In Newton's Third and final law of motion, he described the interaction of two objects when force is applied. Basically, forces do not exist alone—they happen in pairs. Formally, Newton's Third Law states that when an object exerts a force on another object, the other object exerts a force back.
Newton's Third Law of Motion
When an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
These pairs of forces are called action-reaction pairs, but it is not the typical reaction that we think of when we react to some external stimulus. In fact, a reaction seems to imply a separation in time, but there is none in an action-reaction pair. The action force happens at the same time as the reaction force. Once that force is lifted, the reaction force is gone as well. In other words, when you hit a nail with a hammer, the reaction force would be the force of the nail on the hammer. When you push on a wall, the wall pushes back on you.
It will benefit you to consider what action-reaction pairs are and what they are not. So far, when you have drawn free-body diagrams, you may have seen pairs of forces that are equal and opposite. If the free-body diagram was drawn correctly, though, those would NOT be action-reaction pairs of forces, since those forces are all forces acting on the same object. The most common of these is the force of gravity on the object paired with the force of the ground or surface on the object. They are indeed equal and opposite, but since they both act on the same object, they are not action-reaction pairs. Each of those forces, though, has a partner. The force of gravity can be described as the force of the Earth on the object. The action-reaction pair to this force would be the force of the object on the Earth. Notice, to find the reaction pair, you need only switch the order of the objects in the description of the force.
Question
What would be the action-reaction pair to the force of the ground on the object?
The force of the object on the ground would be the action-reaction pair.
Question
Won't these action-reaction pair forces just cancel each other out?
While they are equal and opposite forces, they are acting on different objects thus those forces do not cancel each other out. The forces would be used separately to determine the effect of the force on the two separate objects.
