# ISO 31-3

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 It is superseded by ISO 80000-4.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, $\boldsymbol{\tau}$ 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 alternate 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 $\sigma, \tau$ pascal Pa 1 Pa = 1 N / m2 = 1 kg/(m·s2) Named after Blaise Pascal.
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