Question

A bullet is fired from the rifle. If the rifle recoils freely, then the kinetic energy of the rifle is ___________

(a) Less than the kinetic energy of the bullet

(b) More than the kinetic energy of the bullet

(c) Same as the kinetic energy of the bullet

(d) Equal or less than the kinetic energy of the bullet

This question was posed to me at a job interview.

The origin of the question is Acceleration topic in portion Laws of Motion of Engineering Physics I

Answer 1

Right choice is (a) Less than the kinetic energy of the bullet

For explanation: By conservation of momentum,

MV = mv or V = mv/M

Kinetic energy of the rifle is

Kr = 1/2MV^2 = 1/2M(m^2 v^2)/M^2

Kr = m/M * Kb

As m is lesser than M, Kr is lesser than Kb

Thus the kinetic energy is less than that of the bullet.