Consider a 50 kilogram stationary cannon. A shell of mass 5 kilograms is fired from the cannon which travels with a velocity of 400 meters per second. What will be the velocity of the cannon after firing, ignoring frictional forces? The mass and velocity of the shell and the mass of the cannon are the known quantities. The velocity of the cannon is the unknown quantity. Ignoring frictional forces, the cannon and the shell will form a closed system. The positive x-axis is assumed in the direction of the shot to define the velocity vectors. Since the cannon and shell are stationary, the momentum before the interaction is zero. After firing the shell, the final momentum will be equal to the sum of the cannon's momentum and the shell's momentum. Lastly, equate the momentum before and after the interaction to obtain the unknown quantity. Here, the velocity of the cannon after firing is resolved by substituting the known quantities in the equation and it comes out to be minus 40 meters per second. The negative sign indicates that the cannon recoils in the direction opposite to the direction of the shot.