Newton's third law asserts that for every action, there is an equal and opposite reaction.Newton's third law, also called the law of action and reaction, states: the forces of interaction between two bodies are equal in magnitude and opposite in direction. It is important to understand that they are applied to different bodies, so they do not balance each other. Isaac Newton in the 'Principia' (1687) gave this principle a strict mathematical form, but empirically people always faced opposition: when rowing, the oar pushes the water backward, and the water pushes the boat forward.
The principle of opposition was discussed even by ancient mechanics, but it was systematically studied by Isaac Newton based on experiments and observations. He verified the law in collisions of pendulums (with Christiaan Huygens) and in planetary motion. Newton realized that the Sun and planets mutually attract: if the Sun attracts the Earth, then the Earth pulls the Sun with the same force, but due to the huge mass the reaction is unnoticeable. Later the law was confirmed in all conceivable interactions.
It is applied in jet engines, walking, swimming, in calculating impacts and contact stresses. In rocketry, the third law explains thrust: the ejection of mass at high speed produces an equal force impulse, pushing the rocket forward. The limits of applicability are classical: in electrodynamics with retarded potentials, the third law in the simple form for instantaneous forces is violated; accounting for field momentum is required.
The reactive jet of gases in a rocket engine bursts downward — and the rocket rises upward; without this law, spaceflight would be impossible.
If the third law suddenly turned off, you couldn't walk: your foot pushes off the ground, and the ground pushes you forward.
| F | variable | Net force vector — force newton |
| m | variable | Inertial mass — mass kilogram |
| a | variable | Acceleration vector — acceleration metre per second squared |
| \Gamma | variable | Reaction rate — reaction rate per second |
| E_a | variable | Activation energy — energy joule |
| T | variable | Absolute temperature — temperature kelvin |
| A | constant | Frequency factor, related to collision frequency and steric effects — pre exponential factor = material-dependent per second |
| k_B | constant | Relates temperature to energy — boltzmann constant = 1.380649e-23 joule per kelvin |
| e^{-E_a/k_B T} | operator | Exponential decay with inverse temperature — exponential |
