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Gcse forces physics


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Gcse forces physics

  1. 1. GCSE Forces
  2. 2. Distance, SpeedDistance, Speedand Timeand TimeSpeed = distance (in metres)time (in seconds)DTS1) Dave walks 200 metres in 40 seconds. What is his speed?2) Laura covers 2km in 1,000 seconds. What is her speed?3) How long would it take to run 100 metres if you run at 10m/s?4) Steve travels at 50m/s for 20s. How far does he go?5) Susan drives her car at 85mph (about 40m/s). How long does ittake her to drive 20km?
  3. 3. Distance-time graphsDistance-time graphs40302010020 40 60 80 1004) Diagonal linedownwards =3) Steeper diagonal line =1) Diagonal line =2) Horizontal line =Distance(metres)Time/s
  4. 4. 40302010020 40 60 80 1001) What is the speed during the first 20 seconds?2) How far is the object from the start after 60 seconds?3) What is the speed during the last 40 seconds?4) When was the object travelling the fastest?Distance(metres)Time/s
  5. 5. Speed vs. VelocitySpeed vs. VelocitySpeed is simply how fast you are travelling…Velocity is “speed in a given direction”…This car is travelling at aspeed of 20m/sThis car is travelling at avelocity of 20m/s east
  6. 6. AccelerationAccelerationV-UTAAcceleration = change in velocity (in m/s)(in m/s2) time taken (in s)1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is heracceleration?2) A ball is dropped and accelerates downwards at a rate of 10m/s2for 12 seconds. How much will the ball’s velocity increase by?3) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2.How long did this take?4) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds.What is its acceleration?
  7. 7. Velocity-time graphsVelocity-time graphs80604020010 20 30 40 50Velocitym/sT/s1) Upwards line =2) Horizontal line = 3) Upwards line =4) Downward line =
  8. 8. 8060402001) How fast was the object going after 10 seconds?2) What is the acceleration from 20 to 30 seconds?3) What was the deceleration from 30 to 50s?4) How far did the object travel altogether?10 20 30 40 50Velocitym/sT/s
  9. 9. Balanced and unbalanced forcesBalanced and unbalanced forcesConsider a camel standing on a road.What forces are acting on it?WeightReactionThese two forces would be equal –we say that they are BALANCED.The camel doesn’t move anywhere.
  10. 10. Balanced and unbalanced forcesBalanced and unbalanced forcesWhat would happen if we took theroad away?WeightReaction
  11. 11. Balanced and unbalanced forcesBalanced and unbalanced forcesWhat would happen if we took theroad away?The camel’s weight is no longerbalanced by anything, so the camelfalls downwards…Weight
  12. 12. What would happen if we took theroad away?The camel’s weight is no longerbalanced by anything, so the camelfalls downwards…Balanced and unbalanced forcesBalanced and unbalanced forces
  13. 13. Balanced and unbalanced forcesBalanced and unbalanced forces1) This animal is either________ or movingwith _____ _____…4) This animal is…2) This animal is getting_________…3) This animal is getting_______….
  14. 14. Balanced and unbalanced forcesBalanced and unbalanced forces
  15. 15. Force and accelerationForce and accelerationIf the forces acting on an objectare unbalanced then the object willaccelerate, like these wrestlers:Force (in N) = Mass (in kg) x Acceleration (in m/s2)FAM
  16. 16. Force, mass and accelerationForce, mass and acceleration1) A force of 1000N is applied to pusha mass of 500kg. How quickly doesit accelerate?2) A force of 3000N acts on a car tomake it accelerate by 1.5m/s2. Howheavy is the car?3) A car accelerates at a rate of5m/s2. If it weighs 500kg howmuch driving force is the engineapplying?4) A force of 10N is applied by a boywhile lifting a 20kg mass. Howmuch does it accelerate by?FAM
  17. 17. Terminal VelocityTerminal VelocityConsider a skydiver:1) At the start of his jump the airresistance is _______ so he_______ downwards.2) As his speed increases his airresistance will _______3) Eventually the air resistance will bebig enough to _______ theskydiver’s weight. At this pointthe forces are balanced so hisspeed becomes ________ - this iscalled TERMINAL VELOCITY
  18. 18. Terminal VelocityTerminal VelocityConsider a skydiver:4) When he opens his parachute theair resistance suddenly ________,causing him to start _____ ____.5) Because he is slowing down his airresistance will _______ againuntil it balances his _________.The skydiver has now reached anew, lower ________ _______.
  19. 19. Velocity-time graph for terminal velocity…Velocity-time graph for terminal velocity…VelocityTimeSpeedincreases…Terminalvelocityreached…Parachute opens –diver slows downNew, lower terminalvelocity reachedDiver hits the groundOn the Moon
  20. 20. Weight vs. MassWeight vs. MassEarth’s Gravitational Field Strength is 10N/kg. In otherwords, a 1kg mass is pulled downwards by a force of 10N.WgMWeight = Mass x Gravitational Field Strength(in N) (in kg) (in N/kg)1) What is the weight on Earth of a book with mass 2kg?2) What is the weight on Earth of an apple with mass 100g?3) Dave weighs 700N. What is his mass?4) On the moon the gravitational field strength is 1.6N/kg. What willDave weigh if he stands on the moon?
  21. 21. FrictionFriction1) What is friction?2) Give 3 examples where it is annoying:1) Give 3 examples where it is useful:1) What effect does friction have on the surfaces?
  22. 22. Stopping a car…Stopping a car…BrakingdistanceToo muchalcoholThinkingdistanceTirednessToo manydrugsWet roadsDriving toofastTyres/brakesworn outIcy roadsPoorvisibility
  23. 23. Work doneWork doneWhen any object is moved around work will need to bedone on it to get it to move (obviously).We can work out the amount of work done in moving anobject using the formula:Work done = Force x distance movedin J in N in m WDF
  24. 24. Kinetic energyKinetic energyAny object that moves will have kinetic energy.The amount of kinetic energy an object has can be foundusing the formula:Kinetic energy = ½ x mass x velocity squaredin J in kg in m/sKE = ½ mv2
  25. 25. Some example questions…Some example questions…1) A 70kg boy is running at about 10m/s. What is hiskinetic energy?2) A braking force of 1000N is applied by a driver tostop his car. The car covered 50m before it stopped.How much work did the brakes do?3) What is the kinetic energy of a 100g tennis ball beingthrown at a speed of 5m/s?4) A crane is lifting a 50kg load up into the air with aconstant speed. If the load is raised by 200m howmuch work has the crane done? (The answer isn’t10,000J)
  26. 26. Pressure – some basic ideasPressure – some basic ideasDescribe and draw experiments for each of the followingideas:1) Pressure increases with depth2) Pressure is the same in all directions at a certain depth
  27. 27. PressurePressurePressure depends on two things:1) How much force is applied, and2) How big (or small) the area on which this force isapplied is.Pressure can be calculated using the equation:Pressure (in N/m2) = Force (in N)Area (in m2)FAP
  28. 28. Some example questions…Some example questions…1) A circus elephant weighs 10,000N and can stand on one foot.This foot has an area of 50cm2. How much pressure does heexert on the floor (in N/cm2)?2) A 50kg woman copies the elephant by standing on the heel of oneof her high-heeled shoes. This heel has an area of 1cm2. Howmuch pressure does she exert on the floor?Work through q2 on p58Extension task:Atmospheric pressure is roughly equivalent to 1kg pressing on everysquare centimetre on our body. What does this equate to inunits called Pascals? (1 Pascal = 1N/m2)
  29. 29. Hydraulic systemsHydraulic systems
  30. 30. Hydraulic systemsHydraulic systemsPressure is constant throughout this liquid
  31. 31. Hydraulic systemsHydraulic systemsBasically, a smaller forceon piston A will producea larger force on pistonB because the pressureof the liquid is constant.1) If the area of piston A is 5cm2 and piston B is 20cm2 how muchwill the force be multiplied by?2) If the area of the slave piston is ten times bigger than themaster piston what force will be needed to lift an object weighing1000N?3) A force of 10N is used to lift a weight of 50N. What is the ratioof the master piston area to the slave piston area?Magic!
  32. 32. Pressure and Volume in gasesPressure and Volume in gasesPressure Volume Pressure xvolume
  33. 33. Pressure and volume in gasesPressure and volume in gasesConclusionWhen we multiplied the pressure of a gas by its volume wefound that the answer was always __ _______.In other words, if you DECREASE the volume you _______the pressure and so on.“One goes up, the other goes down”
  34. 34. Pressure and Volume in gasesPressure and Volume in gasesThis can be expressed using the equation:Initial Pressure x Initial Volume = Final Press. x Final Vol.PIVI = PFVF1) A gas has a volume of 3m3at a pressure of 20N/m2. What will thepressure be if the volume is reduced to 1.5m3?2) A gas increases in volume from 10m3to 50m3. If the initialpressure was 10,000N/m2what is the new pressure?3) A gas decreases in pressure from 100,000 Pascals to 50,000Pascals. The final volume was 3m3. What was the initial volume?4) The pressure of a gas changes from 100N/m2to 20N/m2. What isthe ratio for volume change?