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8. 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 75-kg man just before contact
with the ground has a speed of 6.4 m/s. (a)
In a stiff-legged landing he comes to a halt in 2.0 ms.
Find the average net force that acts on him during this time. (b)
When he bends his knees, he comes to a halt in 0.10 s. Find 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 force of the ground on the man in parts (a) and (b). |
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Impulse-Momentum
Theory says |
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Since you are at rest at the
end. The average force points in the
opposite direction of the incoming velocity. |
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Solving for Fave |
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Minus sign on velocity is due
to velocity pointing down. |
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Free Body Diagram |
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From this we can see that |
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Solving for FGround |
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