I am using A.P. French, Mechanics, to supplement my Physics studies (i.e., on my own). I'm stumbling over problem 6-3 (b): A man of 80 kg jumps down from a window ledge 1.5 m above ground, bending his knees so that his center of gravity descends an additional distance h after his feet touch the ground, what must h be so that the average force exerted on him by the ground is only three times his normal weight?(adsbygoogle = window.adsbygoogle || []).push({});

So, obviously, a=3 m/s^2, since we want F=(80 kg)(3 m/s^2). I use v^2 = 2g(1.5) to get the velocity at the point that the knees start to bend, so v^2 = 29.4 (m^2/s^2).

Since I want a=3 m/s^2, I reason that all I need is to use 0 = v^2 - 2ah, with a=3 and v^2=29.4, to find the stopping distance at that acceleration, which should be h.

This gives me 4.9 m, which seems excessive, and French gives h = 0.75 m, and I can't see how he ended up with that. It seems to me this would correspond to a final velocity of 2.12 m/s, which is way too low for a 1.5 m drop.

Well, thank you for any suggestions or pointers.

Dot

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# Homework Help: A.P. French, Mechanics Problem

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