mining geology textbook describes its usage in the particular case of tungsten: In the case of uranium, MTU is sometimes used in the sense of metric ton of uranium (1,000 kg). Use of mass as proxy for energy The tonne of trinitrotoluene (TNT) is used as a proxy for energy, usually of explosions (TNT is a common high explosive). Prefixes are used: kiloton(ne), megaton(ne), gigaton(ne), especially for expressing nuclear weapon yield, based on a specific combustion energy of TNT of about 4.2 MJ/kg (or one thermochemical calorie per milligram). Hence, 1 t TNT = approx. 4.2 GJ, 1 kt TNT = approx. 4.2 TJ, 1 Mt TNT = approx. 4.2 PJ. The SI unit of energy is the joule. One tonne of TNT is approximately equivalent to 4.2 gigajoules. In the petroleum industry the tonne of oil equivalent (toe) is a unit of energy: the amount of energy released by burning one tonne of crude oil, approx, 42 GJ. There are several slightly different definitions. This is ten times as much as a tonne of TNT because atmospheric oxygen is used. Unit of force Like the gram and the kilogram, the tonne gave rise to a (now obsolete) force unit of the same name, the tonne-force, equivalent to about 9.8 kilonewtons. The unit is also often called simply "tonne" or "metric ton" without identifying it as a unit of force. In contrast to the tonne as a mass unit, the tonne-force is not accepted for use with SI. See also Metre–tonne–second system of units Orders of magnitude (mass) Ton Tonnage Ton (volume) Notes and references External links NIST Special Publication 811, Guide for the Use of the International System of Units (SI) Non-SI metric units Units of mass