People who do chin-ups raise their chin just over a bar (the chinning bar), supp

Question

People who do chin-ups raise their chin just over a bar (the chinning bar), supporting themselves only by their arms. Typically, the body below the arms is raised by about 30 in a time of 1.0 , starting from rest. Assume that the entire body of a 710 person who is chinning is raised this distance and that half the 1.0 is spent accelerating and the other half decelerating, uniformly in both cases. Find the force his arms must exert on him during the accelerating part of the chin-up in Newtons. Confused, help would be awesome.

Explanation / Answer

if 'a' is the acceleration, then using the relation S =0 + 1/2 a t^2, we have 0.3 = 1/2 x a x 1/4 so that a = 2.4 m/sec^2, then force of his arms F = m a = (710/9.8)x 2.4 = 173.88 Newtons.

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