# Newton's laws of motion

Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows::49 Newton's laws of motion, combined with his law of gravity, allow the prediction of how planets, moons, and other objects orbit through the solar system, and they are a vital part of planning space travel. During the Apollo 8 mission, astronaut Bill Anders took this photo, Earthrise; on their way back to Earth, Anders remarked, "I think Isaac Newton is doing most of the driving right now."

Law 1. A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.

Law 2. When a body is acted upon by a force, the time rate of change of its momentum equals the force.

Law 3. If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.

The three laws of motion were first stated by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), originally published in 1687. Newton used them to investigate and explain the motion of many physical objects and systems, which laid the foundation for classical mechanics. In the time since Newton, the conceptual content of classical physics has been reformulated in alternative ways, involving different mathematical approaches that have yielded insights which were obscured in the original, Newtonian formulation. Limitations to Newton's laws have also been discovered: new theories are necessary when objects are very fast (special relativity), very massive (general relativity), or very small (quantum mechanics).