An object in equilibrium has no net force acting on it and no net moment (rotational equilibrium).
The moment of a force is its turning effect. It is defined as the force × perpendicular distance of the line of action of the force from the pivot.
moment = F x cos θ
The perpendicular distance of the line of action of the force from the pivot P is xcosθ
Alternatively the force F could be resolved parallel and perpendicular to the line through the pivot P.
The principle of moments states that for a body in (rotational) equilibrium, the sum of clockwise moments about any point is equal to the sum of anticlockwise moments about the same point.
For example, a uniform wooden beam 120cm long with a weight of 15N, carrying a 13N block 80cm from the support, rests on a support at one end and is fixed to a vertical string at the other end.
To find R:
Taking moments about A where the string is tied will mean the moment due to the tension T is zero, so the size of T does not have to be known.
sum of clockwise moments = sum of anticlockwise moments
R X 1.2 = (15 x 0.6) + (13 x 0.4)
so R = 12 N to 2 s.f.
The moment of a force is its turning effect. It is defined as the force × perpendicular distance of the line of action of the force from the pivot.
moment = F x cos θ
The perpendicular distance of the line of action of the force from the pivot P is xcosθ
Alternatively the force F could be resolved parallel and perpendicular to the line through the pivot P.
The principle of moments states that for a body in (rotational) equilibrium, the sum of clockwise moments about any point is equal to the sum of anticlockwise moments about the same point.
For example, a uniform wooden beam 120cm long with a weight of 15N, carrying a 13N block 80cm from the support, rests on a support at one end and is fixed to a vertical string at the other end.
To find R:
Taking moments about A where the string is tied will mean the moment due to the tension T is zero, so the size of T does not have to be known.
sum of clockwise moments = sum of anticlockwise moments
R X 1.2 = (15 x 0.6) + (13 x 0.4)
so R = 12 N to 2 s.f.