When jumping straight down, you can be seriously injured if youland stiff-legged

Question

When jumping straight down, you can be seriously injured if youland stiff-legged. One way to avoid injury is to bend your kneesupon landing to reduce the force of the impact. A 75-kg man justbefore contact with the ground has a speed of 5.7 m/s. (a) In a stiff-legged landing he comes to ahalt in 1.6 ms. Find the average net forcethat acts on him during this time.
N

(b) When he bends his knees, he comes to a halt in 0.09 s. Find the average force now.
N

(c) During the landing, the force of the ground on the man pointsupward, while the force due to gravity points downward. The averagenet force acting on the man includes both of these forces. Takinginto account the directions of these forces, find the force of theground on the man in parts (a) and (b). stiff legged landing N bent legged landing N (a) In a stiff-legged landing he comes to ahalt in 1.6 ms. Find the average net forcethat acts on him during this time.
N

(b) When he bends his knees, he comes to a halt in 0.09 s. Find the average force now.
N

(c) During the landing, the force of the ground on the man pointsupward, while the force due to gravity points downward. The averagenet force acting on the man includes both of these forces. Takinginto account the directions of these forces, find the force of theground on the man in parts (a) and (b). stiff legged landing N bent legged landing N stiff legged landing N bent legged landing N

Explanation / Answer

Momentum of the man = Mass x Velocity = 75 x 5.7 = 427.5 kg.m/s Force = Momentum / Time In first case, Time = 1.6 ms = 1.6 x 10^-3 s So Force = 427.5 / 1.6 x 10^-3 = 267188 N In second case, Time = 0.09 s So Force = 427.5 / 0.09 = 4750 N So you see the force is 56 times greater when you keep you legsstiff than when you bend them....

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