Answer:
8 m/s
Step-by-step explanation:
You want the recoil speed when a 0.1 kg slug leaves the barrel of a 5 kg gun at a speed of 400 m/s.
Momentum
The total momentum of the system remains constant before and after the slug leaves the barrel.
before: mv = 0 . . . . . all parts of the system are at rest
after: m1v1 +m2v2 = 0 ⇒ (0.1 kg)(400 m/s) +(5 kg)(s) = 0
40 kg·m/s + (5 kg)s = 0
s = (-40 kg·m/s)/(5 kg) = -8 m/s
The recoil speed of the gun is 8 m/s in the direction opposite to that of the slug.