gc (engineering)

In engineering, g_c is a unit conversion factor used to convert mass to force or vice versa.^[1] It is defined as

g_{\text{c}}={\frac {ma}{F}}

In unit systems where force is a derived unit, like in SI units, g_c is equal to 1 and is not needed. In unit systems where force is a primary unit, like in English units, g_c is not equal to 1, and is required to obtain correct results.^[2]

Motivations

According to Newton's law of motion, the force F is proportional to mass m and acceleration a

F\propto ma

or in general

F=Kma

If F = 1 lbf, m = 1 lb, and a = 7000981456000000000♠32.2 ft/s², then

1~{\text{lbf}}=K\left(1~{\text{lb}}\cdot 32.2~{\frac {\text{ft}}{{\text{s}}^{2}}}\right)

Leading to

K={\frac {1~{\text{lbf}}}{1~{\text{lb}}\cdot 32.2~{\frac {\text{ft}}{{\text{s}}^{2}}}}}=0.03106~{\frac {{\text{lbf}}\cdot {\text{s}}^{2}}{\text{ft}}}

g_c is defined as the reciprocal of the constant K

g_{\text{c}}={\frac {1}{K}}=32.2~{\frac {\text{ft}}{{\text{lbf}}\cdot {\text{s}}^{2}}}

or equivalently, as

g_{\text{c}}={\frac {ma}{F}}

Specific systems of units

International System	English System	British Customary System
g_c = 1 (kg·m)/(N·s²)	g_c = 32.2 (lb·ft)/(lbf·s²)	g_c = 1 (slug·ft)/(lbf·s²)

References

↑ Janna, William (2015). Introduction to Fluid Mechanics (5th ed.). CRC Press. p. 5. ISBN 9781482211610.
↑ Cengel, Yunus; Boles, Michael (2014). Thermodynamics: An Engineering Approach (8th ed.). McGraw-Hill Education. p. 6. ISBN 9780073398174.

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