Fundamental interaction that underlies some forms of radioactivity and certain interactions between subatomic particles. It acts on all elementary particles that have a spin of 12. The particles interact weakly by exchanging particles that have integer spins. These particles have masses about 100 times that of a proton, and it is this relative massiveness that makes the weak force appear weak at low energies. For example, in radioactive decay, the weak force has a strength about 1/100,000 that of the electromagnetic force. However, it is now known that the weak force has intrinsically the same strength as the electromagnetic force, and the two are believed to be only different manifestations of a single electroweak force (see electroweak theory).

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Fundamental force acting between elementary particles of matter, mainly quarks. The strong force binds quarks together in clusters to form protons and neutrons and heavier short-lived particles. It holds together the atomic nucleus and underlies interactions among all particles containing quarks. In strong interactions, quarks exchange gluons, carriers of the strong force, which are massless particles with one unit of intrinsic spin. Within its short range (about 10⁻¹⁵ m), the strong force appears to become stronger with distance. At such distances, the strong interaction between quarks is about 100 times greater than the electromagnetic force.

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Universal force of attraction that acts between all bodies that have mass. Though it is the weakest of the four known forces, it shapes the structure and evolution of stars, galaxies, and the entire universe. The laws of gravity describe the trajectories of bodies in the solar system and the motion of objects on Earth, where all bodies experience a downward gravitational force exerted by Earth's mass, the force experienced as weight. Isaac Newton was the first to develop a quantitative theory of gravitation, holding that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Albert Einstein proposed a whole new concept of gravitation, involving the four-dimensional continuum of space-time which is curved by the presence of matter. In his general theory of relativity, he showed that a body undergoing uniform acceleration is indistinguishable from one that is stationary in a gravitational field.

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Statement that any particle of matter in the universe attracts any other with a force (math.F) that is proportional to the product of their masses (math.m₁ and math.m₂) and inversely proportional to the square of the distance (math.R) between them. In symbols: math.F = math.G(math.m₁math.m₂)/math.R², where math.G is the gravitational constant. Isaac Newton put forth the law in 1687 and used it to explain the observed motions of the planets and their moons, which had been reduced to mathematical form by Johannes Kepler early in the 17th century.

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In physics, the effect of any of the four fundamental forces—gravitational, electromagnetic, strong, and weak. All known natural forces can be traced to these fundamental interactions. Gravitation is the attractive force between any two objects that have mass; it causes objects to fall to the ground and maintains the orbits of planets around the Sun. Electromagnetic force is responsible for the attraction and repulsion between electric charges and explains the chemical behaviour of atoms and the properties of light. The strong force binds quarks together in protons, neutrons, and other hadrons and also holds the protons and neutrons of an atomic nucleus together, overcoming the repulsion of the positively charged protons for each other. The weak force is observed in certain forms of radioactive decay (see radioactivity) and in reactions that fuel the Sun and other stars.

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One of the four known basic forces in the universe. Electromagnetism is responsible for interactions between charged particles that occur because of their charge, and for the emission and absorption of photons (electromagnetic radiation). The phenomena of electricity and magnetism are consequences of this force, and the relationships between them were first described by James Clerk Maxwell in the 1860s. The physical description of electromagnetism has since been combined with quantum mechanics into the theory of quantum electrodynamics. The electromagnetic force is about 10³⁶ times as strong as the gravitational force (see gravitation), but significantly weaker than both the weak force and the strong force.

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