Lami's theorem

Lami's theorem states that three coplanar, concurrent forces in equilibrium are proportional to the sines of the angles between the other two forces.

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Force: An interaction that changes or tends to change an object's motion, represented by magnitude and direction. Each force in the theorem is one of the three vectors whose balance the relation expresses.
Coplanar forces: Forces whose lines of action lie in the same plane. Lami's theorem is restricted to forces lying in one plane.
Tension (mechanics): The pulling force transmitted through a taut string, rope, cable, or similar connector. Unknown cable tensions are often two of the three forces at a supported junction.
Newton's laws of motion: Three physical laws relating force to motion, inertia, and interaction between bodies. Lami's theorem is a specialized consequence of force equilibrium, not a replacement for the laws.
Static equilibrium: A state in which an object's acceleration is zero and the forces acting on it balance. The theorem applies when the three forces produce zero resultant.
Resultant force: The single vector equivalent to the combined effect of a system of forces. Equilibrium requires the resultant of the three forces to be zero.
Weight: The gravitational force exerted on an object by a nearby celestial body. A suspended body's weight commonly supplies the third force in a Lami equilibrium problem.
Method of joints: A truss-analysis method that applies force equilibrium separately at each pin joint. It handles multiple member forces at a joint, whereas Lami's theorem is limited to three.
Vector addition: The operation of combining vectors by adding their components or placing them head to tail. The three force vectors must form a closed triangle when their resultant is zero.
Angle between vectors: The smaller angle separating two vector directions, measured from their common origin. Each denominator uses the angle between the two forces other than the force in its numerator.