# Physics P3

Notes for P3 section of Physics GCSE (2011)

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## Moments

Forces can be used to turn objects about a pivot. The turning effect of a force is called the moment

You can increase a moment in 2 ways:

1) Increase the force applied

2) Increase the perpindicular distance

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## Centre of Mass

The centre of mass of an object is the point through which the whole weight of the object acts.

To find the centre of mass: a suspended object will always come to rest with its centre of mass directly below the point of suspension.

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## Law of Moments

The law of moments is, that when an object is balanced, or isn't turning...

Total clockwise moments = Total anti-clockwise moments

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## Stability

An object will topple if the line of action of its weight lies outside the base.

The weight of the object causes a resultant moment which makes the object topple.

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## Centripetal Force

An object moving in a circular path is continuously accelerating towards the centre of the circle.

The resultant force causing this acceleration is the centripetal force.

(The acceleration doesn't change the SPEED of the object, but the DIRECTION of its motion)

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## Gravity and Orbiting Speed

As the distance between two objects INCREASES, the forces of gravity between them proportionally DECREASES.

For example, if the distance is:

- Doubled, the force becomes 1/4 of the original force

- Trebled, the force becomes 1/9 of the original force

To stay in orbit at a particular distance, objects must orbit at a particular speed. This balances the gravitational force.

So the further away an orbiting object is, the longer it takes to make a complete orbit.

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