Question

From a rifle of mass 4kg, a bullet of mass 5kg is fired with a initial velocity of 20m/s.Calculating initial velocity of the rifle?

Answer 1

Given: Mass of rifle = 4 kg

Mass of bullet = 5 kg

Initial velocity of the bullet = 20 m/s

Let the recoil velocity of the rifle be = v m/s

Since in the beginning both the rifle and bullet are at the rest (when bullet is not fired)

Therefore, initial momentum = 0

When the bullet is fired both bullet and rifle moves.

Therefore, final momentum = momentum of bullet + momentum of rifle

so,

Final momentum = (5 × 20) + (4 x v)

= 100 + 4v

by applying law of conservation of momentum we get

100 + 4v = 0

or

4v = -100

thus, recoil velocity of the rifle will be

v = -25 m/s