Kinetic energy is ‘some-mass-times-some-speed-squared’^{1} so ‘some-kilograms-times-some-metres-per-second-squared’. This is a big bundle of units:^{2}

$$\mathrm{[kg\cdot (m/s)^2]}=\mathrm{[kg\cdot m^2/s^2]}$$

Let’s give this unit bundle a single name, so it’s easier to talk about the energy unit (it is not unlikely that we will be talking a lot about energy from here on). Why don’t we call it a **Joule**, in honour of the British physicist James P. Joule who was a pioneer in the field of energy,^{3} with the symbol $\mathrm J$:

$$\mathrm{[J]}=\mathrm{[kg\cdot m^2/s^2]}$$

This Joule is now our (derived) SI-unit for energy.

Energy scale | ||
---|---|---|

$\vdots$ | ||

Gigajoules | $\mathrm{GJ}$ | A lightning bolt^{10} |

Megajoules | $\mathrm{MJ}$ | Dynamite^{9} |

Kilojoules | $\mathrm{kJ}$ | A battery^{8} |

Joules | $\mathrm J$ | A falling apple^{7} |

Millijoules | $\mathrm{mJ}$ | A falling piece of paper^{6} |

Microjoules | $\mathrm{\mu J}$ | Descending drizzle rain^{5} |

Nanojoules | $\mathrm{nJ}$ | A flying mosquito^{4} |

$\vdots$ |

**Note**: Energy comes in many forms so it might be unclear to say that an object “has” or ‘contains’ or ‘carries’ energy:

- A juice bottle contains a certain amount of
*chemical energy*stored in the juice’s molecular bonds that make up the protein, sugars etc. - The juice bottle can also contain some
*thermal energy*when we heat it up. - And if you throw the bottle, it also carries
*kinetic energy*. Etc.

When we say that an object “has” some energy, we should always be clear on what type of energy we are talking about.

References:

- ‘
**coefficient**’ (encyclopedia/dictionary),*Margaret Rouse*, WhatIs.com, Tech Target, 2017, whatis.techtarget.com/definition/coefficient (accessed Jun. 11th, 2020)