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A person reaches a maximum height of 65cm when jumping straight up from a crouched position. During the jump itself, the person's body from the knees up rises a distance of around 50cm . To keep the calculations simple and yet get a reasonable result, assume that the entire body rises this much during the jump.

With what initial speed does the person leave the ground to reach a height of 65cm ?
I found this to be 3.6 m/s which is correct.

In terms of this jumper's weight W , what force does the ground exert on him or her during the jump?

I'm not sure how to start this, I've attempted a few ways and each has been wrong.

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Ash

  1. I figured the answer out, but for future reference for others:
    F = reaction of the ground
    F = mg + ma

    V² = 2ax (x = 0.50m)
    a = V²/2x = 2gh/2x = gh/x
    F = mg + mgh/x
    F = W + Wh/x
    F = W(1+h/x)
    here
    F = (1 + 0.64/0.50)W
    F = 2.3W

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    Ash

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