as vertically, hence the 'can't bounce a ball off the bottom of a table without it coming back' paradox in the opening few seconds of the video.
Fascinating stuff physics.
The superballs from Wham-O newer than 2002 use a "new recipe" of Zectron that drop that direction change feature as well as lose 5-10% of vertical cof of restitution. I bought a couple old ones for $25+ and they work like a charm. Their path is so bizarre- for 3-4 bounces they look like a genuine "W" when spinning tangent to their direction of momentum.
@@unwaryquerier Yea, I was thinking about those balls also. I once knocked over a tray of them by accident (at a birthday party and they fell off the 10 ft high deck onto the driveway) and they bounced all over like grass hoppers where half the bounces were back towards me instead of all bouncing away as I had expected. After picking up the tray and collecting all the balls, I tossed it down again a few more times just to make sure I had seen what I thought I had seen.
Some of my favorite undergrad physics problems involved the physics of collisions. Here's a neat one: A solid ball sits on a frictionless plane, subject to a standard gravitational field. It is struck with an impulse 'I' from a direction horizontal to the plane and in line with the center of the ball. Where does the impulse need to be directed so that the ball rolls with zero slippage? The answer, it turns out, is 3/5D (3/5 the way up on the ball.) I use this fact when breaking a racked set of pool balls. It really makes for a solid break.
Applied physics is so much easier to grasp. Thank you for sharing this, I enjoyed the problem and the lesson feels intuitive when applied to a pool ball 😊
Huh. This is information that will stick at the back of my mind until one day I can use it.
Even knowing this information, I know that I'll remain unable to play pool any better :D
Fantastic insight, thank you :)
If the impulse is horizontal to the plane, and also hit the ball in line with its center, how could it strike the ball anywhere except the center?
To hit the center of the ball 3/5 of the way up the ball it needs to be angled down towards the table.
now try to figure where is 3/5 on the ball
1:34 "We're screwed, basically"
- Steve Mould, 2024
Out of context, but still highly relevant.
You have to integrate, and you're screwed? Too bad you don't know another TH-cam creator who specializes in Maths. Someone like that could probably help you out (unless you were trying to come up with a magic square for some reason).
I'm not going to have some Parker Integration on this channel.
@@SteveMouldway to *stand up* to the *maths* !
@@SteveMould I'm both sad that he won't be here, and happy that you're having a jab at him! 🤣
Matt is more for Math. For Physics, call over Dianna Cowern. (Physics Girl) =)
@@SteveMouldMatt is more for Math. For Physics, call over Dianna Cowern. (Physics Girl) =)
Would love to see how a golf ball interacts with a club face. There’s so much compression of both the ball and face, with such a soft cover on higher-end balls.
Edit: compression on metal woods (Driver, 3-wood, fairway metal)…though the interaction with a grooved iron or wedge would be just as interesting.
I wonder if manufacturers of clubs can limit the horizontal COR while maintaining vertical/neutral COR. Low-spin (but non-zero) on a driver is typically ideal, but is typically modified through a difference in swing path and face angle, since you normally hit a stationary target in golf.
All bets are off if it’s a links course in Scotland, you may need to hit a moving ball. 💨
@@dannymac6368 I'm more interested in the compression of irons and wedges.
Longer woods you're getting most of the compression of the ball on the clubface and the elasticity of the face itself. With an iron (particularly some of the newer hollow distance irons) you'll have similar face and ball compression (just less), but also the compression of the ball against the ground. And those two compressive forces are in different directions.
Smarter every day has a video with slow mo footage of golf balls colliding with things! I don't recall if a golf club was used, but they definitely have some cool footage!
I'd love to know the physical properties that attracts a golf ball to trees and bunkers and repels them from the hole
Now explain flubber
And motor boating...
Flabber is highly classified.
Damn, I am old enough to get the reference.
I can't belive he forgot about Tau Day. I thought he was a real tau fan.
Tauists aren't Pi-ous. He needn't be advocating for Tau all the time. He has a family and such.
Besides, tau as a circle constant speaks for itself
Being a Tau fan is the real reason he has more subscribers than that Matt guy. Tauists are just a better class of people.
Hawk Tau
@@shandybassheadComing full circle
10:49 - 12:12 perfectly sums up why billiards games are so tricky. Kick which is when the chalk that was transferred to the cue ball is perfectly positioned so that it's at the contact point with another ball, changing the variables. Billiards and curling keep physicist up at night.
As an enjoyer of pool, I did not understand that sentence. Kick is normally discussed in terms of coming off rails, kicking off the rail. Chalk is "transferred to the cue ball" on a good hit, where they make firm contact (coef. of restitution = 0, in the parlance of this video (sorry, I keep using that word today)), but it's not the chalk being transferred that's the good thing, it's the English or spin you put on the ball that dictates if the cue goes where you need it to after you make your shot. Idk what you mean about contact points and other balls and changing variables. You are right that billiards has a LOT of physics in it, a lot more than most people give it credit for. You can't use much math or physics, really, it's all feel and practice.
1:34 This is the right response, anytime you need to use integrals in physics
Steve can answer ANY physics question we throw at him, and provide high quality demonstrations and footage to show it. What a legend
throw at him? with what velocity and spin?
Great. Imma ask him if gravity is a fundamental force that is carried by bosons, or a downstream effect that arises from more basic elements of the universe.
@@reidflemingworldstoughestm1394it’s a more fundamental force, it results from the way space bends around big stuff, no carrying particle whatsoever
@@BudewFan_ if it's a fundamental force, it is conveyed by a boson... the way the electromagnetic force is carried by photons, and the strong nuclear force is carried by gluons
@@reidflemingworldstoughestm1394 He's got balls to do it.
Try doing the same study for table tennis. Show why "long pips" are kinda unpredictable compared to an inverted rubber, when doing top or backspin.
Screw pips. I want to see a super slow mo of the chinese. I want to see what they are doing, how long the ball stays really on the rubber and how spin is created.
@@JP_Hatecrewneed to get checkered balls popular, so we can get better slow-mo footage of their spin
@@giovane_Diaz great idea!
@@JP_Hatecrew there's a video Fang Bo made, i believe, where he analyzes the rotational speed of Xu Xin's forehand on a slowmo cam.
Also, the ball being hollow means spin is more important.
This actually explains a weird thing I noticed as a kid when I played with bouncy balls and tossed them with some back spin, where it would be spinning slower after it bounced and sometimes even slowly spun in the opposite direction from before. If I remember correctly it usually happened on surfaces with high friction like concrete, and now it makes sense where that force reversing the spin direction comes from.
Ever play around with a superball on a gymnasium floor? The way spin interacts with the bounces will really surprise you at first. Back when I was a kid, you could get superballs in various sizes. The ones that were about the size of a tennis ball, those were the best. Easily could make one bounce over a two-story house. (and put someone's windows out, maybe that's why they are hard to find now...)
I remember having one of those! My brother became a licensed hot-air balloon pilot in that same "era" and the first time he was able to take me up was the last day I saw that ball. Worth it, but now I miss that crazy thing!!! (The ball, of course. lol)
We had an egg-shaped superball which provided much dangerous fun on a squash court. Give it a good belt, cover your head and duck and wait for the pain. (Extra points if you hit the duck).
I've actually been wondering about this forever, thank you for making this video!
One small but important thing I would change about the way you talk about this: "friction always acts in the opposite direction to the direction of motion." I *would not* use the word motion here. The motion of the ball is downwards, but friction doesn't point upwards. Friction doesn't oppose motion, friction opposes *sliding.* Using the word motion here makes it hard for people to understand how friction can actually make things move (like how we walk or cars drive, friction actually pushes us forwards because our legs/wheels are trying to slide backwards).
…” Friction doesn’t oppose motion, friction opposes sliding “…
So, you are saying that “sliding” is not “motion”, or that “motion” is not “sliding”.. ???
And here I always thought that ‘sliding’ involved something ‘moving’…
Well slap my hide and snap my suspenders.. I learned something new today…
@@ernestgalvan9037 I am saying, quite clearly despite your apparent inability to understand, that motion is a broader category and sliding is a particular type. Friction is concerned only with sliding. When you walk, you are moving forward but your foot would be sliding backwards along the ground, so friction pushes you forwards because friction doesn't give a shit about how your body is moving, only how your foot would slide
@@ernestgalvan9037 Sliding is motion constrained to a contact surface.
Friction occurs with air as well, an that is against the direction of motion. Unless you are only talking about things moving over a surface you're technically wrong.
Definitely a great thing to consider. Because walking on slippery ice is hard.
Now you've got me thinking about how the dambusters worked. Great video
"the force must be infinite"
Me: I bet they don't like infinity......
"and in physics we generally don't like infinity"
CALLED IT!
This was a very complicated story. Sorry Steve, sadly I gave up on it and only managed halfway. (I must be more stupid than I thought!) 😳
I’m so stupid I stayed for the whole video and *THOUGHT* I understood it all! I left with no questions! 🥸👏🏼
Thanks for not using a clickbait style text or thumbnail.
Slow motion shots are so fascinating!
Regarding your correction at the end: momentum and angular momentum are only conserved in a closed system. For the ball-in-a-tube to be a closed system you would have to also be accounting the momentum and angular momentum of the tube. If you treat the tube as static, you are giving it infinite inertia which I believe grants it the power to create or destroy momentum. Or alternatively you analyze the ball as a system with the tube being the outside environment, in which case momentum is not conserved, as it can enter and exit the system (of the ball) from the outside environment (of the tube).
10:37 when you store footage for 1000 years and dig it up so you don't have to ruin another sheet of paper XD
WOW! This was the nicest, clearest and most objective masterclass on collisions I've ever watched!!
Very fine work, Steve!
When you have a negative vertical coefficient of restitution, that would probably suggest that the projectile is passing through the target. Bullets have a negative vertical coefficient of restitution against soft materials.
I don't think it works like that. The coefficient of restitution is based on an assumption, and in that assumption the surface against which the projectile bounces is unbreakable and immovable. If a bullet collided with such an object, it would probably bounce but absorb most of the energy, having a really low coefficient of restitution.
Though it is the case that linear momentum and angular momentum are conserved independently of eachother, I think your correction may need to be qualified a bit.
Linear and angular momentum are conserved, but that's because the earth takes the ball's linear momentum and gives the ball some of its angular momentum, but the ball doesn't gain energy from the earth.
We see linear momentum "converted" to angular momentum when a non-spinning ball hits a wall at an angle. The linear kinetic energy of the ball must decrease because the rotating kinetic energy of the ball increased and the only source of energy is the ball.
In this case "converted to" means something more like "allowed for the formation of" which is a common understanding of converted.
e.g. "plants convert light into food" when really the food is not made of the light, but the decrease of light made energy available for the food to form.
Feelz good knowing this video exists (in part) from some points I made =p
*and knowing it it wasn't me, us liking it forces him to look at the irony the mistake
****Mr Steve I've been watching you for a LONG LONG LONG time.. I'm clueless how I was not subscribed... perhaps TH-cam was being funny however I assure you, I love you stuff and sub
Good video, but minor quip. Elastic refers to conservation of kinetic energy, NOT momentum. Momentum is always conserved (in some system) however energy is not always conserved. Conservation of momentum and conservation of kinetic energy are NOT the same thing.
Steve's videos are the best. Don't ever change your format, man.
Not really related, but I couldn't stop thinking about how ABS work.
One would think that the highest stopping power comes from blocking the tires completely, but in fact, due to static vs dynamic frictions and the deformation of the tyres, the most stopping power is achieved at a wheel slip of around 20%.
Ah, missed opportunity to shot handball balls friction, it’s used as a technique in the game.
What if the ball is a particle, eg. a photon? Can a particle be spun like a ball then bounced to a surface and have a non-ordinary return angle? 🤩
Could you use your Newtonian water particles and 'visualize' with a pump how water would flow in a black hole?
Both "spin suction" and with 'vertical flow' lines.
Wouldn't that be fun?
awesome, i've been wondering about this for the last nine days.
Your final conclusion is what I came to after a brief think, that each form of momentum is its own dimension, since objects can move with multiple degrees of freedom. It was also very use to reduce the values to 1-0 or 1-0-1. That reminded me of Planck's use of arbitrary values for things like speed, temperature, etc.
Iirc the coefficient of restitution measure the _energy_ retained, not the speed. A ball bouncing with a coefficient of 0.5 would retain about 71% of its speed (and bounce to 50% of its previous height).
Hey there really love your videos and make awesome content
Just had a request for you man
Recently I was trying to make a hidden storage for a PS5 like a pop up drawer which comes out with a press of a button but was not able to find any reliable info
Would be great if you could show a step by step video about it keeping it in mind that it cost the bare minimum 🙏🙏
Hey there really love your videos and make awesome content
Just had a request for you man
Recently I was trying to make a hidden storage for a PS5 like a pop up drawer which comes out with a press of a button but was not able to find any reliable info
Would be great if you could show a step by step video about it keeping it in mind that it cost the bare minimum 🙏🙏
"I'll be abbreviating it to HCR"
Immediately says the whole phrase again 😂
Where were you Steve when Matt and Grant challenged the pool players?? They could have really needed you with this knowledge there! 😤
3:45 The coefficient of restitution is essential to answering the age old "unstoppable force vs. immovable object" question, which can be modeled as a collision between two objects of infinite mass.
If the coefficient if restitution in this scenario is not 1, there is a tendency for the collision to be quite destructive.
Steve Mould: when physicists realize what engineers do. ;)
"we have to do an integration to get the impulse"
"so basically we're screwed"
yeah
Hey Steve, thanks for this video, as a ping pong king I love backspin.
Hi @SteveMould,
I'm not a scientist by any mention of the subject, I'm actually an illustration student. but whenever i picture this i imagine the ball as bendy lines or sticks coming from the centre of the ball (like a hedgehog or a sea urchin) and whenever the ball is spining and catches the table, the the point of contact is where the sticks stop moving but everything else keeps moving until it snaps back (like a mouse trap or a bow or something along those lines) idk it might not make any sense to anyone else but it does to me.
anyways, love the videos, you always make my day a bit more interesting 😁
I thought the same thing, but in different lines, like, "squash and stretch" animation principle. Amazing!
8:37 why 2/7? that seems like a strange constant in a physics equation
Now I can fully understand Kyiora's insane backspin pass thank you steve 🙏
You know, I always thought mechanics was boring and solved physics but Steve always manages to bring back some weird stuff.
We’re screwed, basically. 😂
*mentions calculus*
"We're screwed."
💀💀💀
Squishy balls are way more fun than the hard ones.
Mehdi is right; you have beautiful blue eyes..
😂
Why is it, that when you apply spin while kicking a leather football (the spherical kind, not the lemon-shaped kind), it will veer off in a certain direction, but if you were to replace that football with one of those plastic toy footballs and do the exact same thing, it will veer off in the opposite direction compared to the leather ball?
If the coefficient of horizontal restitution isn't constant when changing the initial conditions, even between the same two objects, then what's the point? You've just moved the complexity somewhere else and made no progress. I might as well say I've "solved" the problem by defining the Mouldian coefficient to be the horizontal velocity of the ball after the collision? What's the Mouldian coefficient you ask? Well it changes with every collision and there's no known method to calculate it, but it always give the exact right answer!
If you're interested in the whole static/dynamic friction relationship, it's worth exploring the exception to the rule - rubber! Tyres specifically, experience a greater dynamic friction than static friction. It's referred to as the slip ratio or slip angle depending on the circumstance, and it's the core principle behind why cars drive the way they do. For anybody that works in vehicle dynamics or any kind of racing, it's the whole reason the career exists!
the 7 in those hcr equations really rubs me the wrong way. like.. why 7 specifically?
I know what you mean! If it's any consolation it comes from the geometry of a sphere.
3:20 I'm not a physicist, but I thought momentum was always conserved, and that elasticity is whether the kinetic energy is conserved or transferred into alternate forms of energy?
yeah, that was awkward. The thing is, this is not a closed system: the table or other bouncing surface can take up momentum, and since energy is p^2/2M, when you plug in M=mass of earth, the energy is irrelevant.
Now if you were a physicist, you would know momentum is only conserved in systems that are invariant under translation, and a spinning ball bouncing on a table is not. (angular momentum is only conserved if the system is invariant under rotations--gravity breaks these symmetries, and with the spin coupling horizontal translation to rotations, that's broken too).
3:45 One of my favorite things to do in a physics simulator as a kid (can you tell I'm a nerd) was to set the restitution value greater than 1. Pretty much no matter what else you did, everything would eventually implode, how much greater than 1 the value was would just determine how long it took.
Fascinating and insightful video, thank you!
One small comment, though: I don't agree with the statement that the difference between kinetic and static friction is responsible or necessary for the collisions to often give a horizontal restitution coefficient near zero. I think this is rather due to the fact that the tangential force is always dissipative (in particular always antiparallel to the direction of motion). So given enough contact time, the slip velocity is bound to approach zero.
:) last
Aww, I wanted to be last. I usually am. (Esp when things are done alphabetically I'm always near the end. And when teams are being picked in school)
Bravo! Nice analysis of what happens when a rotating object contacts a surface. Also nice would be the effects of top spin or back spin on aerodynamic lift.
Can you please make a part 2 of this where you mess around with some Ping Pong or even just spinning a basketball or bouncy ball as fast as you can in weird ways to see all the unique behavior you can get. It's fun to see all the crazy angles and behavior you can get to happen
Can you please do a video about spin in table tennis? It's cool to see in a bouncy ball, but I'd love to get a much more detailed 2d visual of how the rubber pips deform in a table tennis sponge and why that may affect the angle the ball returns at or the amount of spin in the returned ball vs the total returned kinetic energy, etc.
A fun thing is to drop a basketball vertically with it spinning on its vertical axis. As it presses against the ground during the bounce, the rubber grips and the spinning energy gets converted into an elastic twist. As the ball bounces back the stored elastic energy is converted back into a spin in the opposite direction.
quantititivley 5:10
ever see a baseball pitcher throw a curveball?
Magnus is off-topic today.
I love zero
I feel as if the rate at which comments appear here is somehow related to the length of the video and the total length of slow motion footage. I completely forget what I wanted to comment when slow motion footage appears.
Awesome video! Can you do electron spin next?
You are a marvelous teacher for many !
Fascinating topic.
I always love the surprise in what you choose to talk about in every new video Steve.
"I'm gonna go get the papers, get the papers."
1:07 bro that is so mesmerising to watch
Always answering questions I didn’t ask but I’m glad you did
Since I was a kid, I always imagined a situation where a very sticky ball (like your black one) spinning at an ungodly speed, could convert it's vertical motion almost entirely into horizontal motion after contact with the surface. I don't know if your air cannon would allow for stupid high spin rates, but it would be awesome to see that black ball contacting the surface at around 10,000 rpm (if it can hold itself together at that rotation, lol)
Steve out here flexing his physics crown on people. Be careful as some might see that as a challenge :). Physicists seem to like "hiding" information in equations and "constants". But imo asking what is happening at these friction and contact points is the more fun question than to be given the "solution" and be told "don't worry about the details". But hey that's just me.
If you really want to go into the weeds, look up car tire physics. All that stuff gets exponentially more complex, but is also super important to what makes a tire, a tire, in short.
Car racing simulators, these days, even do some simplified, real-time finite element analysis to get all the little subtleties.
Then once you've done that, look up racing bike physics.
Thanks a lot, Steve. I watched this whole video and now my brain is broken 😵💫
~Trav
Perfect timing
I thought backspins were pretty self explanatory. You have a surface and ball with low friction, you hit the ball the right way to cause it to move forwards while it spins in the reverse direction, the ball has too much forward momentum for the spin to gain any traction until it slows down enough that the ball can grip the surface. At which point, the backwards spinning takes over and causes the ball to move in the reverse direction. You don't really need a high speed camera to work it out. But, I'll see if I'll eat these words after I actually watch the video. =P
Can you do a practical science video demonstrating the chemistry math of burning fossil fuels?
The weight of one liter of petrol (gasoline) and the CO2 produced by its combustion can be understood with a few calculations:
1. **Weight of One Liter of Petrol:**
- Petrol has a density of approximately 0.74 to 0.76 kg/liter. For calculation purposes, let's use an average value of 0.75 kg/liter.
2. **Combustion and CO2 Production:**
- The combustion of petrol is a chemical reaction primarily involving octane (C8H18) with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).
- The balanced chemical equation for the combustion of octane is:
\[ 2C_8H_{18} + 25O_2
ightarrow 16CO_2 + 18H_2O \]
- From this equation, we see that 2 moles of octane produce 16 moles of CO2.
3. **Molecular Weights:**
- Molecular weight of octane (C8H18): \( 8 \times 12 (carbon) + 18 \times 1 (hydrogen) = 114 \) g/mol.
- Molecular weight of CO2: \( 1 \times 12 (carbon) + 2 \times 16 (oxygen) = 44 \) g/mol.
4. **Calculation:**
- For 1 liter of petrol, which weighs about 0.75 kg (750 grams):
\[ \text{Moles of octane} = \frac{750 \text{ grams}}{114 \text{ grams/mol}} \approx 6.58 \text{ moles} \]
- From the balanced equation, 2 moles of octane produce 16 moles of CO2, so 6.58 moles of octane will produce:
\[ \text{Moles of CO2} = 6.58 \times \frac{16}{2} = 52.64 \text{ moles} \]
- The weight of CO2 produced:
\[ 52.64 \text{ moles} \times 44 \text{ grams/mol} \approx 2316.16 \text{ grams} \]
Therefore, the combustion of one liter of petrol, which weighs about 0.75 kg, produces approximately 2.316 kg of CO2.
0 times infinity is not undefined. It is indeterminate. There is an important difference there. Indeterminate basically means there isn’t enough information, and the answer could be anything.
What a pleasant surprise. I was fidgeting with a bouncy ball at work a few days ago and started wondering this exact thing. I had a rudimentary understanding of what was happening, but these visuals help a lot. Thanks for yet another great video!
Dude... you're so good at this I can understand the concepts you are conveying. This is helpful because I'm not very smart. Some people are SO smart I can't understand a fucking word they're saying. Great video, well done.
Angular Momentum is a pseudo vector.
The very definition of Angular Momentum is constructed from Linear Momentum. L = r x (mv)
At the lowest level Angular Momentum only requires Linear Momentum and a choice of Reference Origin.
So there is only Linear Momentum.
en.wikipedia.org/wiki/Angular_momentum
minutephysics ‘What IS Angular Momentum?’
th-cam.com/video/iWSu6U0Ujs8/w-d-xo.htmlsi=dk2Ns-BifknB6Qxr
Great video!
I have been wondering for ages why, in tennis, a GROUNDSTROKE topspin ball bounces higher than a non-spinning ball, whereas in the case of a topspin LOB, the ball accelerates horizontally after the bounce!? Thanks to you I've just understood (at last) that's because the trajectory of the latter is much more vertical (incidence angle closer to 0) and thus the static friction has more time to kick in.
Same phenomenon with a backspin OFFENSIVE low ball (e.g. Federer's slice backhands, especially on slippery surfaces like GreenSet or grass) which bounces lower, versus a backspin DROP SHOT which can stop its horizontal momentum after bouncing (or even bounce backwards).
Thank you for this aha moment!
Is no difference between static and kinetic friction. Pull cloth under the table trick Ff=b*m*a, b>0 - dimensionless friction coefficient between table and cloth. a - acceleration a=v/t^2, if you pull cloth from under the table with objects with mass m on it very fast, let say per 0.001s, acceleration will be a=v/(0.001)^2=v*10^6 m/s^2. Object will remain still if you pull cloth very slowly 10s. Object fall on the ground from table, because acceleration equal to v/10^2=0.01*v.
Feynman as quantum physics theorist can be very controversial, but as classical mechanics explainer he is the best educator ever lived. quote about static and kinetic friction
>
Pull cloth under the table trick is independent from velocity v but is is very sensitive to pull time squared 1/t^2. a∝1/t^2. No static no kinetic frictions is just friction described by b>0 - dimensionless friction coefficient. Static friction can be described described direct through Ff - friction through experiment.
Angular velocity independent from kinetic or potential? Is depended if you right energy conservation condition for example ball rolling from ramp they can be equal. depends from physics experiment conditions.
TH-cam commenter: "I wanna know what's happening in that collision."
Classical physicist: Wouldn't we all.
You can get ever closer to modelling how the collision works, but you'll need quantum electrodynamics to describe the actual surface-to-surface interactions (not to mention the internal interactions that create elasticity, or even just what keeps the molecules of the ball from flying apart due to centripetal forces).
But this is a Steve Mould video, not a Angela Collier or PBS Space Time video. This is an almost exclusively classical physics channel. This is spherical-cow-in-a-vacuum-on-a-frictionless-infinite-plane territory rather than this-is-a-37-minute-video-explaining-why-it-isn't-possible-to-explain-quantum-chromodynamics-in-a-ten-part-series-of-hour-long-videos territory.
Another great video about a surprisingly complicated physics problem. I need to watch it again when I am not sleepy. The one thing that I missed in the video was the interaction between the spinning ball and the air, but I understand that is a completely different story that is better to ignore for the bouncing ball problem.
That's the Magnus effect and Steve has already done that one
I can only imagine your Amazon search history after buying all of these ...spheres.... of various durometers.
What, no fancy editing to show the frame of reference for the surface, where the ball looks to have no spin or drop and the surface flies all around the ball? Pffff, 0/10, worst how backspin ACTUALLY works - in super slow motion video of the year. 😂 Joking, of course, but I'm trying to figure out if the change in direction would look "intuitive", or completely wrong. I might just have to try that on my own.
I've just noticed you've exceeded 3 million subscribers. Congratulations.
The algorithm is wrong about me enjoying that video next...because I've already seen that video LOL