The lever-arm rule is so called since it is based on the same principle of the seesaw moment-arm balance. (weight F1) * distance (F1M) = (weight F2) * distance (MF2)
To have the equilibrium it must be:
The lever-arm rule can be conveniently applied to mixing calculation on enthalpy-composition and on ternary diagrams for extraction.
In this last case the lever-arm rule is used to easily solve overall mass balances for two mixing streams.
By doing the overall mass balance, it can be seen that F1, F2 and M lye on the same straight line and the exact position of the mixing point M can be determined from the mass ratio of the two mixing streams.
The mass of the these two streams F1, F2 is then proportional to the distance of M respectively from the point representing the other stream (MF2, MF1).