Question
From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 ms–1. Calculate the initial recoil velocity of the rifle.
Solution
Mass of bullet, m1 = 50g = 0.05 kg
Mass of rifle, m2 = 4 kg
Initial velocity of bullet, u1 = 0
Initial velocity of rifle, u2 = 0
Final velocity of rifle, v1 = 35 ms-1
Final velocity of rifle, v2 = ?
According to the law of conservation of momentum,
Total momenta after the fire = Total momenta before the fire.
m1v1+ m2v2 = m1u1+ m2u2
0.05 x 35 + 4v2 = 0+0
The negative sign indicates that the direction in which the pistol would recoil is opposite to that of the bullet.
