How Does Bending Your Knees Reduce the Impact Force When Jumping?

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SUMMARY

Bending the knees during landing significantly reduces the impact force experienced by a jumper. In a scenario involving a 68.4-kg man jumping at a speed of 4.99 m/s, the average net force during a stiff-legged landing is calculated to be 83,451.34 N over a deceleration time of 4.09 ms. Conversely, when the knees are bent, the average net force decreases to 1,153.09 N over a longer deceleration time of 0.296 s. The analysis of forces during landing reveals that the upward force from the ground and the downward gravitational force must be considered to determine the total force acting on the jumper.

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When jumping straight down, you can be seriously injured if you land stiff-legged. One way to avoid injury is to bend your knees upon landing to reduce the force of the impact. A 68.4-kg man just before contact with the ground has a speed of 4.99 m/s. (a) In a stiff-legged landing he comes to a halt in 4.09 ms. Find the magnitude of the average net force that acts on him during this time. (b) When he bends his knees, he comes to a halt in 0.296 s. Find the magnitude of the average net force now. (c) During the landing, the force of the ground on the man points upward, while the force due to gravity points downward. The average net force acting on the man includes both of these forces. Taking into account the directions of the forces, find the magnitude of the force applied by the ground on the man in part (b).
2. (EF)= (-m X Vo)/ delta t



I got a) 83451.34 and b)1153.09 but I don't know what to do for c)
 
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J ≡ Pf - Pi = Faverage*Δt
The formula you used is correct, but remember the original form, where Pf =/= 0 for future problems.

Your answers for (a) and (b) are correct.

For (c), make a free body diagram of the man as he impacts the ground (Remember, he's being decelerated from 4.99 m/s). What forces are acting on him, what is the net force, and what is the source of each of the forces?
 
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