# Homework Help: Help with change in internal energy for a real system.

1. Mar 22, 2009

### guitarguy1

1. The problem statement, all variables and given/known data

Tarzan, whose mass is 110 kg, is hanging at rest from a tree limb. Then he lets go and falls to the ground. Just before he lets go, his center of mass is at a height 3.0 m above the ground and the bottom of his dangling feet are at a height 2.0 above the ground. When he first hits the ground he has dropped a distance 2.0, so his center of mass is (3.0 - 2.0) above the ground. Then his knees bend and he ends up at rest in a crouched position with his center of mass a height 0.5 above the ground.
(a) Consider the point particle system. What is the speed v at the instant just before Tarzan's feet touch the ground?
(b) Consider the real system. What is the net change in internal energy for Tarzan from just before his feet touch to the ground to when he is in the crouched position?

2. Relevant equations

Fnet x distance = 1/2mv^2

3. The attempt at a solution

Part (a) was easy. I got 6.261 m/s. I'm having trouble with part (b). I at first thought it might be 0 J since it asks to consider the real system rather than point particle system, but thats not right. I understand that its just the energy exerted to bring him to a stop, but I'm not really sure what to do here.

2. Mar 22, 2009

### Redbelly98

Staff Emeritus
Not 100% sure, but I think you just consider the kinetic and potential energy.

3. Mar 22, 2009

### guitarguy1

alright. But, I'm having trouble calculating the kinetic energy since this is for a real system whereas the other value i calculated was the velocity for the point-particle system.

4. Mar 22, 2009

### Redbelly98

Staff Emeritus
The velocity of the center-of-mass would be used for either the point particle or the real system.

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