Newton's third law of motion, also called the law of interaction, states that each interaction between two objects produces a pair of equal forces. As object A exerts force on object B, object B reacts by exerting an equal and opposite force on object A. This pair of forces is sometimes called an action-reaction.
The forces described by Newton’s third law are either pushes or pulls resulting from interactions between objects. Some interactions come from direct contact between two objects. These are called contact interactions. Other forces act over longer distances, such as magnetism, electricity or gravity. These are called action-at-a-distance forces.
Many examples of the law of interaction take place in nature. Consider a cheetah as it runs. As it moves forward, each of its feet exert a backward force on the ground. As this happens, the ground exerts an equal and opposite force on the cheetah, propelling it forward.
Similarly, when a bus moves down a road, the wheels turn and the tires exert a force on the road. As the tires are exerting a force on the road, however, the road exerts and equal and opposite force on the tires, helping to push the bus forward.
A golf club hitting a ball is another example. Even though the club is doing all of the movement, when the club hits the ball, the ball also hits the club, with an equal and opposite force. When you push a chair to slide it across the floor, it also pushed back on your hand with an equal and opposite force. Every force involves resistance as a part of this symmetrical relationship.
The law of interaction is the third and last in Newton’s series of laws describing how motion happens. The first law in the series, often called the law of inertia, states that objects either at rest or in motion tend to stay in their current state unless they are acted upon by an external, unbalanced force. If an object is currently at rest, it will continue to stay that way, while an object currently moving south at 10 mph will also continue to move in the same direction at the same speed, unless something happens to disrupt the stillness or movement of either of these objects.
People experience the law of inertia when in a car, train, or airplane that is accelerating. As the acceleration quickens, a body will attempt to continue moving at the same speed at which it was moving before the acceleration started. This discrepancy in speeds creates the sensation of being pushed backwards as the acceleration happens.
Newton’s second law describes the forces that act upon an object after it has begun to move. Once the forces have become unbalanced, and the object is in motion, its acceleration is dependent on the net force acting on the object, and the mass of the object. The net force can be defined as the vector sum of all forces acting on an object.