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KS3 Physics. 9L Pressure and Moments. Contents. 9L Pressure and Moments. Pressure. Pressure in liquids. Moments. Summary activities. What is pressure?. 1. 2. Pressure is exerted whenever a force is applied over an area.
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KS3 Physics 9L Pressure and Moments
Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities
What is pressure? 1. 2. 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?
High and low pressure 1. 2. The arm applies a force to the board via a fingertip. The force acts over a small areaand 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.
Calculating pressure f force pressure= area p x a Pressure is the force per unit area and is calculated using this formula: Pressure is measured in:Newtons per square metre (N/m2), which are also calledpascals (Pa). Pressure can also be measured in:Newtons per square millimetre (N/mm2);Newtons per square centimetre (N/cm2).
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?
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.
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.
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.
Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities
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.
Pressure in a liquid The pull of gravity The greater the depth, the higher the pressure The denser the liquid, the heavier it is. 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)
Hydraulics Force applied here Force transferred here 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.
Hydraulic brake hydraulic fluid slave pistons master piston All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston. foot pedal 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 – pressure equations force applied pressure = area of master piston force exerted pressure = area of slave piston force exerted = pressure x area of slave piston The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation: The pressure is transmitted to the slave pistons and so the force exerted by the slave piston can be calculated using: The slave pistonhas a larger areathan themaster 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.
Hydraulic brake – calculations Calculations: 1.At the master piston, p = f = 10N = 2N/cm2 a 5cm2 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. 2.At the slave piston, f = p x a = 2N/cm2 x 50cm2 = 100N So, the force exerted on the brake disc isten times greaterthan the original force applied to the master piston.
Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities
Force and rotation 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
Force and rotation – a moment 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 amoment.
Using moments pivot distance from force to pivot A spanner is a lever that can be used to unscrew a nut. The spanner exerts a moment or turning force on the nut. force If the moment is big enough it will unscrew the nut. If not, there are two ways of increasing the moment.
Using moments – increasing the moment pivot distance from force to pivot 1.Increase the distance from the force to the pivot – apply the force at the end or use a longer spanner. force If the same force is applied over a greater distance, a larger moment is produced.
Using moments – increasing the moment pivot distance from force to pivot 2. Increase the force applied – push/pull harder or get someone stronger to do it! force If a greater force is applied over the same distance, a larger moment is produced.
Moment equation moment = force (N) x distance from pivot (cm or m) moment f x d The moment of a force is given by the equation: Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm).
Moment calculation 0.5m 500N pivot 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
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
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
Principle of moments The principle of moments can be investigated using 10g masses with this balance. moment (right) = (10x3)+(10x4) = 70gcm moment (left) = 10 x 7 = 70gcm Both moments are equal and so the see-saw is balanced.
Principle of moments – calculation 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: total clockwise moment = total anticlockwise moment 200N x 1.5m = 150N x distance 200 x 1.5 = distance 150 distance of second girl = 2m
Why don’t cranes fall over? trolley load arm counterweight loading platform tower Tower cranes are essential at any major construction site. Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?
Why don’t cranes fall over? 3m 6m 10,000N 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?
Why don’t cranes fall over? counterweight x moment of counterweight distance of counterweight from tower = = 10,000 x 3 = 30,000 Nm moment of load load x distance of load from tower = = ? x 6 moment of load = moment of counterweight ? x 6 = 30,000 ? = 3,000 6 ?= 5,000 N
Crane operator activity Atwhatdistancecantheloadingplatformcarryeachloadsafely?
Contents 9L Pressure and Moments Pressure Pressure in liquids Moments Summary activities
Glossary • counterbalance –Aweightusedtobalanceanotherweight. • 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 theforcedividedbyareaandcanbemeasuredinpascals(Pa).
