Hi Chris, Why did you not use mg=kx at the surface of the water to figure out k in the bungee jumper problem? that will give a completely different answer. Why is that wrong?
For the projectile example at 9:20, if we consider the total energy at max height, it would be equaled to only GPE which equals mg(h+H), so based on the law of conservation, can't we equate GPE at max height to KE at the bottom to find the answer C?
@@donerphysics ok thank you so much Mr. Doner, but please may you further explain why? I thought at max height the velocity of a projectile would be equaled to 0 and thus the KE would also be equaled to 0.
The bunjee cord is storing energy. We really only consider PE to be negative if particles are trapped by other particles, so that we need to add energy to set them free.
15:25 Why is the extended length 25 m?
It was 50 to the surface subtract 10 m of unstretched length and another 15 m because if was 15 m off the surface.
Hi Chris, Why did you not use mg=kx at the surface of the water to figure out k in the bungee jumper problem? that will give a completely different answer. Why is that wrong?
mg=kx only in equilibrium where there is no net force. This will also be the point where the object changes from speeding up to slowing down.
18:32, why is the max speed kx=mg? if mg>kx, it would be speeding up so wouldn't the speed be greater if it is accelerating?
Right so it keeps speeding when mg is greater than kx, and begins to slow down when kx>mg. Max speed is when it stops speeding up.
@@donerphysics So when kx=mg the speed is constant and max?
It is not constant because kx becomes bigger than mg right away, so it begins to slow.
@@donerphysics Ok so here is the bungee jumper changing directions?
No, not a change is direction. It begins to slow.
For the projectile example at 9:20, if we consider the total energy at max height, it would be equaled to only GPE which equals mg(h+H), so based on the law of conservation, can't we equate GPE at max height to KE at the bottom to find the answer C?
The projectile is fired at an angle so it also has KE at the top of its path.
@@donerphysics ok thank you so much Mr. Doner, but please may you further explain why? I thought at max height the velocity of a projectile would be equaled to 0 and thus the KE would also be equaled to 0.
It is still moving across at the top of the path even though it is not moving upwards.
@ 16:00 shouldn't the spring energy be subtracted not added
The bunjee cord is storing energy. We really only consider PE to be negative if particles are trapped by other particles, so that we need to add energy to set them free.
at 11:10 why isnt it assumed that KE = 0?
You mean when it hits the sea. That would make it too easy of a question. We want the KE just before it reaches the surface.
@@donerphysics thank you! but then, why isnt KE = 0 when the bunjee jumper is at the top of the cliff, isnt it only potential gravitational energy?
I was referring to a different question. KE equals zero at the top of a bunjee jump.
thank you so much
For the projectile example if we consider the point of h+H, it would only be affected by GPE. Why can't we then assume that the answer is c.
At what time are you referring to?
The projectile is fired at an angle so it has KE at the top of the path.