Meredith, a 60 kg volleyball player, lands on her feet after blocking a shot and immediately jumps back up. When her feet first contact the ground her velocity is 4 m/s downward. When she leaves the ground 0.9 s later she leaves with a velocity of 3 m/s upward. What was is the average ground reaction force exerted upward be the floor on Meredith while she is on the ground in N