Lami’s theorem states that if three forces acting at a point are in equilibrium, then each of the force is directly proportional to the sine of the angle between the remaining two forces. It gives the conditions of equilibrium for three forces acting at a point.

Let consider three forces P, Q and R acting at a point O (figure). Under the action of three forces, the point O is at rest, then by Lami’s theorem,

P ∞ Sin α

Q ∞ Sin β

And R ∞ Sin ϒ then,

**[P/Sin α] = [Q/Sin β] = [R/Sin ϒ] = Constant.**