AQA GCSE Physics Unit 3 (Higher Tier)

Cards covering everying in the Unit 3 chapter of the AQA guide

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Turning Forces - Moments

The turning effect of a force is called a 'moment'.

moment = force x perpendicular distance from the pivot (Nm = N x m)

to increase the moment:

1. increase the distance to the pivot

2. increase the force

key words: moment, turning force, pivot

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Centre of Mass

The centre of mass of a object is where the mass of the object is concentrated

When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension.

The centre of mass of a symmetrical object is along the axis of symmetry.

The position of the centre of mass depends on the shape of the object, and sometimes lies outside the object.

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Centre of Mass (Plumbline Investigation)

How to find the centre of mass of a suspended object:

1. Suspend a thin sheet from a pin (horizontal metal rod) held in a clamp/retort stand. Because it is freely suspended, it can turn.

2. When it comes to rest, hang a plumbline from the same pin.

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

4. Hang the sheet with the pin at another point and repeat the procedure.

5. The centre of mass is where the lines that marked the position of the plumbline cross.

For a diagram, go to page 75 of the AQA GCSE Physics revision guide.

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Moments in Balance

If an object is in equilibrium then it is balanced, and not turning. We can take the moments at any point and will find:

The total clockwise moment = the total anticlockwise moment

There are lots of everyday examples of the Principle of Moments, such as seesaws and balance scales.

Worked Example:

Sam sits 2m away from the centre of a seesaw. Alex weighs twice as much as Sam. How far must he sit from the centre to balance the seesaw?

Ok, so Alex's force = 2x Sam's force. So he needs to sit 2x as far from the centre

As Sam is sat 2m away, we just times that answer by two to get the distance that Alex must sit away from the 'pivot'. 

2 x 2 = 4m.

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The line of action works through the centre of mass.

If it lies outside the base of an object, then there will be a moment and the object will topple over.

The wider the base of an object, and the lower the Centre of Mass, the further it has to tilt before it topples. 

The stability of an object is increased by making its base wider and centre of mass lower. 

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Circular Motion

When an object moves in a circle it is continuously changing direction, so it is continuously changing velocity.

It is accelerating continuously.

This acceleration is called the "Centripetal Acceleration". 

An object only accelerates if a resultant force acts upon it. This is called the "Centripetal Force" and always acts towards the centre of the circle.

If the centripetal force stops acting, the object will move off in a tangent to the the circle

FUN FACT: this is where the saying "go off on a tangent" comes from!! (sorry that was so uninteresting!)

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