A rifle has a mass of 7-kg and the bullet has a mass of 0.7-kg. If the velocity of the bullet is 350-m/s after the rifle is fired, what is the recoil velocity of the rifle?

-35 m/s

Explanation:

Momentum is conserved.

Momentum before firing = momentum after firing

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

Before the bullet is fired, the bullet and rifle have no velocity, so u₁ and u₂ are 0.

0 = m₁v₁ + m₂v₂

Given m₁ = 0.7 kg, v₁ = 350 m/s, and m₂ = 7 kg:

0 = (0.7 kg) (350 m/s) + (7 kg) v

v = - 35 m/s

The rifle recoils at 35 m/s in the opposite direction.