# gc (engineering)

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

$g_{\text{c}}={\frac {ma}{F}}$ In unit systems where force is a derived unit, like in SI units, gc is equal to 1. 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.

## Motivations

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

$F\propto ma$ or

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

$1~{\text{lbf}}=K\cdot 1~{\text{lb}}\cdot 32.2~{\frac {\text{ft}}{{\text{s}}^{2}}}$ $K={\frac {1~{\text{lbf}}}{1~{\text{lb}}\cdot 32.2~{\frac {\text{ft}}{{\text{s}}^{2}}}}}=0.03106~{\frac {{\text{lbf}}\cdot {\text{s}}^{2}}{{\text{lb}}\cdot {\text{ft}}}}$ gc is defined as the reciprocal of the constant K

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

$g_{\text{c}}={\frac {ma}{F}}$ ## Specific systems of units

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