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

Hint:The initial momentum of a stationary body is zero. When no external force is applied then according to the law of conservation of momentum- The momentum of the body before and after the process remains constant.

Step by step solution:
As mentioned In the question the cannon is stationary initially and hence we can assume that the initial momentum of the cannon is zero.
One more assumption we can make is that no external force is applied to the cannon before and after the firing of the cannonballs.
Hence using the above assumption we can easily apply the law of conservation of the momentum.
Since the momentum before firing is zero:
$\begin{gathered}
  0 = {p_{cannon}} + {p_{cannonball}} \\
   \Rightarrow \left| {{p_{cannon}}} \right| = \left| {{p_{cannonball}}} \right| \\
\end{gathered} $
Therefore A is the correct option because the momentum of cannon and cannonball is equal.

Note: while solving this type of question we should have a proper understanding of all the situations so that we can make required assumptions. We should also note that in any case we should not make any wrong assumption.