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Units (Biomechanics)

Common units

Shown below are commonly used physical quantities in biomechanics.

Quantity

Symbol

Formula

Metric Unit

English Unit

Mass

$m$

$\mathrm{kg}$

$\mathrm{slug}$

Force

$F$

$F = ma$

$\mathrm{N}$

$\mathrm{lb}$

Pressure

$P$

$P = \frac{F}{A}$

$\mathrm{}$

$\mathrm{psi}$

Volume (solids)

$V$

$\mathrm{m^3}$

$\mathrm{ft^3}$

Volume (liquids)

$V$

$\mathrm{liter}$

$\mathrm{gallon}$

Density

$\rho$

$\rho = \frac{m}{V}$

$\mathrm{kg/m^3}$

$\mathrm{slugs/ft^3}$

Specific weight

$\gamma$

$\gamma = \frac{W}{V}$

$\mathrm{N/m^3}$

$\mathrm{lb/ft^3}$

Torque

$T$

$T = Fd$

$\mathrm{N-m}$

$\mathrm{ft-lb}$

Impulse

$J$

$J = Ft$

$\mathrm{N \ s}$

$\mathrm{lb \ s}$

Common units of measurement

Shown below are factors for converting between metric units commonly used in biomechanics and their English system equivalents.

Variable

Metric unit

$\leftarrow$ Multiply by Divide by $\rightarrow$

English Unit

Distance

Centimetres

$2.54$

Inches

Metres

$0.3048$

Feet

Kilometres

$1.609$

Miles

Speed

Metres/second

$0.447$

Miles/hour

Mass

Kilograms

$14.59$

Slugs

Force

Newtons

$4.448$

Pounds

Work

Joules

$1.355$

Foot-pounds

Power

Watts

$745.63$

Horsepower

Energy

Joules

$1.355$

Foot-pounds

Linear momentum

Kilogram-metres/second

$4.448$

Slug-feet/second

Impulse

Newton-seconds

$4.448$

Pounds-seconds

Angular momentum

Kilogram-metres$^2$/second

$1.355$

Slug-feet$^2$/second

Moment of inertia

Kilogram-metres$^2$

$1.355$

Slug-feet$^2$

Torque/moments

Newton-metres

$1.355$

Foot-pounds

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