When jumping straight down, you can be seriously injured if youland stiff-legged
ID: 1738439 • Letter: W
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 65.2-kg man justbefore contact with the ground has a speed of 6.74 m/s.(a) In a stiff-legged landing he comes to a haltin 1.62 ms. Find the magnitude of the average net force that actson him during this time. (b) When he bends hisknees, he comes to a halt in 0.248 s. Find the magnitude of theaverage net force now. (c) During the landing, theforce of the ground on the man points upward, while the force dueto gravity points downward. The average net force acting on the manincludes both of these forces. Taking into account the directionsof the forces, find the magnitude of the force applied by theground on the man in part (b).Explanation / Answer
a)mass of man is m = 65.2 kg initial speed of man is u = 6.74 m/s(down direction isnegative ) final speed of man is v = 0 time of halt with stiff legs is t = 1.62 ms =1.62*10-3s then force F = m(v-u)/t =65.2*(0-(6.74))/1.62*10-3 =2.71*105N b) with knees bent time of halt is t = 0.248 sec then f = m(v-u)/t = 65.2*(0-(6.74))/0.248 =1.771*103N c)net force F = Fground+weight weight = mg = 65.2*9.8 = 638.96 N THEN WITH STIFF LEGS Fground = F-weight =2.71*105N-638.96 N = 2.7*105N THEN WITH knees bent Fground = f-weight=1.771*103N -638.96 N = N =1.132*103N THEN WITH knees bent Fground = f-weight=1.771*103N -638.96 N = N =1.132*103NRelated Questions
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