Subjective Type

In about $$1915$$, Henry Sincosky of Philadelphia suspended himself from a rafter by gripping the rafter with the thumb of each hand on one side and the fingers on the opposite side (Fig.). Sinofsky's mass was $$79 kg$$. If the coefficient of static friction between hand and rafter was $$0.70$$, what was the least magnitude of the normal force on the rafter from each thumb or opposite fingers? (After suspending himself, Sinofsky chinned himself on the rafter and then moved hand-over-hand along the rafter. If you do not think Sincosky’s grip was remarkable, try to repeat his stunt.)

Solution

There is no acceleration, so the (upward) static friction forces (there are four of them,
one for each thumb and one for each set of opposing fingers) equals the magnitude of the
(downward) pull of gravity. we have
$$4\mu_sf_N=mg=(79\,kg)(9.8m/s^2)$$
which, with $$\mu_s = 0.70$$, yields $$F_N = 2.8 \times 10^2$$ N.

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