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KS3 Physics 9L Pressure and Moments

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**9L Pressure and Moments Pressure Pressure in liquids Moments**

Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities

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What is pressure? Pressure is exerted whenever a force is applied over an area. 1. 2. If the same force is applied in each picture, which arm exerts the highest pressure on the board?

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**High and low pressure 1. 2. The arm applies a force**

to the board via a fingertip. The force acts over a small area and so produces a high pressure. The same force is now acting over a larger area – the palm has a greater surface area than the fingertip. A lower pressure is produced.

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**f p x a force pressure = area Calculating pressure**

Pressure is the force per unit area and is calculated using this formula: p x a f pressure = area force Pressure is measured in: Newtons per square metre (N/m2), which are also called pascals (Pa). Pressure can also be measured in: Newtons per square millimetre (N/mm2); Newtons per square centimetre (N/cm2).

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Which type of pressure? The same force spread over a larger area means a lower pressure. Which type of shoes would be best for walking over a muddy field – flat soles or heels?

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Which type of pressure? The boots have flat soles and spread the person’s weight over a large surface area. These boots exert a low pressure on the ground. In contrast, the heeled shoes have a smaller surface area and so exert a higher pressure. These shoes are likely to sink into soft ground.

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Using low pressure A force spread over a large area means low pressure, e.g. skis and snowboards. The large surface area of the board means the skier exerts very little pressure on the snow. This means he slides over the top of the snow and does not sink into it.

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Using high pressure A force concentrated on a small area means high pressure, e.g. high heeled shoes, needles, ice skates, sharp knives. The high pressure of the blade of an ice-skate melts the ice and helps the skater slide across the surface. The narrow blade of a knife means that it exerts a high pressure and makes it easier to cut fruit and vegetables.

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**9L Pressure and Moments Pressure Pressure in liquids Moments**

Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities

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**Pressure in a liquid Pressure in a liquid: acts in all directions;**

increases with depth. A liquid can be used to transmit pressure from one place to another.

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**low pressure high pressure**

Pressure in a liquid The relationship between pressure and depth is shown by a water bottle with holes along its length. low pressure high pressure Pressure (N/m2) = 10 N/kg x depth (m) x density (kg/m3) The pull of gravity The greater the depth, the higher the pressure The denser the liquid, the heavier it is.

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**Pressure inside all parts of the hydraulic system is the same**

Hydraulic systems use the principle that pressure is transmitted throughout a liquid. They are used to transfer movement from one part of a machine to another without linking the parts mechanically. All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid. Force applied here Force transferred here Pressure inside all parts of the hydraulic system is the same

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**hydraulic fluid slave pistons foot pedal master piston**

Hydraulic brake All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston. hydraulic fluid slave pistons foot pedal master piston The master piston is used to apply a force. This puts the liquid under pressure. The pressure is transmitted to the pistons on all four wheels of the car.

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**Hydraulic brake – pressure equations**

The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation: pressure = force applied area of master piston The pressure is transmitted to the slave pistons and so the force exerted by the slave piston can be calculated using: pressure = force exerted area of slave piston force exerted = pressure x area of slave piston The slave piston has a larger area than the master piston. So, the force exerted by the slave pistons on the brakes is greater than the force exerted by the driver on the brake pedal.

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**Hydraulic brake – calculations**

The master piston of a car has an area of 5cm2. 1. If a force of 10N is applied to the master piston, calculate the pressure created in the brake pipes. 2. If the slave piston has an area of 50 cm2, calculate the force exerted on the brake disc. Calculations: 1. At the master piston, p = f = 10 N = 2 N/cm2 a cm2 2. At the slave piston, f = p x a = 2 N/cm2 x 50cm2 = 100 N So, the force exerted on the brake disc is ten times greater than the original force applied to the master piston.

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Hydraulics activity

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**9L Pressure and Moments Pressure Pressure in liquids Moments**

Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities

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**Force and rotation pivot 5N**

A force acting on an object can cause it to turn about a pivot. What happens to the see-saw when a force is applied on the left-hand side? Does the seesaw turn? If so, clockwise or anti-clockwise? pivot 5N

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**Force and rotation – a moment**

The left-hand side of the see-saw moves downwards when a force is applied to it – this is an anticlockwise turn. pivot The turning effect of a force is called a moment.

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**distance from force to pivot**

Using moments A spanner is a lever that can be used to unscrew a nut. The spanner exerts a moment or turning force on the nut. pivot distance from force to pivot force If the moment is big enough it will unscrew the nut. If not, there are two ways of increasing the moment.

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**Using moments – increasing the moment**

1. Increase the distance from the force to the pivot – apply the force at the end or use a longer spanner. pivot distance from force to pivot force If the same force is applied over a greater distance, a larger moment is produced.

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**Using moments – increasing the moment**

2. Increase the force applied – push/pull harder or get someone stronger to do it! pivot distance from force to pivot force If a greater force is applied over the same distance, a larger moment is produced.

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**f x d Moment equation The moment of a force is given by the equation:**

moment = force (N) x distance from pivot (cm or m) moment f x d Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm).

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**Moment calculation pivot**

Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot. What moment does she exert? moment = 500 x 0.5 = 250 Nm 0.5 m 500 N pivot

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**Principle of moments pivot The girl on the left exerts**

an anti-clockwise moment, which equals... The girl on the right exerts a clockwise moment, which equals... her weight x her distance from pivot her weight x her distance from pivot

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**Principle of moments pivot**

If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments. When something is balanced about a pivot: total clockwise moment = total anticlockwise moment

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**Both moments are equal and so the see-saw is balanced.**

Principle of moments The principle of moments can be investigated using 10g masses with this balance. moment (left) = 10 x 7 = 70 gcm moment (right) = (10 x 3) + (10 x 4) = 70 gcm Both moments are equal and so the see-saw is balanced.

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**Principle of moments – calculation**

Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. Where must her 150 N friend sit if the seesaw is to balance? When the see-saw is balanced: total clockwise moment = total anticlockwise moment 200 N x 1.5 m = 150 N x distance 200 x 1.5 = distance 150 distance of second girl = 2 m

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**Why don’t cranes fall over?**

Using the principle of moments, when is the crane balanced? 3 m 6 m 10,000 N ? moment of = moment of load counterweight If a 10,000 N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower?

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**Why don’t cranes fall over?**

moment of load = = ? x 6 load x distance of load from tower moment of counterweight distance of counterweight from tower = = 10,000 x 3 = 30,000 Nm counterweight x moment of load = moment of counterweight ? x 6 = 30,000 ? = 3,000 6 ? = 5,000 N

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**Crane operator activity**

At what distance can the loading platform carry each load safely?

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**9L Pressure and Moments Pressure Pressure in liquids Moments**

Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities

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**counterbalance – A weight used to balance another weight. **

Glossary counterbalance – A weight used to balance another weight. effort – The force applied to use a lever. hydraulics – The use of liquid to transmit pressure from one place to another. lever – A simple machine that moves about a pivot and makes work easier by increasing the size of a force. load – The force moved when using a lever. moment – The turning effect of a force. It equals the force multiplied by the distance from the pivot. pascal – A unit of pressure (Pa). 1 Pa = 1 newton per square metre (N/m2). pivot – The point around which a lever turns. pressure – The force pushing on a certain area. It equals the force divided by area and can be measured in pascals (Pa).

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**Why don’t cranes fall over?**

Tower cranes are essential at any major construction site. load arm trolley counterweight loading platform tower Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?

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Multiple-choice quiz

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