# The bullet takes off from a Kalashnikov assault rifle at a speed of 715 / s. Determine the speed of the machine at recoil

**The bullet takes off from a Kalashnikov assault rifle at a speed of 715 / s. Determine the speed of the machine at recoil if the masses of the bullet and machine are 7.9 g and 3.6 kg, respectively.**

Given:

m1 = 7.9 grams = 0.0079 kilograms – bullet mass;

v1 = 715 meters per second – the speed of the bullet from the Kalashnikov assault rifle;

m2 = 3.6 kilograms – the mass of the Kalashnikov assault rifle.

It is required to determine v2 (meter per second) – the speed of the machine during recoil.

To determine the speed of the machine during recoil, you need to use the following formula (the law of conservation of momentum):

m1 * v1 = m2 * v2;

v2 = m1 * v1 / m2 = 0.0079 * 715 / 3.6 = 5.65 / 3.6 = 1.6 m / s.

Answer: the speed of the Kalashnikov assault rifle during recoil will be 1.6 m / s.