Note 8 — Chapter II · Exploring the Atom

This is expressed in the mathematical formula ½mv² for the energy of motion of a body of weight m moving with a speed v. If m is measured in grammes, and v in centimetres per second, the energy of motion of the body is said to be ½mv² “ergs.” Thus an “erg” is the energy of motion of a body of 2 grammes weight (so that ½m= 1) moving with a speed of one centimetre a second. As an example, the energy of an express train of 300 tons’ weight (3 × 10⁸ gms.) moving at 60 miles an hour (2682 cms. a second) is 1079 × 10¹⁴ ergs; a cannon-ball or shell weighing a ton and moving at 1520 feet a second has precisely the same energy.