A person of mass 60 kg jumps from a height of 5 m, onto the ground. If he does not bend his knees on touching the ground, he comes to rest in `(1)/(10)` s. But if he bends his knees, he takes 1 s to come to rest. Find the force exerted by the ground on him in the two cases. [ g = 10 m `cdot s^(2)` ]

Explanation

Let v = velocity acquired during the free fall through 5 m.
`therefore " " v^(2) = 0 + 2 xx 10 xx 5 " " or, v = 10 " m"cdot s^(-1)`
This velocity becomes zero due to the impact with the ground. Then, change in momentum
= `mv - mv = 60 xx 10 - 0 = 600 N`s
Force applied by the earth for this change,
in 1st case, `F_(1) = (600)/((1)/(10)) = 6000 N `
in 2nd case, `F_(2) = (600)/(1) = 600 N`
Hence, the impact in the 1st case is more severe than in the 2nd case, where the momentum changed at a shower rate.