Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

A Cannon

A cannon of mass 1000 kg fires a 10-kg shell at a velocity of 200 m/s. At what speed does the cannon recoil?

The law of conservation of momentum tells us that

pcannon,i + pshell,i = pcannon,f + pshell,f .

Choosing a coordinate system in which the cannon points in the positive direction, the given information is

pcannon,i = 0

pshell,i = 0

pshell,f = 2000 kgm/s .

We must have pcannon,f = -2000 kgm/s, so the recoil velocity of the cannon is -2 m/s.

Last Update: 2009-06-21