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I am puzzled with the following problem.

A man starts to jump. First he will exert force on the floor and he starts accelerating upwards. Then he is off the floor. What is the relationship between the force exerted by him and the force of the floor towards him while he is still on the floor?

My argument is, since the man is still not off the floor, there is no movement at all; as in, his net acceleration towards upwards is zero. Thus, the force of his legs = the force from the floor according to the 3rd law.

However, the problem says that the floor pushes the man upwards with a stronger force than his legs.

To me this is counter intuitive because the floor does not move. I guess I can think of it as that the net force must be upwards because the man can actually jump, but I don't know why this problem is puzzling me so much. Can someone help me out?

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  • $\begingroup$ Notice the man is accelerating upwards before he leaves the floor, this is because his legs are initially bent, but by the time he is about to leave the floor, his legs are straight. Thus we has accelerated upward and the force causing this acceleration comes from the floor. Is this something you were confused about? $\endgroup$ – Brian Moths Dec 9 '13 at 19:32
  • $\begingroup$ Hmm... I am still not confident if I fully understand the general concept, but it at least answers this particular question. Thank you ! $\endgroup$ – hyg17 Dec 9 '13 at 19:36
  • $\begingroup$ "the floor pushes the man upwards with a stronger force than his legs." what? The forces should be equal. $\endgroup$ – Luke Burgess Dec 9 '13 at 20:38
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However, the problem says that the floor pushes the man upwards with a stronger force than his legs.

This can not be true. The floor pushes the man upwards with as much force as his legs exert downwards. However, this happens to be more than his weight, which is why he accelerates upwards.

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    $\begingroup$ That makes perfect sense. The force exerted by the man's leg does not count the man's weight. Since the force of the leg+weight = force from the floor, the force of the leg translates into acceleration towards up. That was very helpful/ $\endgroup$ – hyg17 Dec 10 '13 at 4:19

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