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
Importance of Newton's laws of motion
Newton's laws of motion, together with his Law of Universal Gravitation and the mathematical techniques of calculus, provided for the first time a unified quantitative explanation for a wide range of physical phenomena such as: the motion of spinning bodies, motion of bodies in fluids; projectiles; motion on an inclined plane; motion of a pendulum; the tides; the orbits of the Moon and the planets. The law of conservation of momentum, which Newton derived as a corollary of his second and third laws, was the first conservation law to be discovered.
Newton's laws were verified by experiment and observation for over 200 years, until 1916, when they were superseded by Einstein's theory of relativity.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.
Newton's First Law
This law is also called the Law of Inertia or Galileo's Principle.
Alternative formulations:friction usually acts upon moving objects. Newton's law indicates that some force (gravity) must be acting upon the planets, as they do not travel in a straight line.
In the equation, F = ma, 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.
Newton's Third Law
If you strike an object with a force of 200 N, then the object also strikes you (with a force of 200 N). Not only does a bullet exert force upon a target; but, the target exerts equal force upon the bullet. Not only do planets accelerate toward stars; but, stars accelerate toward planets. The reaction force has the same line of action, and is of the same type and magnitude as the original force.