This article is about the SI unit of force. For other uses see Newton (disambiguation)
In physics, the newton (symbol: N) is the SIunit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics. It was first used around 1904, but not until 1948 was it officially adopted by the General Conference on Weights and Measures (CGPM) as the name for the mks unit of force.
Since one definition of weight is the force acting between two objects due to gravity, the newton is also the unit of weight. A mass of one kilogram near the Earth's surface has a weight of approximately 9.81 newtons, although this figure varies by a few tenths of one per cent over the Earth's surface. Conversely, an object with a mass of 9.81-1 kg (≈101.94 grams) weighs roughly one newton. Rather fittingly, given the story about Newton's discovery of gravity, this is about the mass of a small apple.
Newton made contributions to all branches of mathematics then studied, but is especially famous for his solutions to the contemporary problems in analytical geometry of drawing tangents to curves (differentiation) and defining areas bounded by curves (integration).
Newton's work on pure mathematics was virtually hidden from all but his correspondents until 1704, when he published, with Opticks, a tract on the quadrature of curves (integration) and another on the classification of the cubic curves.
Correspondence with Hooke (1679-1680) redirected Newton to the problem of the path of a body subjected to a centrally directed force that varies as the inverse square of the distance; he determined it to be an ellipse, so informing Edmond Halley in August 1684.
The two forces in Newton's third law are of the same type, e.g., if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward on the road.
The force on the dipole due to the point charge is perpendicular to the line connecting them, so there is a reaction force on the point charge in the opposite direction, but these two force vectors are parallel and, even when extended to a line, they never cross each other in space.
Newton's first law appears to be a special case of the second law, and Newton may have stated the first law separately simply in order to throw down the gauntlet to the Aristotelians.
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