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Information about 9 l pressure and moments

KS3 Physics

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© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20052 of 37 9L Pressure and Moments Contents Pressure in liquids Moments Pressure Summary activities

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20053 of 37 Pressure is exerted whenever a force is applied over an area. If the same force is applied in each picture, which arm exerts the highest pressure on the board? 1. 2. What is pressure?

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20054 of 37 The arm applies a force to the board via a fingertip. The force acts over a small area and so produces a high pressure. 1. High and low 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. 2.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20055 of 37 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 ). pressure = area force p x a f Pressure is the force per unit area and is calculated using this formula: Calculating pressure

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20056 of 37 The same force spread over a larger area means a lower pressure. Which type of pressure? Which type of shoes would be best for walking over a muddy field – flat soles or heels?

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20057 of 37 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. Which type of pressure? 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.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20058 of 37 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. Using low pressure

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20059 of 37 A force concentrated on a small area means high pressure, e.g. high heeled shoes, needles, ice skates, sharp knives. The narrow blade of a knife means that it exerts a high pressure and makes it easier to cut fruit and vegetables. The high pressure of the blade of an ice-skate melts the ice and helps the skater slide across the surface. Using high pressure

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200510 of 37 9L Pressure and Moments Contents Pressure in liquids Moments Pressure Summary activities

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200511 of 37 Pressure in a liquid: acts in all directions; increases with depth. Pressure in a liquid A liquid can be used to transmit pressure from one place to another.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200512 of 37 high pressure low pressure The relationship between pressure and depth is shown by a water bottle with holes along its length. 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. Pressure in a liquid

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200513 of 37 Hydraulic systems use the principle that pressure is transmitted throughout a liquid. Force applied here Pressure inside all parts of the hydraulic system is the same Force transferred here Hydraulics 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.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200514 of 37 All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave 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. Hydraulic brake foot pedal master piston slave pistons hydraulic fluid

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200515 of 37 The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation: pressure = force applied area of master piston Hydraulic brake – pressure equations 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. 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

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200516 of 37 The master piston of a car has an area of 5cm2 . Hydraulic brake – calculations Calculations: 1. At the master piston, p = f = 10N = 2N/cm2 a 5cm2 1. If a force of 10N is applied to the master piston, calculate the pressure created in the brake pipes. 2. At the slave piston, f = p x a = 2N/cm2 x 50cm2 = 100N So, the force exerted on the brake disc is ten times greater than the original force applied to the master piston. 2. If the slave piston has an area of 50 cm2 , calculate the force exerted on the brake disc.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200517 of 37 Hydraulics activity

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200518 of 37 9L Pressure and Moments Contents Pressure in liquids Moments Pressure Summary activities

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200519 of 37 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 Force and rotation

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200520 of 37 pivot The left-hand side of the see-saw moves downwards when a force is applied to it – this is an anticlockwise turn. The turning effect of a force is called a moment. Force and rotation – a moment

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200521 of 37 A spanner is a lever that can be used to unscrew a nut. force pivot distance from force to pivot Using moments If the moment is big enough it will unscrew the nut. If not, there are two ways of increasing the moment. The spanner exerts a moment or turning force on the nut.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200522 of 37 1. Increase the distance from the force to the pivot – apply the force at the end or use a longer spanner. Using moments – increasing the moment force If the same force is applied over a greater distance, a larger moment is produced. pivot distance from force to pivot

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200523 of 37 2. Increase the force applied – push/pull harder or get someone stronger to do it! Using moments – increasing the moment force If a greater force is applied over the same distance, a larger moment is produced. pivot distance from force to pivot

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200524 of 37 moment = force (N) x distance from pivot (cm or m) The moment of a force is given by the equation: Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm). moment f x d Moment equation

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200525 of 37 Gina weighs 500N and stands on one end of a seesaw. She is 0.5m from the pivot. What moment does she exert? moment = 500 x 0.5 = 250 Nm 0.5m 500N pivot Moment calculation

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200526 of 37 Principle of moments The girl on the right exerts a clockwise moment, which equals... The girl on the left exerts an anti-clockwise moment, which equals... her weight x her distance from pivot her weight x her distance from pivot pivot

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200527 of 37 Principle of moments When something is balanced about a pivot: total clockwise moment = total anticlockwise moment If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments. pivot

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200528 of 37 The principle of moments can be investigated using 10g masses with this balance. moment (left) = 10 x 7 = 70gcm moment (right) = (10 x3)+(10x4) = 70gcm Both moments are equal and so the see-saw is balanced. Principle of moments

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200529 of 37 Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200N and is 1.5m from the pivot. Where must her 150N friend sit if the seesaw is to balance? When the see-saw is balanced: Principle of moments – calculation total clockwise moment = total anticlockwise moment 200N x 1.5m = 150N x distance 200 x 1.5 = distance 150 distance of second girl = 2m

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200530 of 37 Tower cranes are essential at any major construction site. load arm trolley loading platform tower Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads? counterweight Why don’t cranes fall over?

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200531 of 37 Using the principle of moments, when is the crane balanced? moment of = moment of load counterweight If a 10,000N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower? 6m 3m 10,000N? Why don’t cranes fall over?

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200532 of 37 moment of counterweight distance of counterweight from tower = = 10,000 x 3 = 30,000 Nm counterweight x moment of load = = ? x 6 load x distance of load from tower moment of load = moment of counterweight ? x 6 = 30,000 ? = 3,000 6 ? = 5,000 N Why don’t cranes fall over?

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200533 of 37 At what distance can the loading platform carry each load safely? Crane operator activity

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200534 of 37 9L Pressure and Moments Contents Pressure in liquids Moments Pressure Summary activities

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200535 of 37 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). 1Pa = 1newton 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).

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200536 of 37 Anagrams

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200537 of 37 Multiple-choice quiz

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