The bullet flies out of the rifle at a speed of 780 m / s. The rifle gains a speed of 1.5 m / s upon recoil.

The bullet flies out of the rifle at a speed of 780 m / s. The rifle gains a speed of 1.5 m / s upon recoil. What is the mass of a bullet if the mass of the rifle is 4.8 kg?

Given: u1 = 780 m / s
u2 = 1.5 m / s
m2 = 4.8 kg
Find: m1 =?
Solution: m1 * u1x + m2 * u2x = 0
u1x = u1, u2x = -u2 (since the recoil is directed in the opposite direction)
m1 * u1-m2 * u2 = 0
m1 * u1 = m2 * u2
780 * m1 = 1.5 * 4.8
780 * m1 = 7.2
m1 = 7.2 / 780
m1≈0.0092 (kg)
Answer: ≈0.0092 kg