Principle of Moments

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In physics the concept of moments has nothing to do with time but is rather a measure of the energy in a system. Moments, or the torque of a force, about a turning point are measured in Newton meters (Nm). Moments capture the turning effect that a force (measured in Newtons, N)  has. Whether a force can turn an object depends on the size of the force and the perpendicular distance the force is applied from the turning point.

Moments are important to calculate when estimating the effect of a force on a system. For example, a force applied perpendicularly to the hinges of a door will cause the door to open or close. If the force is not perpendicular to the turning point (i.e. it is applied aligned with the hinge direction) then the door will not move. Neither will it move if the size of the force is zero. Otherwise the door will move.

The moment of a force can be calculated by multiplying the force (F) by the perpendicular distance to the force from the turning point (d). For example, a 10N force operating at a perpendicular distance of 0.50m from the turning point would have a moment of 10 multiplied by 0.5 giving a moment of 5.0Nm.

The principle of moments is about the equilibrium of a system. When a system is in balance the forces in the system are equal and the energy is constant. The system is stable because of the balanced forces, or moments in the system. The principle states that "When an object is in equilibrium the sum of the anticlockwise moments about a turning point must be equal to the sum of the clockwise moments.” (see

The principle of moments explains why a see-saw can be made to balance when two forces (weights) are applied in just the right spots. Normally a see-saw swings up and down as the weights on one end push the see-saw down and the weight the other end reverses the swing. There is no equilibrium and the see-saw moves because of the energy in the system being produced by the weights as forces bearing down around the pivot

The principle of moments can help tell us where to position weights on the see-saw so that it can be made to rest in a stable balanced state. If the weights are equal then they will need to be placed equal distances from the pivot. If the weights are objects of different amounts then the principle can be used to tell us where the weights need to be placed (either nearer or further from the pivot point) to balance the system and achieve equilibrium of its moments.

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