Newton's Third Law
Newton's Third Law of Motion states that for every action force, there is an equal and opposite reaction force. These two forces always act on different objects simultaneously, forming an action-reaction pair. The law explains how rockets propel themselves through space, how we walk by pushing the ground backward, and why swimming is possible by pushing water backward to move forward.
Action-Reaction Pairs — Newton's Third Law Examples
| Action Force | Acts On | Reaction Force | Acts On |
|---|---|---|---|
| Foot pushes ground backward | Earth | Ground pushes foot forward | Person |
| Rocket expels gas downward | Gas | Gas pushes rocket upward | Rocket |
| Swimmer pushes water backward | Water | Water pushes swimmer forward | Swimmer |
| Earth pulls apple downward | Apple | Apple pulls Earth upward | Earth |
| Gun propels bullet forward | Bullet | Bullet pushes gun backward (recoil) | Gun |
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Related Terms
Newton's First Law
Newton's First Law of Motion states that an object at rest remains at rest, and an object in motion continues in motion at constant velocity, unless acted upon by a net external force. This principle is also known as the Law of Inertia and forms the conceptual foundation of classical mechanics. It explains why passengers lurch forward when a bus brakes suddenly, or why a hockey puck slides indefinitely on a frictionless ice surface.
Newton's Second Law
Newton's Second Law of Motion states that the net force acting on an object equals the product of its mass and acceleration. It is the most quantitative of the three laws and provides the mathematical relationship between force, mass, and motion. This law is used in virtually every engineering and physics calculation involving dynamics, from designing car brakes to launching spacecraft.
Normal Force
The normal force is the contact force exerted by a surface on an object, acting perpendicular (normal) to the surface at the point of contact. It is a reaction force that prevents objects from passing through solid surfaces and adjusts in magnitude to balance components of other forces. On a flat horizontal surface, the normal force on a stationary object equals its weight; on an inclined surface, it equals the component of weight perpendicular to the slope.
Published by Isaac Newton in 'Philosophiae Naturalis Principia Mathematica' (1687). 'Action' comes from Latin 'actio' (a doing), and 'reaction' from 'reactio' — a back-acting force. The principle was later found to be related to the conservation of momentum.