Question

A boy (30 kg) sitting on his horse whips it. The horse speeds up at an average acceleration of 2.0 m/s 2.If the boy does not slide back, what is the force of friction exerted by the horse on the boy ?

(a) 20 N

(b) 30 N

(c) 40 N

(d) 60 N

This question was addressed to me by my college professor while I was bunking the class.

The question is from Laws of Solid Friction and Limiting Friction in section Friction of Machine Kinematics

Answer 1

Right choice is (d) 60 N

Easiest explanation: The forces acting on the boy are

(i) the weight Mg.

(ii) the normal contact force N and

(iii) the static friction fs

As the boy does not slide back, its acceleration a is equal to the acceleration of the horse. As friction is the only horizontal force, it must act along the acceleration and its magnitude is given by Newton’s second law

fs = Ma = (30 kg) (2.0 m/s^2) = 60 N.