## A Conversation for Units of Measurement

### An ugly unit

Pete, never to have a time-specific nick again (Keeper of Disambiguating Semicolons) - Born in the Year of the Lab Rat Started conversation Nov 5, 2001

Here's an obscure, odd-looking, but (in its field) commonly used unit:

GeV/c^2

This is derived from that famous equation E=mc^2 -- or, rather, its rearrangement m=E/c^2. E is the kinetic energy of an object, and m is its mass defect -- that is, the difference between its mass while in motion and its rest mass (see http://www.bbc.co.uk/h2g2/guide/A287381). The electron volt (eV) is used to express the kinetic energy of a particle (usually an electron or proton) accelerated by a potential difference. This unit therefore represents the mass defect of a particle in a particle accelerator. (The G is the standard prefix for 10^9.)

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### An ugly unit

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