# Physics - Forces and Pressure

This set is for IGCSE Physics and deals specifically with Forces and Pressure using the Stephen Pople. This topic has five subtopics which are:

Moments          Centre of Mass          Stretching and Compressing          Pressure and Pressure in Liquids          Gas Pressure and Volume

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## Moment

The turning effect of a force is called a moment. Moments are calculated by this equation:

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

If there is an anit-clockwise moment and clockwise moment, you minus the bigger moment from the smaller one and from there you will have your overall anticlockwise / clockwise moment. If the moments are the same then the state of balance is in equilibrium.

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## Centre of Mass

An object's (ex. card) centre of mass / gravity can be found by pinning the peice of card to a surface allowing it to swing freely.

Then draw a line vertically downwards which can be done by dangling a weight on a string and the string will be vertical.

Repeat this with another point of the card and once you have two lines, where they cross is where the centre of mass / gravity is.

If the object is something that cannot be pinned then attatch one corner to a peice of string and draw a verticle line from the string. Then repeat this but attatch it from another corner and where the two lines cross is where the centre of mass / gravity is.

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## Stretching and Compressing

If you have weights hanging from a spring, their weight is known as a load. The more load the bigger the extension, the extension is the difference from its original and new length.

If the extension was plotted on the graph the line would be directly proportional to a certain point. Until this point the rule of the extension follows the limit of proportionality.

This point is the elastic limit of the spring.

Hookes law was that a spring would return to its original shape if the load did not exceed the spring's elastic limit.

if the load does not exceed the elastic limit then the extension can be worked out using:

extension (mm) = load (N) / spring constant

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## Pressure and Pressure in Liquids

The amount of pressure depends on the force and are of an object. Pressure can be increased by reducing the area and increasing the force. Pressure is found using:

Pressure (Pa) = force (N) / area (m2) or p = F / A

Pressure can also be found in liquids. This pressure can be increased by increasing the height of the water, the density, height and gravity. Pressure of a liquid is found using:

Pressure (Pa) = density (kg/m³) x height (m) x gravity (N/kg) or p = pgh

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## Gas Pressure and Volume

Pressure and volume are inversely proportional. If you have a bottle with a known volume and pressure and the pressure (or volume) changes which is also known, then the new, unknown volume (or pressure) can be calculated using this equation:

primary pressure (Pa) x primary volume (m³) = secondary pressure (Pa) x secondary volume (m³) or p1 x V1 = p2 x V2

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