The triangle, parallelogram, and polygon of forces

If three forces which act at a point are in equilibrium, they can be represented by the sides of a triangle taken in order (Fig. 1.10). This is called the principle of the triangle of forces, and the so-called parallelogram of forces is really the same thing, two sides and the diagonal of the parallelogram corresponding to the triangle.

If there are more than three forces, the principle of the polygon of forces is used – when any number of forces acting at a point are in equilibrium, the polygon formed by the vectors representing the forces and taken in order will form a closed figure, or, conversely, if the polygon is a closed figure the forces are in equilibrium.