Newton's laws of motion
Newton first defined these laws in Philosophiae Naturalis Principia Mathematica (1687) and, using his newly developed calculus, proved many results concerning "idealised" particles. 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. Newton's laws were modified, in 1916, by Einstein's theory of relativity.
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2 Newton's Second Law
3 Newton's Third Law
Newton's First Law (Law of Inertia)
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.