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Principle of Moments
An object is in equilibrium, i.e. it is balanced and not turning or moving when:
The sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the point.
The see-saw is balanced because the clockwise moment is equal to the sum of the anticlockwise moments, i.e.
Moment = Force x Distance
Moment at A = 50N x 2m = 100 Nm anticlockwise moment
Moment at B = 50N x 1m = 50 Nm anticlockwise moment
Anticlockwise moment at A + B = 150 Nm
Moment at C = 150N x 1m = 150 Nm clockwise moment
The see-saw is balanced because the clockwise moment is equal to the sum of the anticlockwise moments.