The weight W = mg is the gravitational force that the Earth exerts on an object with mass (m). In general, there is an attractive force between two masses m_{1} and m_{2} separated by the distance r.

We have two masses m_{1} and m_{2}. The forces F_{12} = – F_{21} are action-reaction pairs. The magnitude of the force is proportional to both masses and inversely proportional to the square of the radius r,

**F _{12} = F_{21} = G (m_{1} m_{2 }/r^{2})**

Where, G = 6.67 x 10^{-11} N • m^{2}/kg^{2} is the universal gravitational constant.

We consider the case of mass m and the Earth M_{E} , so that m_{1} = m and m_{2} = M_{E} . Since r = R_{E} = 6.38 x 10^{6} m is the radius of the Earth, we get for the mass of the Earth: mg = GM_{E }/R_{E}^{2}: