Introduction: Imagine a heavy block, like the one shown to the left. If you have ever kicked your foot against a wall or a heavy block in frustration, you'll be able to speak to the amount of pain that it causes your foot. This is but one of Newton's Laws of Motion. This example, in particular, refers to Newton's Third Law, which involves an action-reaction pair. Newton's Laws are fundamental to an understanding of how forces interact, as well as what is behind a force to begin with.

Newton's First Law of Motion states that an object that is at rest will remain at rest and that an object that is in motion will remain in motion, unless acted upon by an unbalanced force. This law is often called the law of inertia, and inertia refers to the ability of an object to resist changes in its motion.

Newton's Second Law of Motion states that acceleration is produced when a force acts on a mass. In general, the greater that the mass of the object being accelerated is, the greater the amount of force is needed to accelerate that given object. This can be thought of by the idea that lighter objects require less force to be moved or accelerated, while heavier objects require more force to be moved or accelerated.

Newton's Third Law of Motion states that, for every action, there is an equal but opposite reaction. In the case of your foot kicking against the wall, the force you apply to the wall comes from your foot, whereas the pain in your foot as a result of kicking the wall comes from the wall acting on your foot. In this way, the magnitude of the force that your foot applies to the wall is equal to the magnitude of the force that the wall applies to your foot, though they act in opposite directions on one another.