• A body under coplanar concurrent forces is in equilibrium if both of the following conditions hold together:

• ΣX = 0, that is, the sum of the horizontal components is zero.
• ΣY = 0, that is, the sum of the vertical components is zero.

The closed triangle of forces

• If a body is in equilibrium under three forces only, then when these forces are drawn as arrows placed head to tail, they form a closed triangle.
• So the three forces satisfy the equilibrium condition, and the triangle can be used to find the unknowns.