In Progress
Skill 4 of 13
In Progress

# The Joule unit

Kinetic energy is ‘some-mass-times-some-speed-squared1 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 bolt10
Megajoules$\mathrm{MJ}$Dynamite9
Kilojoules$\mathrm{kJ}$A battery8
Joules$\mathrm J$A falling apple7
Millijoules$\mathrm{mJ}$A falling piece of paper6
Microjoules$\mathrm{\mu J}$Descending drizzle rain5
Nanojoules$\mathrm{nJ}$A flying mosquito4
$\vdots$

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:

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