Home > Courses > 1P21_Sternin > Dynamics Gravity Is an example of a non--contact force Newton's law of universal gravitation $F = G\> {{m \, m'}\over{r^2}} \, , \qquad G=6.67\times10^{-11} {\mbox{Nm^2}\over\mbox{kg^2}}$ On Earth's surface, $m'=M_E$ and $r=R_E$ $F = m\, \left({{GM_E}\over{R_E^2}}\right) = mg = W = \mbox{weight}$ Weight $\vec{W}$: a force, a vector, measured in N (or lb) vs. mass $m$: a scalar, measured in kg (or slug)