Question

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

Answer 1

force applied at `1`s: `160`N
force would increase to `1600`N at `0.1`s

Explanation:

`F = (mDeltav)/t`

force = mass * change in velocity / time

at `1`s:

mass: `40`kg
change in velocity (taking downwards as a positive direction):
`4` m/s to rest `=4-0 = 4` m/s
time: `1` s

`(mDeltav)/t = 160/1 = 160`
force applied `=160`N

at `0.1`s:

`(mDeltav)/t = 160/0.1 = 1600`
force applied = `1600`N

note that in the equation for force applied, time is the denominator.
this means that force is inversely proportional to time - as force increases, time decreases, and vice versa.

if the time is divided by a certain amount, the new force can be found by multiplying the previous force by that same amount.

here, dividing the time by `10`, from `1`s to `0.1`s, multiplies the force by `10`, from `160`N to `1600`N.