Question

when you jump from an elevated position you usually bend your knees upon reaching the ground. by doing this, you make the time of the impact about 10 times as great as for a stiff legged landing. in this way, the average force your body experiences is...

Answer 1

about 1/10th as great by bending your legs 

Time extended decreases acceleration value. 

solve using Newton's 2nd and Uniform Acceleration Laws. plug in values for descending. 

Vf = 0.0 m/s 

Vi = 10 m/s 

t = 1 s 

Acceleration is given by 

a = [Vf - Vi] / t 

a = [ (0.0 m/s) - (10 m/s) ] / (1 s) 

a = [ -10 m/s ] / (1 s) 

a = -10 m/s^2 

Solve the same thing, but with time 10x, so t = 10 s 

a = [ (0.0 m/s) - (10 m/s) ] / (10 s) 

a = [ -10 m/s ] / (10 s) 

a = -1 m/s^2 

Solve for first force and second force, using a mass of 100 kg 

F = m * a 

F = (100 kg) * (-10 m/s^2) 

F = -1,000 N 

F = (100 kg) * (-1 m/s^2) 

F = -100 N 

So divide long-force by short-force 

(-100 N) / (-1,000 N) = 0.1 (which is 1/10)