A girl wearing roller skates is standing in front of a wall. Assume that the whe

Question

A girl wearing roller skates is standing in front of a wall. Assume that the wheels on the skates are good enough that friction is negligible between the skates and the ground. The girl pushes off the wall and begins traveling away from the wall. Call the initial time when the girl was standing at rest in front of the wall with her hand touching the wall. The final time is when the girl has traveled 2.8 m away from the wall and is moving at a constant speed of 0.58 m/s. Pick energy is conserved or energy is not conserved for the following:

1. System: wall
2. System: girl
3. System: girl+wall

A girl is jumping on a trampoline. After coming off of the trampoline, she is in the air for 1.2 seconds. Call the initial time when the trampoline is at its lowest point with the girl still on the trampoline. The final time is 1.0 seconds after the girl comes off the trampoline. Pick energy is conserved or energy is not conserved for the following:

1.System: girl + trampoline
2. System: girl
3. System: girl + trampoline + earth
4. System: trampoline
5. System: girl + earth

Explanation / Answer

Problem1)

1. System : Wall - conserved. By third Newtons third law. Action and reaction are equal.

2. System ; Girl - Not conserved. The work done in pushing off from the wall has been converted to KE of motion.

3. system: Girl + Wall - Conserved, As there is no friction no energy going out or coming in out of the system consisiting of girl and wall

Problem 2)

1.System: girl + trampoline - Conserved, the system has GPE, which remains conserved.
2. System: girl - Not Conserved - As elastic energy of trampoline converted to KE
3. System: girl + trampoline + earth - conserved, All forms of engery elastic, gravitational, KE conserved. Nothing is coming in or out of system comprising of girl , trampoine and earth
4. System: trampoline - Not conserved. It will osicallte as wave and dampen after sometime if undisturbed
5. System: girl + earth - conserved, the GPE of girl is converted to KE, when she jumps off.

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