From a rifle weighing 5 kg, suspended on cords, a bullet weighing 4 g takes off at a speed of 520

From a rifle weighing 5 kg, suspended on cords, a bullet weighing 4 g takes off at a speed of 520 m / s. The recoil speed of the rifle is equal to …

Given:
M = 5kg
m = 4g = 0.004kg
v1 = 520m / s
Find v2 – rifle recoil speed
From the law of conservation of momentum:
m * v1 – M * v2 = 0
(The second term is taken with a minus sign, because the velocities of the bullet and rifle are opposite)
From here
v2 = (m / M) * v1 = 0.004 * 520/5 = 0.416m / s