Question

A child of mass 40 kg jumps off a wall and hits the ground at 4 m/s. He bends his knees and stops in 1 s. Calculate the force required to slow him down. How would this force be different if he didn't bend his knees and stopped in 0.1 s?​

Answer 1

`1600`

Fnet

`fnet = (m * v)/(?)`

Answer 2

The normal force exerted by the ground applies an average acceleration of

a[ave] = (0 - 4 m/s) / (1 s) = -4 m/s²

(where upward is the negative direction) so the average force felt by the child is

F[ave] = (40 kg) (-4 m/s²) = -160 N

or 160 N directed upward.

Without bending his knees, the average acceleration would instead by

a[ave] = (0 - 4 m/s) / (0.1 s) = -40 m/s²

which makes the average force increase tenfold,

F[ave] = (40 kg) (-40 m/s²) = -1600 N