Newton first published these laws in *Philosophiae Naturalis Principia Mathematica* (1687) and used them to prove many results concerning the motion of physical objects. In the third volume (of the text), he showed how, combined with his Law of Universal Gravitation, the laws of motion would explain Kepler's laws of planetary motion.

Table of contents |

2 Newton's First Law 3 Newton's Second Law 4 Newton's Third Law |

- Nature and Nature's laws
- lay hid in night;
- God said, "Let Newton be!"
- And there was light.
- -- Alexander Pope

- -- Alexander Pope

Newton's laws were verified by experiment and observation for over 200 years, until 1916, when they were superseded by Einstein's theory of relativity. Newton's laws still provide a completely adequate approximation for the behaviour of objects in "everyday" situations.

Alternative formulations:

*Every object persists in its state of rest, or uniform motion (in a straight**line); unless, it is compelled to change that state, by forces impressed on it.**A body remains at rest, or moves in a straight line (at a constant velocity), unless acted upon by a net outside force.*

*The**time rate of change in momentum is proportional to the applied force and takes place in the direction of the force.**The acceleration of an object is proportional to the force acting upon it.*

*F*=*ma**F*= force*m*= mass*a*= acceleration.

In the equation, **F** = m**a**, a is directly measurable but **F** is not. The second law only has meaning if we are able to assert, in advance, the value of **F**. Rules for calculating force include Newton's Law of Universal Gravitation.

Taken together with Newton's Third Law of Motion, it implies the Law of Conservation of Momentum.

*Whenever one body exerts force upon a second body, the second body exerts an equal and opposite force upon the first body.**For every action, there is an equal and opposite reaction.*