Lesson 10: Newton's Third Law

​For every action, there is an equal and opposite reaction. This is Newton's Third Law of Motion. What that means is if you push on a wall, then the wall pushes back against you with an equal amount of force. When you hit a baseball with a bat, the ball pushes back against the bat with the same amount of force.

​A force pair is the forces two objects apply to each other. Force pairs are equal and opposite, but they do not cancel each other out (causing a net force of zero) because the forces act on different objects. Think of the baseball and the bat. They hit each other with the same amount of force, but the two forces act on different objects.

​When you push on an object, the object pushes back with an equal amount of force. The force you apply is called the action force. The force applied by the object in response is called the reaction force. Newton's Third Law explains how we can swim We push against water (action) and are propelled forward by the water pushing back on us (reaction).

​Since action and reaction forces don't cancel each other out, they can cause motion in the objects involved. Momentum, a measure of how hard it is to stop an object, is a product of the velocity and mass of an object.

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​The equation for momentum is:

​p = m x v where p stands for momentum.

​If a small sedan and an 18-wheeler are moving with the same velocity, then will they have the same momentum? No. The 18-wheeler is harder to stop because it has more mass than the sedan. It has greater momentum.

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Newton's first two laws relate to momentum. The first law states that if there is no net force on an object, it will not change momentum. The second law says the force on an object is a product of its mass and acceleration. Since momentum is a product of an object's mass and velocity (a component of acceleration), the force on an object equals its change in momentum.

​If two objects collide, then they transfer their momentum to each other. In the case of a cue ball hitting other pool balls, the cue ball transfers its momentum to the other balls, causing them to gain momentum and the cue ball to lose momentum.

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​According to the law of conservation of momentum, the total momentum in a group of objects stays the same unless they are acted upon by outside forces.

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​So, why do the balls stop moving? Because of friction, an outside force, which slows them to a stop.

There are two types of collisions: elastic and inelastic collisions. In elastic collisions, the two objects do not stick together, but bounce off each other. Think of the pool balls in the last slide. In inelastic collisions, the two objects stick together.​ Think of a football player tackling another player. In either of these cases, the total momentum of the group from before the collision is conserved.