You have been hired by the US Army as a biomechanics consultant. New recruits ar

Question

You have been hired by the US Army as a biomechanics consultant. New recruits are injuring their ankles and knees when they go through an obstacle course in basic training. Most of the injuries occur when the recruits climb over a 5.0m wall and jump to the ground.

a. How much kinetic energy would a 165-lbs soldier have at the instant he makes contact with the ground after going over the wall? Soldiers hang from the top of the wall before falling down to the ground, so assume that the center of mass of the soldier is 1.7-m from the top of the wall when he lets go. At the instant he makes contact with the ground, the soldier’s legs are extended so his center of mass is about 0.9m from the ground.

b. How large was the ground reaction impulse on the soldier during his landing?

c. If the landing took about 0.25 seconds, what was the average force on the soldier’s legs during the landing?

Explanation / Answer

H = 5 m. h(com) = 1.7 m ; m = 165lb = 74.84 kg

h = 5 - 1.7 = 3.3

The potential energy of the soldier at h = 3.3 m is

PE = m g h = 74.84 x 9.81 x 3.3 = 2422.8 J

from conservation of energy, the PE of soldier gets converted to kinetic energy of motion. So when he touches the ground he has KE equal to the PE at height h.

KE = 2422.8 J

b)Ground reaction should be equal to the weight.

W = mg = 74.84 x 9.81 = 734.2 N

Hence, R = 734.2 N

c)I = F x t = mv

F = mv/t

mgh = 1/2 m v^2

v = sqrt (2gh) = sqrt (2 x 9.81 x 3.3) = 8.05 m/s

F = 74.84 x 8.05/0.25 = 2409.9 N

Hence, F = 2410 N

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