Review actions and reaction forces. Determine interaction which occurs when a cannon fires a cannon ball. Determine why the interactions are not equal, as should be by Newton's 3rd law. If force equals mass x acceleration, then acceleration equal force/mass. The cannonball exhibits a large force and small mass while the cannon exhibits a small force and a large mass. The forces remain equal, the acceleration changes. Introduce systems. Determine why a force must be applied outside the system in order for motion to occur. The forces will not cancel each other when opposing the system. Use examples to explain why.