You probably think of wet surfaces as being slippery. Surprisingly, the opposite is true for human skin, as you can demonstrate by sliding a dry versus a slightly damp fingertip along a smooth surface such as a desktop. Researchers have found that the static coefficient of friction between dry skin and steel is 0.27, while that between damp skin and steel can be as high as 1.4. Suppose a man holds a steel rod vertically in his hand, exerting a 400 N grip force on the rod. A. What is the heaviest rod he can hold without slipping if His hands are dry?b)What is the heaviest rod he can hold without slipping if his hands are wet?

(A) 108 N

(B) 560 N

Explanation:

From the law of solid friction, for a body in contact with a flat surface,

F is the maximum frictional force between the body and surface, μ is the coefficient of friction and R is the reaction between the body and the surface.

The maximum frictional force is the maximum weight of the rod the man can hold without slipping.

(A) His hands are dry. Then μ = 0.27.

R is the reaction of the rod against his hands. Because he presses on the rod perpendicular to the surface of the vertical rod, the reaction is the same as his grip force.

R = 400 N

Then, F = 0.27 × 400 N = 108 N

(A) His hands are wet. Then μ = 1.4.

R = 400 N

Then, F = 1.4 × 400 N = 560 N

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