Reaction (physics)

Lore & Background

The third law is often stated as: 'To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.' The terms 'action' and 'reaction' have the misleading suggestion of causality, as if the action is the cause and reaction is the effect. This is incorrect; the forces are perfectly simultaneous and exist for the same reason. Examples include the ground reaction force when a person jumps: the person exerts a downward force on the ground (action), and the ground exerts an upward force on the person (reaction). Similarly, spinning wheels of a vehicle exert a backward friction force on the ground, and the ground exerts a forward friction force on the wheels. In gravitational interactions, the Earth exerts a pull on the Sun, and the Sun exerts an equal and opposite pull on the Earth, though the Sun's large mass makes its motion less apparent. A common misinterpretation is applying Newton's third law to forces acting on the same object. For example, a book on a table experiences a downward gravitational force and an upward normal force; these are equal and opposite but act on the same object, so they are not action-reaction pairs. The actual action-reaction pairs involve forces on two different objects, such as the Earth's pull on the book and the book's pull on the Earth.

Reader's Guide

The concept of reaction is fundamental to understanding interactions in classical mechanics. It clarifies that forces always come in pairs, each acting on a different object, and that these pairs are simultaneous and equal in magnitude but opposite in direction. This principle is essential for analyzing systems ranging from everyday interactions (like walking or driving) to celestial mechanics (like planetary orbits around a barycenter). A key pedagogical point is that students often confuse balanced forces on a single object (which obey Newton's second law) with action-reaction pairs (which obey the third law). The third law does not require that the forces on an object be balanced; it only relates forces between two objects. For instance, a mass oscillating on a spring has unequal gravitational and elastic forces on the mass, yet the gravitational force on the mass equals the gravitational force on the Earth, and the elastic force on the mass equals the elastic force on the spring. The causal misinterpretation of action and reaction is also important: the forces are not cause and effect but simultaneous mutual interactions. This understanding helps avoid errors in physics education and application, particularly in distinguishing real forces from fictitious ones like centrifugal force.

Did You Know?

• The attribution of which force is the action and which is the reaction is arbitrary; either can be considered the action.
• In a supported mass at rest, there are four forces of equal magnitude: gravitational forces between Earth and object, and support forces between object and support.
• The forces in an action-reaction pair are perfectly simultaneous, not cause and effect.
• A common student error is applying Newton's third law to equal and opposite forces acting on the same object, which instead relates to Newton's second law.