Rotational Forces
What is a Moment?
When a force acts around a point (a pivot) it is called a Moment. Levers can be used to increase the size of a moment by increasing the distance between the force and the pivot.
A moment can be calculated as follows:
Moment = force X perpendicular distance from pivot
Principle of Moments
When an object is balanced, the clockwise and anticlockwise moments are equal.
Centre of Gravity
The centre of gravity of an object is the point through which all the weight of the object can be take to act. It is the point at which an object can be balanced.
Objects are stable when the centre of gravity remains vertically within the base, so objects with a wide base and a low centre of gravity are the most stable.
Forces on a beam
To calculate the force exerted by an object on the supports of a beam we look at the fraction of the length of the beam where the object sits and the weight of the object.
In this example we look at somebody standing at different positions on a beam.
| In the first case the weight is split equally between the two supports, he is halfway between them. | |
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In the next example, all of the weight is acting through one support; he is all the way along the beam. |
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In the last example, he is 3/4 of the way along the beam. This means that 3/4 of his weight is acting on the left hand support and 1/4 of his weight is acting on the right hand support. |
Resources
You should know:
- that forces can have a turning effect on a body
- how to calculate the moment of a force using moment = force x perpendicular distance from the pivot
- and understand the Principle of Moments
- what is meant by the Centre of Gravity
- how to determine the Centre of Gravity of an object
- that when an object is resting on a beam, the upward forces at the ends of the beam depend upon the position of the object.