Discuss how to change momentum, an impulse force must by outside the system. Use as an example a cannon ball fired from a cannon. Elicit that at rest, the system has a zero momentum. The velocity is zero therefore the momentum is zero. Discuss what effects occur when the cannon is fired. Discuss the momentum of the cannon and the momentum of the cannonball. Discuss how the cannon has a large mass but a small velocity (recoil), while the cannonball has a small mass. but a high velocity. Show why the momentum of the system becomes zero. Called the conservation of momentum law-