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) ]

A person of mass 60 kg jumps from a height of 5 m, onto the ground. If

1.	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) ]
Answer» Solution :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.