P3 - Moments and stability

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What is a moment?

A moment is the turning effect.

Moment= Force x Perpendicular distance (from line of action of force to the pivot)

What is the moment measured in?

Nm (newton metres)

How can we increase the moment?

If we increase the force or perpendicular distance the moment will also increase.

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Recap again what was on the last card - you must learn this by heart !

Forces that act on objects that have a fixed point - such as the hinge of a door, or the axle of a wheel - may make that object rotate. We call the turning effect of a force the moment.

The fixed point is often called the pivot or fulcrum.

How strong the turning effect is depends on:

·  the size of the force

·  its distance from the pivot

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Balancing moments

clockwise moment being balanced by what force? (http://www.gcse.com/fm/images/moment5.png)

If we have two sets of moments, we can find a missing force if the system is balanced.

If the system is balanced what does the clockwise moment equal?

clockwise moment=anticlockwise moment

(it is just like momentum where if you can work out the p on one side it will be the same on the other side)

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Finding the centre of mass

Outline the experiment to find an irregular shape's centre of mass:

1. Suspend the shape from a pin held in a clamp. Freely suspended so that it can turn.

2. When the object comes to rest, providing there is no external forced interrupting, hang a plumbline from the same pin.

3. Mark the position of the plumbline against the shape.

4. Hang the irregular shape from another point and repeat the process.

5. The point where the lines intersect are the shape's centre of mass.


When an object is suspended it will come to rest with its centre of mass directly below the point of suspension.

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Why would an object topple over?

It's line of action of weight is outside its base.

What factors affect stability?

A wider base means an object is more stable because there is more area for the line of action of weight (force) to act in. This means the object has a lower centre of mass.

If an object has a narrow base it has a high centre of mass.

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