Newton's laws of motion

Lore & Background

Newton used them to investigate and explain the motion of many physical objects and systems. The laws can be paraphrased as follows: A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force. At any instant of time, the net force on a body is equal to the body's acceleration multiplied by its mass or, equivalently, the rate at which the body's momentum is changing with time. If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.

Reader's Guide

Newton's laws of motion are often stated in terms of point or particle masses, a reasonable approximation for real bodies when the motion of internal parts can be neglected and when the separation between bodies is much larger than the size of each. The mathematical description of motion, or kinematics, is based on specifying positions using numerical coordinates. In the time since Newton, new insights, especially around the concept of energy, built the field of classical mechanics on his foundations. In modern times, limitations to Newton's laws have been discovered; new theories were consequently developed, such as quantum mechanics and relativity to address the physics of objects in more extreme cases.

Did You Know?

• Newton used the laws to investigate and explain the motion of many physical objects and systems.
• In modern times, limitations to Newton's laws have been discovered, leading to new theories such as quantum mechanics and relativity.
• The laws are often stated in terms of point or particle masses, where the body's volume is negligible.