<P> Classically, a bound system is at a lower energy level than its unbound constituents . Its mass must be less than the total mass of its unbound constituents . For systems with low binding energies, this "lost" mass after binding may be fractionally small . For systems with high binding energies, however, the missing mass may be an easily measurable fraction . This missing mass may be lost during the process of binding as energy in the form of heat or light, with the removed energy corresponding to removed mass through Einstein's equation E = mc . Note that in the process of binding, the constituents of the system might enter higher energy states of the nucleus / atom / molecule, but these types of energy also have mass . It is necessary that they be removed from the system before its mass may decrease . Once the system cools to normal temperatures and returns to ground states in terms of energy levels, there is less mass remaining in the system than there was when it first combined and was at high energy . In that case, the removed heat represents exactly the mass "deficit" and the heat itself retains the mass which was lost (from the point of view of the initial system). This mass appears in any other system which absorbs the heat and gains thermal energy . </P> <P> As an illustration, consider two objects attracting each other in space through their gravitational field . The attraction force accelerates the objects and they gain some speed toward each other converting the potential (gravity) energy into kinetic (movement) energy . When either the particles 1) pass through each other without interaction or 2) elastically repel during the collision, the gained kinetic energy (related to speed), starts to revert into potential form driving the collided particles apart . The decelerating particles will return to the initial distance and beyond into infinity or stop and repeat the collision (oscillation takes place). This shows that the system, which loses no energy, does not combine (bind) into a solid object, parts of which oscillate at short distances . Therefore, in order to bind the particles, the kinetic energy gained due to the attraction must be dissipated (by resistive force). Complex objects in collision ordinarily undergo inelastic collision, transforming some kinetic energy into internal energy (heat content, which is atomic movement), which is further radiated in the form of photons--the light and heat . Once the energy to escape the gravity is dissipated in the collision, the parts will oscillate at closer, possibly atomic, distance, thus looking like one solid object . This lost energy, necessary to overcome the potential barrier in order to separate the objects, is the binding energy . If this binding energy were retained in the system as heat, its mass would not decrease . However, binding energy lost from the system (as heat radiation) would itself have mass . It directly represents the "mass deficit" of the cold, bound system . </P> <P> Closely analogous considerations apply in chemical and nuclear considerations . Exothermic chemical reactions in closed systems do not change mass, but become less massive once the heat of reaction is removed, though this mass change is much too small to measure with standard equipment . In nuclear reactions, however, the fraction of mass that may be removed as light or heat, i.e., binding energy, is often a much larger fraction of the system mass . It may thus be measured directly as a mass difference between rest masses of reactants and (cooled) products . This is because nuclear forces are comparatively stronger than the Coulombic forces associated with the interactions between electrons and protons, that generate heat in chemistry . </P> <P> Mass change (decrease) in bound systems, particularly atomic nuclei, has also been termed mass defect, mass deficit, or mass packing fraction . </P>

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