Commons

e the same as they always were. The change is due to the change inγ with heating, not to a change in mass. The heat energy, however,seems to be acting as if it was equivalent to some extra mass.But this whole argument was based on the fact that heat is aform of kinetic energy at the atomic level. Would E = mc 2 apply toother forms of energy as well? Suppose a rocket ship contains someelectrical energy stored in a battery. If we believed that E = mc 2applied to forms of kinetic energy but not to electrical energy, thenwe would have to believe that the pilot of the rocket could slowthe ship down by using the battery to run a heater! This wouldnot only be strange, but it would violate the principle of relativity,because the result of the experiment would be different dependingon whether the ship was at rest or not. The only logical conclusion isthat all forms of energy are equivalent to mass. Running the heaterthen has no effect on the motion of the ship, because the totalenergy in the ship was unchanged; one form of energy (electrical)was simply converted to another (heat).The equation E = mc 2 tells us how much energy is equivalentto how much mass: the conversion factor is the square of the speedof light, c. Since c a big number, you get a really really big numberwhen you multiply it by itself to get c 2 . This means that even a smallamount of mass is equivalent to a very large amount of energy.We’ve already seen several examples of applications of E = mc 2 ,on page 32.Section 4.3 Dynamics 91