# 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 other side (see the figure). Sincosky's mass was 79.0 kg. If the coefficient of static friction between hand and rafter was 0.720, what was the least magnitude of the normal force on the rafter from each thumb or opposite fingers?

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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 other side (see the figure). Sincosky's mass was 79.0 kg. If the coefficient of static friction between hand and rafter was 0.720, what was the least magnitude of the normal force on the rafter from each thumb or opposite fingers?

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Step 1

Net force on Sincosky is zero since he is suspended on the rafters. As he is gripping the rafter with both the hands, there are 4 contact forces – two for thumb finger and two for other fingers. Therefore,

Step 2

Re-arranging and substituting the values,

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