### Newton's laws

(physical laws of dynamics and gravitation)

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.

(*physics,dynamics*)
**Further reading:**

http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

http://hyperphysics.phy-astr.gsu.edu/hbase/Newt.html

**Referenced by pages:**

black-hole information paradox

celestial mechanics

general relativity (GR)

gravity

Hamiltonian

homologous collapse

Laplace-Lagrange secular theory

Navier-Stokes equations (NS equations)

post-Newtonian formalism (PN formalism)

parameterized post-Newtonian formalism (PPN formalism)

quantum mechanics (QM)

relativity

special relativity (SR)

Index