The joule (J) is the SI unit of energy and work, and is defined as 1 kg m2 s-2 = 1 N m = 1 W s. It is named in honour of the physicist James Prescott Joule.
The work required to lift 102 g (e.g. a small apple) for one metre is 1 joule.
See also conversion of units, kWh.