1 Joule (J) = 1 Newton × 1 metre (N·m) = 1 kg·m²/s². The Joule is the SI unit of energy, work, and heat. One joule is the work done when a force of one newton displaces an object by one metre in the direction of the force.
The kinetic energy of a 70 kg person walking at 1 m/s is 35 joules. A fast-moving car has millions of joules of kinetic energy — which is why car crashes are so destructive.
From the work-energy theorem: Work = Force × Displacement W = F × d
When F = 1 Newton and d = 1 metre: 1 Joule = 1 Newton × 1 metre = 1 N·m
In base SI units:
The Joule is named after English physicist James Prescott Joule (1818–1889) who established the mechanical equivalent of heat.
| Equivalent | Value |
|---|---|
| 1 J = | 1 N·m |
| 1 J = | 1 kg·m²/s² |
| 1 J = | 1 W·s (watt-second) |
| 1 J = | 0.2389 calories |
| 1 J = | 10⁷ ergs (CGS unit) |
| 1 J = | 6.242 × 10¹⁸ electron volts (eV) |
| 1 calorie = | 4.184 J |
| 1 kilowatt-hour = | 3.6 × 10⁶ J |
| 1 electronvolt = | 1.602 × 10⁻¹⁹ J |
[Joule] = [Work] = [Force × Distance] = [MLT⁻²] × [L] = [ML²T⁻²]
This is the same dimensional formula as energy and heat — confirming they are all the same physical quantity measured in different contexts.
To make the abstract unit concrete:
1 calorie = 4.184 joules. The calorie used in food (kilocalorie or kcal) = 4184 joules. So a 100-calorie snack contains 418,400 joules of chemical energy.
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