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Chapter 9

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Chapter 9 Chapter 9 Presentation Transcript

  • Work and Energy Changes
    • How do we do work and how does it affect energy?
  • We will find out
    • How to calculate changes in energy types in different scenarios
    • How Newton’s Laws of Motion can be used to calculate the braking forces and distances (work done in stopping a vehicle)
    • Newton's Laws of Motion
    • Objects continue at the same velocity unless acted on by a resultant force
    • Force = Mass x Acceleration
    • Every Action has an equal and opposite reaction
  • Falling Masses - Acceleration
    • Why do 2 different masses fall at the same rate?
    • Think about Newton’s 2 nd Law!!!
    • Force = Mass x Acceleration
  • Falling Masses - Acceleration
    • Why do 2 different masses fall at the same rate?
    • F = ma therefore a = F / m
    • But Force is the weight of the mass = mg
    • Acceleration = Force / Mass = Mass x g / Mass = g
    • Acceleration is always g independent of Mass!!!
    LARGE MASS - M Small mass - m
  • Free Fall
    • What are the forces acting on a 100Kg parachutist?
      • When he first jumps
      • When he stops accelerating
      • What is the maximum air resistance?
    • How long will it take him to fall 4km? (ignore air resistance)
    • Can you hurry up?!?!?!
  • Free Fall
    • What are the forces acting on a 100Kg parachutist?
      • When he first jumps
      • WEIGHT mg = 1000N
      • When he stops accelerating – WEIGHT and DRAG Both = 1000N
      • What is the maximum air resistance? 1000N
  • Free Fall
    • It takes him this long…
    • We know
      • Distance – s = 4000m
      • Acceleration – g = 9.8 ms -2
      • Initial Velocity – u = 0
      • Use s = u + ½ at 2
      • 4000 = 0 + ½ (9.8) t 2
      • 9.8 x 8000 = t 2
      • t = 280s = 4.7minutes
  • Forces at Angles
    • What happens if forces act at angles to each other?
    • We look at the components of each force – like we did with vectors…
    • Example – what is the tension in the cable in the picture?
    • The car has a mass of 1500Kg and the helicopter is flying level.
  • Forces at Angles
    • What happens if forces act at angles to each other?
    • We look at the components of each force – like we did with vectors…
    • Tension = Weight = mg = 1500 x 9.8 = 14.7kN
  • Forces at Angles
    • What happens if forces act at angles to each other?
    • We look at the components of each force – like we did with vectors…
    • The helicopter starts to accelerate vertically at 3m/s 2 .
    • What is the tension in the cable now?
  • Forces at Angles
    • What happens if forces act at angles to each other?
    • We look at the components of each force – like we did with vectors…
    • Force (Resultant) = ma = 1500 x 3 = 4500N
    • But it also has to overcome the weight, so add it!
    • 4500 + 14700 = 19.2kN
  • Forces at Angles
    • The helicopter now moves forward.
    • The cable tilts – but the mass obviously does not change!
    • What is the tension now?
  • Forces at Angles
    • The helicopter now moves forward.
    • The cable tilts – but the mass obviously does not change!
    • Weight = Component of Tension
    • mg = T Cos 15
    • 14700 = T Cos 15
    • T= 14700 / 0.76 = 19300N
  • Forces at Angles
    • The helicopter now moves forward.
    • The cable tilts – but the mass obviously does not change!
    • What is the Air Resistance on the car?
    • Which direction is it acting?
  • Forces at Angles
    • The helicopter now moves forward.
    • The cable tilts – but the mass obviously does not change!
    • Air Resistance = T Sin 15
    • = 19300 Sin 15
    • = 12500N
    • Which direction is it acting?
  • Gravitational Field
    • Gravity is proportional to mass
    • It acts on all masses
    • ‘ Gravitational field Strength’ and ‘Acceleration due to Gravity’ are the same thing!
    • The units are either m/s 2 or N/Kg
    • Can you derive it from the equation:
    • F = ma ?
    • N = Kg x a therefore a = N/Kg
  • Energy Changes
    • Potential Energy = mgh
    • Kinetic energy = ½ m v 2
    • If potential energy is converted to kinetic, how does v vary with the change in h?
    • mgh = ½ m v 2 therefore gh = 1/2 v 2
    • Therefore v 2 = 2gh
  • Energy Changes
    • Draw a ramp on a piece of graph paper – side view
    • Draw a mass at the bottom of the ramp. Label it 150Kg
    • Measure or calculate the height at 3 different points on the ramp – it can be to scale…
    • Draw the mass at these points
  • Skateboard Worksheet
  • Forces and Momentum
    • Can you re-arrange the following to make a new equation?
    • F = ma
    • a = v /t
    • Force x Time = mv
    • This tells us how large a force is needed over a given time to stop something
  • Stopping Distances To stop an object, a force has to be applied for a certain time proportional to its moments – mv Force x Time = mv
  • Stopping Distances e.g. how long does a force of 100N have to be applied to stop a car of mass 1000Kg travelling at 10m/s? Time = mv / Force = 1000 x 10 / 100 = 100s How far will it travel in this time? – s = (u + v / 2) x t = 50 x 100 = 5000m