The stiff legged jump?

Question

When jumping straight down, you can be seriously injured if you land stiff-legged. One way to avoid injury is to bend your knees upon landing to reduce the force of the impact. A 75 kg man just before contact with the ground has a speed of 6.2 m/s.

(a) In a stiff-legged landing he comes to a halt in 1.7 ms. Find the average net force that acts on him during this time.

273529.41 N

(b) When he bends his knees, he comes to a halt in 0.08 s. Find the average force now.

5812.5 N

(c) During the landing, the force of the ground on the man points upward, while the force due to gravity points downward. The average net force acting on the man includes both of these forces. Taking into account the directions of these forces, find the force of the ground on the man in parts (a) and (b). stiff legged landing and bent legged landing.

i've found a and b. just can't seem to get the force of the ground. any help?

Answer 1

The net force on the man is the sum of the force of gravity and the force on the ground. You've already calculated the net force, so simply subtract the man's weight from it. Remember that gravity is pulling him down, so you'll be subtracting a negative number (i.e. the final result should be greater, not less, than the net force).