How is mechanical energy measured?

How is mass measured in relation to energy?

• How would I measure mass in relation to energy. For example in e=mc2, how would the measurement of energy stated if mass was equal to 1 kg? And how would it be stated if it was 1 gram. Saying e=1xc2 wouldn't really tell me what E equals. To be honest, I don't know if this is even a vaild question. I'm at work and my job provides no mental stimuli so I end up coming up with these odd questions.

• Answer:

  If c (the speed of light in a vacuum) is expressed in meters/sec and m is expressed in kg, then m*c^2 has units of kg*(m/s)^2. A joule is defined as one kg*(m/s)^2, so if you use the above units for m and c, the energy you calculate will be in joules (J). The energy equivalent of 1 kg would be: 1 kg * (2.998*10^8 m/s)^2 = 8.99 * 10^16 J, which is an enormous amount of energy. By convention, one megaton of TNT is defined as being equivalent to 4.184 × 10^15 J, so the energy equivalent of 1 kg of mass is approximately 21.5 megatons of TNT. Physicists commonly express the mass of fundamental particles in terms of their energy, but the energy unit used to do this is typically the electron volt (eV). 1eV = 1.60217646 * 10^ -19 joules (1 joule = 6.24150974 * 10^18 eV). For example, an electron has a rest mass of 9.10938188 * 10^-31 kilograms. It's rest mass expressed in electron volts is 9.1094*10^-31 kg * (2.998 *10^8 m/s)^2 * 6.242 *10^18 eV/J = 5.11*10^5 ev = 511 keV.