Explosions
An object being fired from a cannon is also a collision where momentum must be conserved. As the momentum before the 'collision' is zero, the momentum after the collision is zero. In physics, this type of event is termed an explosion.
Watch this video for a practical demonstration of conservation of momentum in explosions.
Example
Consider a cannon ball of mass, \({m_B}\), 4 kg fired at velocity \({v_B}\) 120 ms-1 from left to right from a cannon of mass, \({m_C}\), 96 kg. This information lets you determine the recoil velocity of the cannon \({v_ C}\).
The total momentum before is zero so by the law of conservation of momentum the momentum after the ball is fired is also zero.
\({m_B}{v_B}+ {m_C}{v_C}= 0\) so:
\(4\times 120 + 96 \times {v_C} = 0\)
\(480 + 96 \times {v_C} = 0\)
\({v_C} = \frac{{ - 480}}{{96}}\)
\(= - 5m{s^{ - 1}}\)
The negative sign means the cannon moves backwards to conserve the momentum in the explosion, this effect is known as 'recoil'.
Question
Cannons on old wooden ships were allowed to roll backwards after firing. If the cannons were attached to the deck what would have happened?
This recoil is an essential part of firing any object. If the cannon was attached to the deck the recoil could have ripped apart the ship's deck.