**Newton’s laws of motion** are three physical laws that, together, laid the foundation for classical mechanics. They describe the relationship between a body and the forces acting upon it, and its motion in response to those forces. More precisely, the first law defines the force qualitatively, the second law offers a quantitative measure of the force, and the third asserts that a single isolated force doesn’t exist. These three laws have been expressed in several different ways, over nearly three centuries,^{[1]} and can be summarised as follows.

First law: |
In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.^{[2]}^{[3]} |

Second law: |
In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. |

Third law: |
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body. |

The three laws of motion were first compiled by Isaac Newton in his *PhilosophiÃ¦ Naturalis Principia Mathematica* (*Mathematical Principles of Natural Philosophy*), first published in 1687.^{[4]} Newton used them to explain and investigate the motion of many physical objects and systems.^{[5]} For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler’s laws of planetary motion.