The Feynman Lectures on Physics, Vol. I: The New Millennium Edition: Mainly Mechanics, Radiation, and Heat (Volume 1)

by Richard P. Feynman

Then, although we might at first expect the mass to be , we have found that it is not , but . Since is what is put in, but are the rest masses of the things inside, the excess mass of the composite object is equal to the kinetic energy brought in.