If pulling the rope horizontally makes it roll backward, and near-vertically makes it roll forward, is there some angle in between where it won't roll at all?
If you pull the spool at an angle such that string intersects the ground at the contact point of the larger wheels, then it will either not move, or it will slide across the ground. Either way, the spool does *not* rotate
1:18 "Not wrong, *surprising*!" Woah, that's an genius way to turn something that could hurt your self esteem to something that makes you excited and interested in learning more!
If the spool were to be completely filled with rope, would it roll backwards due to how close the rope you are pulling is to the ground? And if so, would there be a point in which it would not roll at all (assuming the same force is applied)?
Rick Restagno it will go back only if the diameter of the circle on which rope is wound is more than the diameter of the circle in contact (something like train tracks)
Heh, you do this exact riddle in first year engineering physics and it is just as confusing even after you learn all the rules do all the math and know how to work the diagrams. One thing I learned about physics is NEVER to trust my instincts. Draw a diagram, map the forces and really dig into the problem cause your instants, Chris...they are just so often wrong.
That is a really good picture. The first time I rode a horse bare back I fell off when the horse sped up and made a quick turn while crossing a creek. I landed flat on my back in water. So lucky I didn't land on any of the protruding rocks.
I love the format of these riddle series, this was one of my favorite yet. I thought a bit about the spool riddle after seeing the first two but was surprised. This would be a fun one to replicate with different sizes of spools.
You don't need different sizes of spools. Just spools that are wound up to different proportions. It's the ratio of the two radiuses/radii that is important: one to point where the rope leaves the spool, one to the point where the spool touches the ground.
Let's find it mathematically: Draw the FBD of the given situation (1:50). Assume radius of cylinder is R and tension force(T) is acting at r perpendicular distance from COM. Clearly r
This is what my 100 level university physics classes were like. We all had clickers and gave answers so the professors could instantly gauge our overall level of understanding. I ended up with a pharmacy degree but these were probably my most memorable and enjoyed courses.
Having dropped a lot of spools under the desk, it depends on the surface, I have unwound spools, lost them further back, and mostly chased them in circles! I think the angle of the pull, the surface, and the rate of pull all weigh into this for the answer!!
If you pull the spool at an angle such that string intersects the ground at the contact point of the larger wheels, then it will either not move, or it will slide across the ground. Either way, the spool does *not* rotate
Is there really the need to solve this with torque? I just made a sketch: 2 concentric circles. The inner circle rolls on the rope, the outer one on the floor. Now just let the whole system roll to one side and calculate (using the circumference) how the length of the rope changes. Because the inner circle is smaller than the outer one, the change of length of the rope is less than the travelled distance of the spool. So in order to move the rope relative to the floor it has to go the same direction as the spool. Now you can even see, that if the inner circle would be larger than the outer one, the spool would move in the opposite direction. You could test this if you use some system on rails. :)
The force and the motion of the center of mass has to be in same direction. This solves it completely. If it's com moves where you pull it, it can't roll the other way.
cool! I watched this video when it came out and it was interesting. Now I'm rewatching because I'm going to coach teens in a mech eng summer school projecty thing, and this video is my inspiration :D thanks Diana
I founded the same problem on my mechanics exercise book just two or three weeks ago! It's wonderful to see that it really works! (I mean: Physics is an approssimation of reality, so lot of things that happen in a problem won't work in real life). Ps: I'm sorry for my bad english but I wanted to show my appreciation!) thank you!
this physics is mastered industrially with a spool turning on a winch and a manilla rope looped around the spool. one end in your hand, the other tied on a 400lb side hammer with a rock core spoon. The setup is called a cats head. its used to get a core sample of bedrock. you tug the rope applying friction to the spool, and the motor supplies extra torque. it is very fast powerful and dangerous.
Rolling along the ground constitutes a rotation about the contact point between the spool and the ground at any moment. The net torque around this point is what determines which way it rolls. If the spool is reasonably symmetric, gravity cannot provide such a torque around the ground contact point because it has no offset lever arm around that point (its line of force goes right through that point at all times, and thus cannot create torque around it). So the only force that can provide a torque around the ground contact point is the applied force. Draw the line of force along the direction of that pulling force, and see if the torque generated by that force makes the object rotate clockwise or counterclockwise around the ground contact point. If the applied force makes provides a counterclockwise torque around the ground contact, it will roll to the left, as in the first example. If it provides a clockwise torque around the ground contact, it will roll to the right, as in the second case. You can make this quick, one-step determination with any force.
I remember the first time I heard this riddle I thought the same thing as Anne Wojcicki, that the spool would unwind. It's interesting how intuition (at least mine and hers anyway) goes straight to torque rather than force when applied to circles and rotating objects. If you think about force, it is ridiculous to expect pulling one way on an object to cause it to move the other way, but when you're thinking about spin, this rather silly answer seems perfectly obvious.
I've done this before, but it's always fun to watch. Here's something to give Dianna simmering to ponder on; the friction on the outer side doesn't matter! Pull it too fast for fiction to act on it and it'll still do the same thing, come right at you. Pulling it straight up though, hmmm? 🤔 I've got to try this one again.
I'd say that the considerations about where the force is applied is not key. As long as the distance traveled in any direction is directly proportional to the outer rim length, the circumference of the circle, and the string comes from a smaller circle, the movement will *always* be in the same direction as the the pull on the string, irrespective of angles. Of course, it would take extra tricks to keep any upward movement directly proportional with the rotation of the spool, as it normally slips *very* easily if lifted from the floor. A movement opposite of the pull would require much more string than unwound.
I have Physics knowledge on a highschool level, so I'm not that familiar with torques and I solved it in a different way. If it doesn't slide, it means that the velocity of th point of the wheel on the spot which touches the ground must be 0. To negate the velocity of the object to the left, the whole wheel has to rotate anti-clockwise
Another way of looking at it. Suppose the spindle has radius r and the rim has radius R. For one rotation of the spool, the centre moves 2 * pi * R and the extra rope is 2 * pi * r. Thus the end of the rope moves 2 * pi * (R-r). Turn it around and pull on the rope instead. Since the rim is larger than the spindle, the amount of free rope decreases and the spool moves towards you. Now consider when the rim is supported on rails with a gap between them so that the rope can be wrapped to a larger radius than the rim. When you start pulling, the spool will move away. When it reaches the point where the spool and the rim have the same radius then, assuming infinite friction, the spool will stop experiencing a net force. However, if it has momentum then it will continue to roll. Then the rim will be larger than the rope on the spindle so the spool will start moving back. Rinse & repeat. Congratulations! You have just built a horizontal yo-yo.
Do something regarding spools and tangles for the 3D printing community, most people feel like it should be impossible for a spool to be tangled during the manufacturing process, others disagree, I am not so sure due to the amount of external variables that could come into play. A slight slip in the manufacturing process or otherwise could easily cause a tangle and there are almost always exceptions to any given rule, but it's a highly debated topic and the sort of one where both sides of the argument stick to it so strongly that it leaves everyone else either lost or frustrated with the lack of logic people use in their reasoning for things. Or someone else do it, this is the first video I came across regarding spools.
So what if the same spool was completely coiled with rope? You said that if the pulling force was lower to the ground, then it would roll the other way. So does that mean if the spool was completely filled, it would roll one way, then reverse back to the opposite direction as you keep pulling?
I thought about it differently. This is more intuitive to me: Lets say you don't pull, but hold the rope horizontally and then roll the spool towards yourself. You will lose some length of the rope as it is wrapped around the spool. However, because of the different radii/radiuses, you lose rope more slowly than what you win in closeness to the spool. So, if you roll the spool towards me, you have to move backwards to keep the rope straight. Reversing this thought, if you move backwards, the spool moves towards me. You can play the same game in the other direction. Let's say you roll the spool away from yourself. Then you "win" some rope as it unwraps, but the spool is moving away faster than the rope unwinds (because of the different radii/radiuses). So, you will have to run/walk along behind the spool or let go of the rope.
I was always thinking of this problem the opposite way around. I imagine holding the rope horizontally on the one end with my hand while the rest of the rope and the wheel is exactly as presented on this riddle. Then, I push the wheel the other way keeping the rope stretched. Because on a rotating disc the points closer to the edge travel a greater distance than the points closer to the centre, the rope unwinds at a speed smaller than the linear speed of the wheel. While the wheel rolls it unwinds some rope but because it is somewhere in the middle of the wheel the released rope is less than the distance the wheel travels, so the wheel pulls the rope and my hand to the same direction. if the rope was on the center of the wheel then it would pull my hand with the same speed the wheel moves horizontally. If the rope was winded on the edge of the wheel, it would simply unwind and my hand would stay in place. In the opposite perspective, like when pushing the wheel pulls the rope to the same direction, pulling the rope will push the wheel to that direction too. The ratio between the velocity of the rope and the velocity of the wheel is dependent on the distance fron the center of the wheel where the rope winds and unwinds
If the tension force goes through the center of mass there will be no torque aka the spool just rolls with perfect slip (is it even rolling at that point?)
Hai physics girl.... i Am from India and am very very impressed about your science explanations and also ur cute expression....iam love it.. Not only ur talent.. Also u
So... there is a point where if the thread is rolled up so much that it's close to the edge of the spool ends that it rolls away from the puller, but eventually the spool empties enough to make it want to go towards the puller, so the spool effectively stops. I want a spool of thread. NOW!
Ok. Let's get some math down. We assume that the spool rolls without slipping, so there is static friction, not kinetic friction. By applying newton's second law of motion, we get ΣF=ma=> F-2Τστ=m*a. This is equation 1.By applying the law for rotational movement, we get Στ=Ιολ*a/R. Στ is the sum of all torques, Ιολ is the total rotational inertia of the spool. So we have 2Τστ*R - F*r=Ioλ*a/R. We'll call this equation 2. Keep in mind that Forces and torques are vectors, as is acceleration. If we add the two, and solve for a(acceleration), we get : a=F*(1-r/R)/(mολ+Ιολ/R²). This is always bigger than 0. It's always a positive value. Thus the spool goes backwards (We set the direction of F as the positive one). That's the solution in a nutshell. No reason to further complicate things.
shes a physics major not a biology major. she probably only took 1 year of bio and didn't really retain all the information because physics is so rich in information.
Thus episode was so cool because two incredibly intelligent people didn't have a clue about stuff from the other person's field of study, which I guess is normal, but it wasn't super specialized knowledge... anyways, I loved this episode.
Bla Blum The threshold angle depends on the relative radii of the inner and outer parts of the spool. If you extend the line the string makes (the tangent to the inner circle) until it touches the ground, the threshold is the point at which this line hits the ground where the spool is contacting the ground. I.e. in the video, when the string is pulled straight up, the line of the string points to the ground to the left of the contact point. When the string is pulled at a flatter angle, the line points towards a point to the right of contact. That make sense? :)
If there's an angle at which the spool does roll away from you, then this is my question: Is there an angle at which you can pull the rope where the spool does not move?
why would friction over power the pulling force isn't it equal. friction is never more than the applied force??? Or is it because the friction force has a greater radius and thus a greater torque? I really don't like how you made it seem like the friction is greater than the pull, which you didn't directly say but it sounded like it. P.S. love the vids :)
I wonder if being back in Hawaii breathing the fresh, clean ocean air would help you. If I had a private jet, I would fly you back for free for rest and relaxation. Since I’m not there yet, the best I can do is pray for you, watch all of your videos from the very beginning (I’ve watched all of the ones so far up to this one.) through the most recent one (which I will get to soon) and interact with them as much as possible by liking, commenting and sharing. Please stay strong mentally!!! ❤️🙏🏻
I think it would have been simpler to describe the radii as levers. The longer lever wins, like a pry bar. Maybe that is just my "left handed" brain. I have a riddle I bet Diana can answer without Google: Who said, "Give me a long enough lever and a place to stand and i will move the earth?"
Electrician here... Unrolling wire. You can only unroll a spool so far without it spinning around, hence putting it on an axle to get the wire off without twisting. Just lying it on the ground, at first the wire pulls off, but as the wire spins off... poof, the wire starts trying to roll up the spool, spins around and twists up.
The higher up the weaker gravity gets relative to the surface of the ground. The earth spins 1000 mph on its axis. If she pulls it will move towards her, but if she pushes it it will not because the line will go slack and it wont go anywhere.
That's not the entire answer... let's call pulling force "Fp"; friction force "Ff"; radius of the big wheel "R"; radius of the cilinder "r". We can get 4 cases: 1) Fp*r > Ff*R ----> This means primarily that Fp > Ff and the object will move forward. In this case the torque producted by Fp wins and the spool will rotate clockwise. In a realistic world it would probably be very bouncy. 2) Fp*r = Ff*R ----> Fp > Ff like before. While the spool will still go forward the torque by Fp will be equal to the Ff one and we will get no rotation. 3) Fp*r < Ff*R ----> two cases: 3a) Fp =< Ff ----> no movements. 3b) Fp > Ff ----> the spool go forward but the torque producted by Ff will overwhelm the Fp one and it will rotate counterclockwise
If pulling the rope horizontally makes it roll backward, and near-vertically makes it roll forward, is there some angle in between where it won't roll at all?
Hedgehog Well, it could rotate in place.
there should be yeah
MindLaboratory no I don't have time to explain it but it's impossible
yes yes there is
If you pull the spool at an angle such that string intersects the ground at the contact point of the larger wheels, then it will either not move, or it will slide across the ground. Either way, the spool does *not* rotate
1:18 "Not wrong, *surprising*!"
Woah, that's an genius way to turn something that could hurt your self esteem to something that makes you excited and interested in learning more!
..and that's why the mule is sterile.
Steve Gould I was about to ask! That makes sense
My god. You definitely need more likes!!...
"You have 23 pairs... * most likely *"
I knew this from playing with yo-yo's. It's a fast way to rewind them.
If the spool were to be completely filled with rope, would it roll backwards due to how close the rope you are pulling is to the ground? And if so, would there be a point in which it would not roll at all (assuming the same force is applied)?
Rick Restagno it will go back only if the diameter of the circle on which rope is wound is more than the diameter of the circle in contact (something like train tracks)
Heh, you do this exact riddle in first year engineering physics and it is just as confusing even after you learn all the rules do all the math and know how to work the diagrams. One thing I learned about physics is NEVER to trust my instincts. Draw a diagram, map the forces and really dig into the problem cause your instants, Chris...they are just so often wrong.
The best thing to do is not to not trust your instincts ever, but to rebuild them to make them better.
What do you mean, confusing? It's just torque.
OMG that horse pic. In love more than I already was! :)
Wake up Warrior Son, girls don't use the internet.
That is a really good picture.
The first time I rode a horse bare back I fell off when the horse sped up and made a quick turn while crossing a creek. I landed flat on my back in water. So lucky I didn't land on any of the protruding rocks.
I love u
I love everyone
I am also love u jay kenley
Was that Biology Girl?
Nope, that was Biology Legend.
BiologyLegend27
+Hector Kayman I love you guys. You're seriously the best.
*Meme*
ThreXys there's a meme about a League of Legends player named TheLegend27.
You and Ann are so natural and awesome together. You MUST do more videos together!!
I love the format of these riddle series, this was one of my favorite yet. I thought a bit about the spool riddle after seeing the first two but was surprised. This would be a fun one to replicate with different sizes of spools.
You don't need different sizes of spools. Just spools that are wound up to different proportions.
It's the ratio of the two radiuses/radii that is important: one to point where the rope leaves the spool, one to the point where the spool touches the ground.
She is cooler than Santa.
Because she brings me the gift of knowledge! Best gift imaginable!
Let's find it mathematically:
Draw the FBD of the given situation (1:50). Assume radius of cylinder is R and tension force(T) is acting at r perpendicular distance from COM. Clearly r
SquareSpace sponsored this not 23andMe? Seems odd.
yeah I was just thinking that
why?
The person she's interviewing works at 23andMe and they have sponsored her in the past.
I just found you and now I love Physics Girl. :)
This is what my 100 level university physics classes were like. We all had clickers and gave answers so the professors could instantly gauge our overall level of understanding. I ended up with a pharmacy degree but these were probably my most memorable and enjoyed courses.
Having dropped a lot of spools under the desk, it depends on the surface, I have unwound spools, lost them further back, and mostly chased them in circles! I think the angle of the pull, the surface, and the rate of pull all weigh into this for the answer!!
I tried this on a near-frictionless surface and the spool barely rolled, but did go slightly backwards.
If you pull the spool at an angle such that string intersects the ground at the contact point of the larger wheels, then it will either not move, or it will slide across the ground. Either way, the spool does *not* rotate
Is there really the need to solve this with torque? I just made a sketch: 2 concentric circles. The inner circle rolls on the rope, the outer one on the floor. Now just let the whole system roll to one side and calculate (using the circumference) how the length of the rope changes. Because the inner circle is smaller than the outer one, the change of length of the rope is less than the travelled distance of the spool. So in order to move the rope relative to the floor it has to go the same direction as the spool.
Now you can even see, that if the inner circle would be larger than the outer one, the spool would move in the opposite direction. You could test this if you use some system on rails. :)
The force and the motion of the center of mass has to be in same direction. This solves it completely. If it's com moves where you pull it, it can't roll the other way.
cool! I watched this video when it came out and it was interesting. Now I'm rewatching because I'm going to coach teens in a mech eng summer school projecty thing, and this video is my inspiration :D thanks Diana
I founded the same problem on my mechanics exercise book just two or three weeks ago! It's wonderful to see that it really works! (I mean: Physics is an approssimation of reality, so lot of things that happen in a problem won't work in real life).
Ps: I'm sorry for my bad english but I wanted to show my appreciation!) thank you!
this physics is mastered industrially with a spool turning on a winch and a manilla rope looped around the spool. one end in your hand, the other tied on a 400lb side hammer with a rock core spoon. The setup is called a cats head. its used to get a core sample of bedrock. you tug the rope applying friction to the spool, and the motor supplies extra torque. it is very fast powerful and dangerous.
Rolling along the ground constitutes a rotation about the contact point between the spool and the ground at any moment. The net torque around this point is what determines which way it rolls.
If the spool is reasonably symmetric, gravity cannot provide such a torque around the ground contact point because it has no offset lever arm around that point (its line of force goes right through that point at all times, and thus cannot create torque around it).
So the only force that can provide a torque around the ground contact point is the applied force. Draw the line of force along the direction of that pulling force, and see if the torque generated by that force makes the object rotate clockwise or counterclockwise around the ground contact point.
If the applied force makes provides a counterclockwise torque around the ground contact, it will roll to the left, as in the first example. If it provides a clockwise torque around the ground contact, it will roll to the right, as in the second case. You can make this quick, one-step determination with any force.
If the spool goes forwards full and backwards when close to empty, is there a place in between in which it doesn’t move or switched directions?
I remember the first time I heard this riddle I thought the same thing as Anne Wojcicki, that the spool would unwind. It's interesting how intuition (at least mine and hers anyway) goes straight to torque rather than force when applied to circles and rotating objects. If you think about force, it is ridiculous to expect pulling one way on an object to cause it to move the other way, but when you're thinking about spin, this rather silly answer seems perfectly obvious.
I've done this before, but it's always fun to watch. Here's something to give Dianna simmering to ponder on; the friction on the outer side doesn't matter! Pull it too fast for fiction to act on it and it'll still do the same thing, come right at you.
Pulling it straight up though, hmmm? 🤔 I've got to try this one again.
Like your videos! A big shout out from Argentina!
I'd say that the considerations about where the force is applied is not key. As long as the distance traveled in any direction is directly proportional to the outer rim length, the circumference of the circle, and the string comes from a smaller circle, the movement will *always* be in the same direction as the the pull on the string, irrespective of angles. Of course, it would take extra tricks to keep any upward movement directly proportional with the rotation of the spool, as it normally slips *very* easily if lifted from the floor. A movement opposite of the pull would require much more string than unwound.
So funny! I like your guest, she's so nice!
her guest is the CEO of youtube
Anne (her guest) is the sister of the CEO of youtube ✌
This is such a cool channel! So glad I found it!!
I have Physics knowledge on a highschool level, so I'm not that familiar with torques and I solved it in a different way.
If it doesn't slide, it means that the velocity of th point of the wheel on the spot which touches the ground must be 0. To negate the velocity of the object to the left, the whole wheel has to rotate anti-clockwise
YASS FINALLY! I'VE BEEN WAITING SINCE THE LAST 24 HOURS! aand i was wrong! :/
Harleen Dhunda. Me too. Waaaaay wrong.
Solid explanation animation. Nice!
The genetics portion takes me back to when I taught Human Origins as a TA and we discussed meiosis.
Another way of looking at it. Suppose the spindle has radius r and the rim has radius R. For one rotation of the spool, the centre moves 2 * pi * R and the extra rope is 2 * pi * r. Thus the end of the rope moves 2 * pi * (R-r). Turn it around and pull on the rope instead. Since the rim is larger than the spindle, the amount of free rope decreases and the spool moves towards you.
Now consider when the rim is supported on rails with a gap between them so that the rope can be wrapped to a larger radius than the rim. When you start pulling, the spool will move away. When it reaches the point where the spool and the rim have the same radius then, assuming infinite friction, the spool will stop experiencing a net force. However, if it has momentum then it will continue to roll. Then the rim will be larger than the rope on the spindle so the spool will start moving back. Rinse & repeat. Congratulations! You have just built a horizontal yo-yo.
Do something regarding spools and tangles for the 3D printing community, most people feel like it should be impossible for a spool to be tangled during the manufacturing process, others disagree, I am not so sure due to the amount of external variables that could come into play.
A slight slip in the manufacturing process or otherwise could easily cause a tangle and there are almost always exceptions to any given rule, but it's a highly debated topic and the sort of one where both sides of the argument stick to it so strongly that it leaves everyone else either lost or frustrated with the lack of logic people use in their reasoning for things.
Or someone else do it, this is the first video I came across regarding spools.
I loved these riddle videos. Please can you do some more? I miss your weekly videos. It feels like they are to far apart now. X
So what if the same spool was completely coiled with rope? You said that if the pulling force was lower to the ground, then it would roll the other way. So does that mean if the spool was completely filled, it would roll one way, then reverse back to the opposite direction as you keep pulling?
So you want to do physics?
Step 1: Get a pair of Chucks 2:29.
Same as a bicycle. Veritasium did that a while back. It's all gear ratios or leaver lengths.
I thought about it differently. This is more intuitive to me:
Lets say you don't pull, but hold the rope horizontally and then roll the spool towards yourself. You will lose some length of the rope as it is wrapped around the spool. However, because of the different radii/radiuses, you lose rope more slowly than what you win in closeness to the spool. So, if you roll the spool towards me, you have to move backwards to keep the rope straight.
Reversing this thought, if you move backwards, the spool moves towards me.
You can play the same game in the other direction. Let's say you roll the spool away from yourself. Then you "win" some rope as it unwraps, but the spool is moving away faster than the rope unwinds (because of the different radii/radiuses). So, you will have to run/walk along behind the spool or let go of the rope.
I believe the speed in which you pull the rope will play a significant roll in what it does as well.
I was always thinking of this problem the opposite way around. I imagine holding the rope horizontally on the one end with my hand while the rest of the rope and the wheel is exactly as presented on this riddle. Then, I push the wheel the other way keeping the rope stretched. Because on a rotating disc the points closer to the edge travel a greater distance than the points closer to the centre, the rope unwinds at a speed smaller than the linear speed of the wheel. While the wheel rolls it unwinds some rope but because it is somewhere in the middle of the wheel the released rope is less than the distance the wheel travels, so the wheel pulls the rope and my hand to the same direction. if the rope was on the center of the wheel then it would pull my hand with the same speed the wheel moves horizontally. If the rope was winded on the edge of the wheel, it would simply unwind and my hand would stay in place.
In the opposite perspective, like when pushing the wheel pulls the rope to the same direction, pulling the rope will push the wheel to that direction too. The ratio between the velocity of the rope and the velocity of the wheel is dependent on the distance fron the center of the wheel where the rope winds and unwinds
I'm not rich, but I'd be willing to chip in $5 to the "buy Anne some shoelaces" fund. Good video, as always, Dianna.
The horse hair wig joke made me laugh more than would've expected... Kudos...
If the tension force goes through the center of mass there will be no torque
aka the spool just rolls with perfect slip (is it even rolling at that point?)
IF you do the same with a spool of stiff cat5 cable it will roll back at first then the other way the More you pull.
I think it depends on many variables.... like axle to disc ratio, friction , angle of pulling, and mass of the whole wheel....
Hai physics girl.... i Am from India and am very very impressed about your science explanations and also ur cute expression....iam love it.. Not only ur talent.. Also u
4:37 "That doesn't sound right, but I don't know enough about stars to dispute it..."
for a second i was like “oh that’s the ceo of youtube right?”
Got it right. Same as in Physics class last term. :)
if the spool was full of rope would it spin the other way? Does it roll forward because the rope is closer to the center axis?
SquareSpace sponsors the 23andMe episode. Life is so lovely. :-)
if friction toward the right is larger than the tension to left, then shouldn't the string and spool go to the right?
Riddle me this, Batman. If I pulled on a spool...
The Great Courses has a 72 lecture biology course you should get. If you want to understand microbiology just watch lectures 5 through 11.
Please tell me, can two circles or spheres touch?If yes what is the area of cross section?
So... there is a point where if the thread is rolled up so much that it's close to the edge of the spool ends that it rolls away from the puller, but eventually the spool empties enough to make it want to go towards the puller, so the spool effectively stops. I want a spool of thread. NOW!
Ok. Let's get some math down. We assume that the spool rolls without slipping, so there is static friction, not kinetic friction. By applying newton's second law of motion, we get ΣF=ma=> F-2Τστ=m*a. This is equation 1.By applying the law for rotational movement, we get Στ=Ιολ*a/R. Στ is the sum of all torques, Ιολ is the total rotational inertia of the spool. So we have 2Τστ*R - F*r=Ioλ*a/R. We'll call this equation 2. Keep in mind that Forces and torques are vectors, as is acceleration. If we add the two, and solve for a(acceleration), we get : a=F*(1-r/R)/(mολ+Ιολ/R²). This is always bigger than 0. It's always a positive value. Thus the spool goes backwards (We set the direction of F as the positive one). That's the solution in a nutshell. No reason to further complicate things.
I'm a mule, therefore she has ridden me. Logic FTW
I was right in the Veritasium's bicycle riddle but failed at this one. I was tricked by the other cylindrical objects that do go forward.
There's no way Diana didn't know how chromosomes worked
shes a physics major not a biology major. she probably only took 1 year of bio and didn't really retain all the information because physics is so rich in information.
She must have too many
Most people have modular intelligence and certainly no one has equal depth of talent and education across all subjects.
lol thank you
im just trying to help her out... OK. lol
"Not wrong, surprising" - I Love That. I hope the two of you do this again. Stay Awesome and Have a Great Weekend. : )
So is this riddle meant for that particular spool? Cuz the answer could be different based on the weight of the spool right?
Thus episode was so cool because two incredibly intelligent people didn't have a clue about stuff from the other person's field of study, which I guess is normal, but it wasn't super specialized knowledge... anyways, I loved this episode.
A guy in my college dynamics class got this wrong too. You gotta use your mental free body diagram not your gut intuition.
Friction makes a difference from just in space. Tried with small spool and string.
Where can I get that couch?
How about if u do this on a very smooth surface with a very low friction?
I'm a little underwhelmed - no angle discussion? Please do one, assume perfect friction, is the threshold at 45 degrees?
Bla Blum without friction it wont even roll...
Of course I meant "ignore additional friction complexity", edited it. My question is answered in MindLaboratory's thread, nice :)
Bla Blum The threshold angle depends on the relative radii of the inner and outer parts of the spool. If you extend the line the string makes (the tangent to the inner circle) until it touches the ground, the threshold is the point at which this line hits the ground where the spool is contacting the ground.
I.e. in the video, when the string is pulled straight up, the line of the string points to the ground to the left of the contact point. When the string is pulled at a flatter angle, the line points towards a point to the right of contact. That make sense? :)
+Superiorer Without friction, it will still roll lol. The torque from the string is still there.
I got the riddle right. I am so happy. I did not consider all the math tho.
If there's an angle at which the spool does roll away from you, then this is my question: Is there an angle at which you can pull the rope where the spool does not move?
Hi dianna ! Love your vids
Do one with Clint from LGR cos that would be an odd pairing.
I solved infinity. Just move the needle on a protractor from degree 1 to degree 2. There are an an infinite number of degrees between them.
Doesn't this depend heavily on the spool, rope, and ground used? Different materials would have very different coefficients of friction.
This really got me I used a yarn spool as an example lol
Why did you upload it twice?Im very confused...
@physicsgirl could you do a video on the physics behind a boomerang please 😎
Do a riddle video with hank green please
derek riddled hank in a video but he arrogantly refuse to answer.
pride or reputation, idk which one
Don't know how this show went from bicep flexing to riddles.
Opposite to the bike pedal riddle!
Yay. The third video is up
That poor guy on the couch in the background suddenly gets frozen near the end.
why would friction over power the pulling force isn't it equal. friction is never more than the applied force??? Or is it because the friction force has a greater radius and thus a greater torque? I really don't like how you made it seem like the friction is greater than the pull, which you didn't directly say but it sounded like it.
P.S. love the vids :)
I wonder if being back in Hawaii breathing the fresh, clean ocean air would help you. If I had a private jet, I would fly you back for free for rest and relaxation. Since I’m not there yet, the best I can do is pray for you, watch all of your videos from the very beginning (I’ve watched all of the ones so far up to this one.) through the most recent one (which I will get to soon) and interact with them as much as possible by liking, commenting and sharing. Please stay strong mentally!!! ❤️🙏🏻
2:30 = converse advertisement XD
So if you pull the rope hard enough to overcome friction then it would unwind, right?
I think it would have been simpler to describe the radii as levers. The longer lever wins, like a pry bar. Maybe that is just my "left handed" brain. I have a riddle I bet Diana can answer without Google: Who said, "Give me a long enough lever and a place to stand and i will move the earth?"
It seems like that should break newtons law! I assumed the torque from the spool would be equal and opposite and therefore wouldn't move!
Electrician here...
Unrolling wire. You can only unroll a spool so far without it spinning around, hence putting it on an axle to get the wire off without twisting. Just lying it on the ground, at first the wire pulls off, but as the wire spins off... poof, the wire starts trying to roll up the spool, spins around and twists up.
4:49 what happened to that girl in the background 😂😂😂😂
I love Physics Girl.
Funny enough i didn't expect the result in the spool experiment, but i nailed the mule chromosome riddle ;)
I think it will move opposite of inertia of all three forces. The person pulling the spool the earths spin on its axis and gravity
The higher up the weaker gravity gets relative to the surface of the ground. The earth spins 1000 mph on its axis. If she pulls it will move towards her, but if she pushes it it will not because the line will go slack and it wont go anywhere.
so her force would be canceled out and it is the only force that can move it. Gravity and Centrifugal force are holding it in place.
The gravitation field must spin opposite centrifugal force.
Am I correct?
I only know because I am Real Fly the Science Guy.
hey diana can u make a video recommending some good books on physics in general..?
That's not the entire answer... let's call pulling force "Fp"; friction force "Ff"; radius of the big wheel "R"; radius of the cilinder "r". We can get 4 cases:
1) Fp*r > Ff*R ----> This means primarily that Fp > Ff and the object will move forward. In this case the torque producted by Fp wins and the spool will rotate clockwise. In a realistic world it would probably be very bouncy.
2) Fp*r = Ff*R ----> Fp > Ff like before. While the spool will still go forward the torque by Fp will be equal to the Ff one and we will get no rotation.
3) Fp*r < Ff*R ----> two cases: 3a) Fp =< Ff ----> no movements.
3b) Fp > Ff ----> the spool go forward but the torque producted by Ff will overwhelm the Fp one
and it will rotate counterclockwise