In statics, Lami's theorem is an equation relating the magnitudes of three coplanar, concurrent and non-collinear forces, which keeps an object
in static equilibrium, with the angles directly opposite to the corresponding
forces. According to the
theorem,
A/sin(a)=B/sin(b)=C/sin(c)
where A, B and C are the magnitudes of three coplanar, concurrent and non-collinear forces, which keep
the object in static equilibrium, and a, b and c are the angles directly
opposite to the forces A, B and C respectively.
Lami's theorem is applied in static analysis of mechanical and structural systems. The theorem is named
after Bernard Lamy.
Proof of Lami's Theorem:
Suppose there are three coplanar, concurrent and non-collinear forces, which keeps the object in
static equilibrium. By the triangle law, we can re-construct the diagram as
follow:
By the
law of sines,
A/sin(PI-a)=B/sin(Pi-b)=C/sin(Pi-c)=>A/sin(a)=B/sin(b)=C/sin(c)
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