ISO 31-3 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to mechanics. It is superseded by ISO 80000-4.
Its definitions include (note boldfaced symbols mean quantity is a vector):
| Quantity | Unit | Remarks | |||
|---|---|---|---|---|---|
| Name | Symbol | Name | Symbol | Definition | |
| Force | F | newton | N | 1 N = 1 kg·m/s2 | Unit named after Isaac Newton |
| Moment of force, Torque | M, \( {\displaystyle {\boldsymbol {\tau }}} \) | N·m | 1 N·m = 1 kg·m2/s2 | The unit is dimensionally equivalent to the units of energy, the joule; but the joule should not be used as an alternative for the newton metre. | |
| Linear momentum | p | kg·m/s or N·s | |||
| (Linear) impulse | J | N·s or kg·m/s | |||
| Angular momentum | L | kg·m2/s or N·m·s | |||
| Mechanical energy, Work | E, W | joule | J | 1 J = 1 kg·m2/s2 = 1 Pa·m3 = 1 W·s | Unit named after James Joule. The joule is dimensionally equivalent to the units of torque and moment of force but should be used in preference to the newton metre (N·m). |
| Power | P | watt | W | 1 W = 1 J/s = 1 N·m/s = 1 kg·m2/s3 | Unit named after James Watt. |
| Pressure | p | pascal | Pa | 1 Pa = 1 N / m2 = 1 kg/(m·s2) | Named after Blaise Pascal. |
| Normal stress, Shear stress | \( {\displaystyle \sigma ,\tau } \) | pascal | Pa | 1 Pa = 1 N / m2 = 1 kg/(m·s2) | Named after Blaise Pascal. |
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