So far we've learned that objects will move with constant, possibly zero velocity, unless an externally applied force causes their velocity to change. We've learned how objects respond when forces are applied to them (they accelerate) and how to mathematically relate the net force on an object to its mass and observed acceleration. These are Newton's first and second laws. We haven't yet completely described how objects exert forces on each other. This is the subject of Newton’s third law. Let's look at what happened in the exploration activities.
In the first activity you looked at a video of a car crash. In the video two cars crash into each other. The critical thing to observe is that when the car crashes and comes to a stop it must accelerate, because its velocity is changing. In the case of the two cars crashing it is easy to see that they each exert a force on each other, and because their masses are equal and their accelerations are equal it is easy to see that those two forces are equal in magnitude and opposite in direction. This is what Newton's third law tells us: when an object exerts a force on something it feels an equal and opposite force in return. We'll look at one car crashing into a wall later
In the second activity we again see Newton's third law in action. In this case the skater applies a force to the bowling ball causing it to accelerate forward, but as a result she can clearly be seen to accelerate backwards. This video demonstrates that Newton’s law holds true even with the masses are not equal.