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Newton's third law
Beginner
Forces come in equal and opposite pairs acting on different bodies.
F_AB = −F_BAIf A pushes on B, B pushes back on A equally and oppositely. The two forces are of the same type and act on different bodies, so they never cancel in a single free-body diagram.
A rocket pushes gas back; the gas pushes the rocket forward.
Weight and the normal force on a resting book are not a third-law pair.
In more detail
Newton's third law states that when object A exerts a force on B, B exerts an equal and opposite force on A. The two forces act on different objects.
Real-world uses
- Rowing a boat pushes water backward while the water pushes the boat forward.
- Jumping pushes the ground down, which pushes the body up.
- When a rocket pushes gas backwards, the gas pushes the rocket forwards with an equal and opposite force.
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Frequently asked questions
Why do action and reaction not cancel?
Because they act on different objects, so they do not add up on the same body.
Are these forces always equal?
Yes, in magnitude, even when the objects have very different masses or accelerations.
If the forces are equal and opposite, why does anything move?
Because the forces act on different objects. Each body accelerates according to the net force on that body alone.
Related topics in Newton's Laws & Forces
Inertia · Newton's second law · Units of force · Weight and normal force · Free-body diagrams · Net force and equilibrium