Newton's Third Law

This law says forces always come in pairs: if object A pushes or pulls on object B, object B pushes or pulls back on A with the same size force in the opposite direction. The two forces act on different objects, so they do not cancel each other on one object; they are a direct result of interactions like contact, gravity, or magnetic forces. It works because forces arise from mutual interactions, so each body experiences the interaction from the other body equally and oppositely.

Use this law whenever two objects interact and you want to identify the paired forces, such as walking, pushing a wall, collision problems, or objects resting on surfaces. It is especially useful for drawing free-body diagrams and keeping the forces on each object separate.

• thinking the action and reaction forces cancel because they are equal and opposite, even though they act on different objects
• mixing up which object is doing the pushing and which object is receiving the force
• using Newton's Third Law to find motion directly without also considering the other forces on each object