Transcript: The equivalence between mass and energy denoted by the equation E = mc2 has profound consequences for the way we look at the physical world. We know that energy comes in many forms, and mass is just one of these. In a sense mass is potential energy since normally it is frozen in the form of stable particles. In the equation E = mc2 the arrows go both ways. Just as mass is a form of energy, so energy is a form of mass. A rapidly moving car has slightly more mass than a stationary car. A spent battery has slightly less mass than a fully charged battery. These effects are tiny but real and measurable, and so instead of the conservation of energy we should really talk about the conservation of mass-energy.