First slide

Friction

Question

A boy of mass m is sliding down a vertical pole by pressing it with a horizontal force f. If $μ$ is the coefficient of friction between his palms and the pole, the acceleration with which he slides down will be

Moderate

A

$g$
B

$\frac{μf}{m}$
C

$g + \frac{μf}{m}$
D

$g - \frac{μf}{m}$

Solution

Normal reaction R = f.
Therefore, force of friction $= μR = μf .$
The net downward force $F = mg - μf$ .
Hence, the acceleration $a = \frac{F}{m} = \frac{mg - μf}{m} = g - \frac{μf}{m}$ .
Hence the correct choice is (d).

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