Peter, a 100 kg basketball player, lands on his feet after completing a slam dunk and then immediately jump up again to celebrate his basket. When his feet first touch the floor after the dunk,
his velocity is 5 m/s downward; when his feet leave the floor 0.50 s later, as he jumps back up,
his velocity is 4 m/s upward. What is the average reaction force exerted upward by the floor on Peter during this 0.50 s?