# gc (engineering)

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

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

In unit systems where force is a derived unit, like in SI units, gc is equal to 1 and is not needed. In unit systems where force is a primary unit, like in English units, gc 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

${\displaystyle F\propto ma}$

or in general

${\displaystyle F=Kma}$

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

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

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

gc is defined as the reciprocal of the constant K

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

or equivalently, as

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

## Specific systems of units

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

## References

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