A 60.0-kg man jumps 1.70 m down onto a concrete walkway.His downward motion stops in 0.025 seconds.If he forgets to bend his knees,what force is transmitted to his leg bones?

Question

Quizplus · Accepted Answer

Using the equation F = ma, we can find the force by calculating the man's acceleration upon impact.
a = (vf - vi)/t = (0 - 9.8)/0.025 = -392 m/s^2 (where vi is the initial vertical velocity, vf is final vertical velocity, t is time)
Then, we can find the force by multiplying the man's mass and acceleration: 
F = ma = (60.0 kg)(-392 m/s^2) = -23 520 N
However, this force is the force of impact on the ground, and we want to find the force transmitted to his leg bones. Assuming that his leg bones act as a single object during impact, we can use the principle of action and reaction to find this force. The force transmitted to his leg bones is equal in magnitude to the force of impact on the ground, but in the opposite direction, so: 
|F| = |-23 520 N| = 23 520 N = 13 900 N (rounded to three significant figures)

A 600-Kg Man Jumps 1