Question

A heavy box of mass 20 kg is pulled on a horizontal surface by applying a horizontal force. If the coefficient of kinetic friction between the box and the horizontal surface is 0.25, find the force of friction exerted by the horizontal surface on the box.

(a) 29 N

(b) 39 N

(c) 49 N

(d) 59 N

I had been asked this question in a national level competition.

My question comes from Laws of Solid Friction and Limiting Friction in section Friction of Machine Kinematics

Answer 1

The correct option is (c) 49 N

The best explanation: As the box slides on the horizontal surface, the surface exerts kinetic friction on the box. The magnitude of the kinetic friction is

f = μN

  = μmg

  = 0.25 x (20 kg) x (9.8 m/s ^2) = 49 N.

This force acts in the direction opposite to the pull.