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 3.1 m away from the wall and is moving at a constant speed of 0.59 m/s.
Energy is constant Energy is not constant  System: wall
Energy is constant Energy is not constant  System: girl+wall
Energy is constant Energy is not constant  System: girl

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 0.9 seconds after the girl comes off the trampoline.
Energy is constant Energy is not constant  System: girl + Earth
Energy is constant Energy is not constant  System: girl + trampoline + Earth
Energy is constant Energy is not constant  System: trampoline
Energy is constant Energy is not constant  System: girl + trampoline
Energy is constant Energy is not constant  System: girl

Explanation / Answer

Wall : energy is not constant

Wall + girl : energy is constant as the work done by hand on the wall appear as the kinetic energy

girl : not constant

System: girl + Earth

energy not constant as trampoline does work on girl which is not included in the system

System: girl + trampoline + Earth

energy is constant .

System: trampoline

not constant since trampoline at the lowest position has some energy stored but no energy is stored when girl leaves contact with trampoline

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