Posted by **phys** on Wednesday, March 31, 2010 at 2:34am.

A 60.0 kg person bends his knees and then

jumps straight up. After his feet leave the floor, his motion is

unaffected by air resistance and his center of mass rises by a maximum

of 14.9 cm. Model the floor as completely solid and motionless.

(a) Does the floor impart impulse to the person?

Yes, am i correct to this part

(b) Does the floor do work on the person?

yes, since the floor exerts a force along the distance he jump, am i correct?

(c) With what momentum does the person leave the floor?

This part i'm stuck since p=mv, but i don't know how to find v in this case. Can you help me how to start it

(d) Does it make sense to say that this momentum came from the floor? Explain your answer.

Yes, since the guy exerts momentum on the floor, the floor also exerts same momentum in opposite direction

(e) With what kinetic energy does the person leave the floor?

do we use k=1/2mv^2, but again im stuck with finding velocity

(f) Does it make sense to say that this energy came from the floor? Explain your answer.

hm..how do you explain this part then

- physics(check my answer) -
**bobpursley**, Wednesday, March 31, 2010 at 8:36am
The assumption of the floor being motionless violates Newtons laws of motion.

The floor is connected to Earth. When one jumps, the Earth moves in the opposite direction. Mv=mV Assumptions cant change that. Nor can after leaving the floor, Gravitational force cannot be forgotten....Earth is attracted to you as you fall, Earth moves toward you, and has momentum toward you. If your center of mass rises, the Earth's center of mass declines, so that the cg of the system remains the same.

I don't know what to tell you. Your teacher has asked you to answer the questions with that fundamentally flawed assumption. If you answer those questions with my explanations from above, the questions have an entirely new light, and suddenly, Truth brings light on the answers.

If you view experimental situations with blinders on, such as the assumption the floor does not move, then one can make any conclusion, including erroneous conclusions to any work-energy situation, including the second law of thermodynamics.

Why don't you challenge the instructors assumptions, in view of well accepted laws of motion and work-energy?

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