Newton's laws

The term Newton's laws generally refers to Newton's laws of motion, three propositions laid out by Isaac Newton that underly dynamics, physics of force and motion. A fourth law is Newton's law of gravitation, describes gravity, a source of force. These four laws, in addition to Newton's development of calculus, underlie much of physics ever since.

• Newton's first law: objects retain their velocity unless a force is applied (inertia).
• Newton's second law: the force on an object is equal to the mass of the object times the acceleration resulting from the force (F=ma).
• Newton's third law: when one body exerts a force on another, it experiences the same amount of force in the opposite direction ("for every action, there is an equal and opposite reaction").
• Newton's law of gravitation: the force of gravity between two objects is proportional to the product of their masses and the reciprocal of the square of the distance between them.

Einstein's relativity theories affected all these, showing that Newton's laws merely approximate more complex laws that can differ significantly (measurably) under extreme circumstances. Astrophysics and modern technology include examples of such extreme circumstances, but Newton's laws are still widely useful approximations, generally used unless there is reason to believe the discrepancy is significant.