2.  Coplanar parallel force: In this system, all forces are parallel to each other and lie in a single plane. Consider a See-Saw. Two children are sitting in the See-Saw and it is in equilibrium. If the weights of both the boys are W₁ and W₂ and they are sitting at a distance of d₁ and d₂ from the fulcrum, then for equilibrium, the fulcrum reaction should be

R = W₁+W₂ .................(2.2)

Taking moment about fulcrum,

W₁d₁ = W₂ d₂...........(2.3)