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2 Teacher’s NotesA slide contains teacher’s notes wherever this icon is displayed -To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000).Notes Page ViewNormal ViewFlash FilesA flash file has been embedded into the PowerPoint slide wherever this icon is displayed –These files are not editable.
4 Force and rotation pivot 5N A force acting on an object can cause it to turn about a pivot.What would happen to the see-saw above ?Would it turn? If so, clockwise or anti-clockwise?
5 A turning force is called a moment. Force and rotationpivotThe left goes down - an anticlockwise turn.A turning force is called a moment.
6 Moments Pivot Distance from force to pivot Force Suppose you were trying to unscrew a nut using a spanner. The spanner exerts a moment or turning force on the nut.If the moment is big enough it will unscrew the nut. If not there are 2 ways of increasing the moment.
7 Increasing the moment1. Increase the distance from the force to the pivot - apply the force at the end or use a longer spanner.PivotDistance from force to pivotForce
8 Increasing the moment2. Increase the force applied - push/pull harder or get someone stronger to do it!pivotDistance from force to pivotForce
9 F x d Moment The moment of a force is given by the relationship: Moment = Force (N) x Distance (cm or m).Moments are measured in Newton centimetre (Ncm) or Newton metre (Nm).momentFxd
10 Moments calculationGina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot.What moment does she exert?Click for solutionmoment = 500 x 0.5= 250 Nm0.5 m500 Npivot
11 Principle of momentsThe green girl exerts an anti-clockwise moment equal to ...her weight x distance from pivot.The yellow girl exerts a clockwise moment equal to...her weight x distance from pivot.pivot
12 Principle of momentspivotIf the two moments are equal then the seesaw is balanced. This is known as the principle of moments.When balancedTotal clockwise moment = total anti-clockwise moment“c.m.” = “a-c.m.”
13 Both moments are equal and so the seesaw 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 seesaw is balanced.
14 Why don’t cranes fall over? Tower cranes are essential at any major construction site.load armtrolleyConcrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?counterweightloading platformtower
15 Why don’t cranes fall over? Using the principle of moments, when is the crane balanced?3 m6 mN?moment of = moment ofload counterweightIf a N counterweight is 3 metres from the tower, what weight can be lifted when the loading platform is 6 metres from the tower?
16 Why don’t cranes fall over? moment ofload== ? x 6load x distance of load from towermoment ofcounterweightdistance of counterweight from tower== x 3= Nmcounterweight xmoment of load = moment of counterweight? x 6 =? =6? = 5000 N
17 Crane operator activity Where should the loading platform be on the loading arm to carry each load safely?
19 Using moments in calculations 1. Two girls are on a seesaw. One weighs 200N and is 1.5m from the pivot. Where must her 150N friend sit if the seesaw is to balance ?.Click for solutionAt balancetotal “c.m.” = total “a-c.m.”200 x 1.5 = 150 x distance200 x 1.5 = distance150distance = 2 m“c.m.” = clockwise moment“a-c.m.” =anti-clockwise moment
21 Pressure Pressure is exerted whenever a force is applied over an area. 1.2.Which one exerts the biggest pressure, 1 or 2?
22 1.Case 1.The arm applies a force onto a board via a finger tip.The force applied produces a high pressure because the force acts over a small area.
23 2.Case 2.The arm applies the same force onto the board.The force is now acting over a larger area - the area of the palm is greater than the finger tip.Thus, a lower pressure is produced.
24 Force Pressure = Area Pressure F P x A Pressure is the force per unit area so is calculated using the expression shown below:Pressure =AreaForcePressure is measured in:Newtons per metre squared (N/m2) which is calleda PASCAL (Pa)Pressure can also be measured in:Newtons per millimetre squared (N/mm2);Newtons per centimetre squared (N/cm2).
25 The same force spread over a big area means low pressure. Which shoes would you choose for walking over a muddy field?
26 The boots on the right spread the weight over a larger area The boots on the right spread the weight over a larger area. Therefore, the pressure exerted on the ground is low.In contrast, fashion shoes have a smaller area and exert a higher pressure. These shoes are likely to sink into soft ground.
27 Application of 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 boy exerts very little pressure on the snow. He therefore slides over the top and does not sink in.
28 Application of pressure A force concentrated on a small area means high pressure, e.g. razor blades, needles, high heeled shoes, spurs, ice skates, sharp knives.The high pressure on the cutting edge of an ice-skate melts the ice and helps the skater slide across the surface.On the cutting edge of a knife a very high pressure is exerted - this makes it easier to cut.
29 Pressure in liquids In a liquid: Pressure acts in all directions and pressure increases with depth.
30 Pressure (N/m2) = 10 N/Kg x depth (m) x density (Kg/m3) The 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 pull of gravityThe deeper you go, the higher the pressureThe denser the liquid, the heavier it is!
31 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 them mechanically.All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid.ForceappliedhereForcetransferredherePressure inside all parts of the hydraulic system is the same
32 Hydraulics brakesAll 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.Pressure =Force appliedArea master pistonThe pressure is transmitted to the pistons on all four wheels.
33 Force exerted = Pressure x Area slave piston The slave piston always has a much larger area than the master piston. The force exerted can be calculated from the same equation:Much larger than master pistonPressure =Force exertedArea slave pistonForce exerted = Pressure x Area slave pistonSo, a greater force is exerted by the brakes than the driver exerted on the pedal.
35 The hydraulic brake - example The car master piston has an area of 5cm2. If a force of 10N is applied to it, calculate the pressure created in the brake pipes. If the slave piston has an area of 50 cm2, calculate the force exerted on the brake disc.Click for solutionAt the master piston, P=F/A= 10/5 = 2 N/cm2At the slave piston, F= PxA =2x50 = 100 N(10 times the original force applied to the master piston).