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.Summary:At equilibrium, there is no resultant force and no resultant moment
W1d1 = W2d2W1 + W2 = R
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.Summary:At equilibrium, there is no resultant force and no resultant moment
W1d1 = W2d2W1 + W2 = R| Sum of Clockwise moments | = | Sum of Anticlockwise moments |