Question

A stationary cannon fires a cannonball. Which of the following is NOT true immediately after the cannonball is fired?

• A. The cannon and cannonball do not have the same magnitude of momentum
• B. The force the cannon exerts on the cannonball is the same magnitude as the force of the cannonball on the cannon
• C. The impulse the cannon exerts on the cannonball is the same magnitude as the impulse the cannonball exerts on the cannon
• D. The cannon and cannonball do not have the same kinetic energy after firing
• E. The cannon and cannonball do not have the same kinetic energy before firing

Answer 1

Answer: (A)

The cannon and cannonball do not have the same magnitude of momentum

Since no external force acts on the cannon-cannonball system during the process, the momentum of the system before and after the firing is constant.
Since the momentum before firing is zero,

$0=\overrightarrow{p_{cannon}}+\overrightarrow{p_{cannonball}}$

$⟹\left|\overrightarrow{p_{cannon}}\right|=\left|\overrightarrow{p_{cannonball}}\right|$

Hence correct answer is option A.