29L Pressure and Moments Pressure Pressure in liquids Moments Contents9L Pressure and MomentsPressurePressure in liquidsMomentsSummary activities
3What 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?
4High and low pressure 1. 2. The arm applies a force to the board via a fingertip.The force acts over asmall area and soproduces a high pressure.The same force is now acting over a larger area – the palm has a greater surface areathan the fingertip.A lower pressure is produced.
5f p x a force pressure = area Calculating pressure Pressure is the force per unit areaand is calculated using this formula:p x afpressure =areaforcePressure 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).
6Which 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?
7Which 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 likelyto sink into soft ground.
8Using low pressureA 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.
9Using high pressureA 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.
109L Pressure and Moments Pressure Pressure in liquids Moments Contents9L Pressure and MomentsPressurePressure in liquidsMomentsSummary activities
11Pressure 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.
12low pressure high pressure Pressure in a liquidThe relationship between pressure and depth is shown by a water bottle with holes along its length.low pressurehigh pressurePressure (N/m2) = 10 N/kg x depth (m) x density (kg/m3)The pullof gravityThe greater the depth, the higherthe pressureThe denser the liquid, the heavier it is.
13Pressure 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.ForceappliedhereForcetransferredherePressure inside all parts of thehydraulic system is the same
14hydraulic fluid slave pistons foot pedal master piston Hydraulic brakeAll hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston.hydraulic fluidslave pistonsfoot pedalmaster pistonThe 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.
15Hydraulic brake – pressure equations The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation:pressure =force appliedarea of master pistonThe pressure is transmitted to the slave pistons and so theforce exerted by the slave piston can be calculated using:pressure =force exertedarea of slave pistonforce exerted = pressure x area of slave pistonThe 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.
16Hydraulic 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/cm2a cm22. At the slave piston, f = p x a = 2 N/cm2 x 50cm2 = 100 NSo, the force exerted on the brake disc is ten times greater than the original force applied to the master piston.
189L Pressure and Moments Pressure Pressure in liquids Moments Contents9L Pressure and MomentsPressurePressure in liquidsMomentsSummary activities
19Force 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?pivot5N
20Force 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.pivotThe turning effect of a force is called a moment.
21distance from force to pivot Using momentsA spanner is a lever that can be used to unscrew a nut.The spanner exerts a moment or turning force on the nut.pivotdistance from force to pivotforceIf the moment is big enough it will unscrew the nut.If not, there are two ways of increasing the moment.
22Using 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.pivotdistance from force to pivotforceIf the same force is applied over a greater distance, a larger moment is produced.
23Using moments – increasing the moment 2. Increase the force applied– push/pull harder or get someone stronger to do it!pivotdistance from force to pivotforceIf a greater force is applied over the same distance, a larger moment is produced.
24f x d Moment equation The moment of a force is given by the equation: moment = force (N) x distance from pivot (cm or m)momentfxdMoments are measured in Newton centimetres (Ncm) or Newton metres (Nm).
25Moment 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 Nm0.5 m500 Npivot
26Principle 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 distancefrom pivother weight x her distancefrom pivot
27Principle 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
28Both moments are equal and so the see-saw is balanced. Principle of momentsThe principle of moments can be investigated using 10g masses with this balance.moment (left) = 10 x 7= 70 gcmmoment (right) = (10 x 3) + (10 x 4)= 70 gcmBoth moments are equal and so the see-saw is balanced.
29Principle 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 moment200 N x 1.5 m = 150 N x distance200 x 1.5 = distance150distance of second girl = 2 m
30Why don’t cranes fall over? Using the principle of moments, when is the crane balanced?3 m6 m10,000 N?moment of = moment ofload counterweightIf 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?
31Why don’t cranes fall over? moment ofload== ? x 6load x distance of load from towermoment ofcounterweightdistance of counterweight from tower== 10,000 x 3= 30,000 Nmcounterweight xmoment of load = moment of counterweight? x 6 = 30,000? = 3,0006? = 5,000 N
32Crane operator activity At what distance can the loading platform carry each load safely?
339L Pressure and Moments Pressure Pressure in liquids Moments Contents9L Pressure and MomentsPressurePressure in liquidsMomentsSummary activities
34counterbalance – A weight used to balance another weight. Glossarycounterbalance – A weight used to balance another weight.effort – The force applied to use a lever.hydraulics – The use of liquid to transmit pressure fromone 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).
35Why don’t cranes fall over? Tower cranes are essential at any major construction site.load armtrolleycounterweightloading platformtowerConcrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?