The problem statement, all variables and given/known data:
A 20 kg child jumps in the air to try to shoot free throws while his older sister is practicing basketball. His center of mass is normally 30cm off the ground. The lowest he can get his center of mass is 20cm off the ground, and the highest he can get it before he can no longer push is 40cm. He can push against the floor with a force 3.5x his own weight.
a) If you identify just the child as your system, what is the net external force on the child as he is jumping?
b) Assuming the child was at rest when he started pushing on the floor, what is his kinetic energy when his feet leave the floor?
c) How much work did the floor do on the child?
d) How high can he jump? (May want to change your system here.)