A 100-kg cannon at rest contains a 10-kg cannon ball. When fired, the cannon ball leaves the cannon with a speed of 90 m/s. What is the recoil speed of the cannon?
A)4.5 m/s
B)9 m/s
C)45 m/s
D)90 m/s
E)zero m/s

Question

Quizplus · Accepted Answer

According to conservation of momentum, the momentum of the cannon and cannon ball before firing is zero, and after firing the momentum of the cannon and cannon ball system is still zero. Thus, the momentum of the cannon and ball after firing is equal and opposite to the momentum of the ball before firing. Let 'v' be the recoil speed of the cannon, then
100 kg x v = -10 kg x 90 m/s
Solving for v, we get v = -9 m/s or 9 m/s in the opposite direction. Since the question is asking for the speed (not velocity), the answer is 9 m/s or B.

A 100-Kg Cannon at Rest Contains a 10-Kg Cannon Ball