<P> The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons . Work (energy) is required to bring charged protons together against their electric repulsion . This energy is stored when the protons and neutrons are bound together by the nuclear force to form a nucleus . The mass of a nucleus is less than the sum total of the individual masses of the protons and neutrons . The difference in masses is known as the mass defect, which can be expressed as an energy equivalent . Energy is released when a heavy nucleus breaks apart into two or more lighter nuclei . This energy is the electromagnetic potential energy that is released when the nuclear force no longer holds the charged nuclear fragments together . </P> <P> A quantitative description of the nuclear force relies on equations that are partly empirical . These equations model the internucleon potential energies, or potentials . (Generally, forces within a system of particles can be more simply modeled by describing the system's potential energy; the negative gradient of a potential is equal to the vector force .) The constants for the equations are phenomenological, that is, determined by fitting the equations to experimental data . The internucleon potentials attempt to describe the properties of nucleon--nucleon interaction . Once determined, any given potential can be used in, e.g., the Schrödinger equation to determine the quantum mechanical properties of the nucleon system . </P> <P> The discovery of the neutron in 1932 revealed that atomic nuclei were made of protons and neutrons, held together by an attractive force . By 1935 the nuclear force was conceived to be transmitted by particles called mesons . This theoretical development included a description of the Yukawa potential, an early example of a nuclear potential . Mesons, predicted by theory, were discovered experimentally in 1947 . By the 1970s, the quark model had been developed, by which the mesons and nucleons were viewed as composed of quarks and gluons . By this new model, the nuclear force, resulting from the exchange of mesons between neighboring nucleons, is a residual effect of the strong force . </P> <P> While the nuclear force is usually associated with nucleons, more generally this force is felt between hadrons, or particles composed of quarks . At small separations between nucleons (less than ~ 0.7 fm between their centers, depending upon spin alignment) the force becomes repulsive, which keeps the nucleons at a certain average separation, even if they are of different types . This repulsion arises from the Pauli exclusion force for identical nucleons (such as two neutrons or two protons). A Pauli exclusion force also occurs between quarks of the same type within nucleons, when the nucleons are different (a proton and a neutron, for example). </P>

What is the exchange particle for strong nuclear force
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