Question

A bullet is fired from a riffle. If the riffle recoils freely, then the kinetic energy of the rifle will be

• A. same as that of the bullet
• B. more than that of the bullet
• C. less than that of the bullet
• D. none of these

Answer 1

The correct option is C less than that of the bullet

Since the total momentum of the bullet + riffle system is zero initially. Thus final momentum of the system must also be zero. So, the bullet and riffle move with equal magnitude of momentum but in opposite direction i.e. $|P_{bullet}|=|P_{riffle}|$

Kinetic energy $K=\frac{P^2}{2m}$

$⟹$ $K\propto \frac{1}{m}$ $(\because$ $|P_{bullet}|=|P_{riffle}|)$

As, mass of riffle is more than that of bullet. Thus kinetic energy of riffle is less than that of bullet.