# Mass-energy equivalence

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Mass-energy equivalence is where mass has an energy equivalence, and energy has a mass equivalence. This is expressed quantitatively using Einstein's equation:

E=mc²

where

The conversion factor c2 is 89.88 PJ/kg = 21.48 Mt TNT per kg = 149.3 pJ/u = 931.5 MeV/u

## Conservation of mass and energy

With the concept of mass-energy equivalence, we combine together the conservation of mass and the conservation of energy, allowing mass to be converted to other forms of mass or to a form of energy, and energy to be converted to other forms of energy or a form of mass. The total amount of mass/energy in a system remains constant, that is to say, mass and energy cannot be destroyed or created.

## Fast moving object

A fast-moving object moving at near to the speed of light cannot be accelerated to, or faster than, the speed of light, regardless of how much energy we put into the system. As we apply a force, and hence do work on the object, its speed does not appear to increase by the amount specified by Ekinetic = 1/2 mv2. Instead, the energy provided to it is converted to mass, and its relativistic mass increases, in what is known as mass dilation. The relativistic mass of an object is expressed as a function of its relative speed.