Balanced moments

The pivot doesn’t have to be in the centre of an object for a moment to be in balance. For example, imagine a scale. There are two weights: weight 1 (W1) and weight 2 (W2). W1 hangs on a hook at a distance (D1) from the pivot. W1 is closer to the pivot than W2, which is hanging at a distance (D2). The weights are different, however, the moment is in balance.

This means that:

W1 x D1 = W2 x D2

This is an example of the Principle of Moments which states that for an object in equilibrium:

the sum of all clockwise moments about a point = the sum of all anti-clockwise moments

Using the equation W1 x D1 = W2 x D2 you can carry out calculations which involve moments.

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Centre of mass

The centre of mass or the centre of gravity is an imaginary point within an object where we say the weight is acting. It could be where the mass of an object is concentrated. If an object is symmetrical then the centre of mass is located along the axis of symmetry. If an object has more than one line of symmetry then it’s where these lines meet up.

A hanging object, like a hanging basket of flowers, has a centre of mass which lies directly under the point of suspension. This means that the object is in equilibrium: as the centre of mass is directly under the point of suspension no turning weight is being exerted by its weight.

If you were to pull or push a hanging object, moving it from its original position so that it’s in non-equilibrium, it would swing back to its equilibrium. This is because of the turning effect caused by the object’s weight: the turning effect causes the object to return to its original position.