Physics - P2.3 - Work, Energy and Momentum

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P2.3.1 - Energy and Work

  • Force is applied to anything when it moves - energy is transferred and work is done - object stops moving: supplied energy is transferred to object so work done = energy transferreed
  • Work and energy both measured in joules (J)
  • W = F x d
    • W - work done - J
    • F - force - N
    • d - distance moved in direction of force - m (no distance moved = no work done on object)
  • Work done to overcome friction mainly transferred into energy by heating - brakes applied to vehicle, friction between brake pads and wheel discs oppose wheel motion - kinetic energy of vehicle transferred into energy that heats brake pads, wheel discs and surroundings
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P2.3.2 - Gravitational Potential Energy

  • Gravitational potential energy - energy stored in object because of position in Earth's gravitational field - object moves vertically upwards: gains gravitational potential energy = to work done by lifting force
  • Ep = m x g x h
    • Ep - change in gravitational potential energy - J
    • m - mass - kg
    • g - gravitational field strength - N/kg
    • h - change in height - m
  • Power - rate of transfer of energy
  • P = E / t
    • P - power - W
    • E - energy - J
    • t - time - s
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P2.3.3 - Kinetic Energy

  • All moving objects have kinetic energy - greater mass = faster speed of object = more kinetic energy
  • Ek = 0.5 x m x v²
    • Ek - kinetic energy - J
    • m - mass - kg
    • v - speed - m/s
  • Elastic - regains shape after stretched or squashed - when work is done on elastic object to stretch or squah it, energy transferred is stored as elastic potential energy - when it returns to original shape, energy is released
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P2.3.3 - Kinetic Energy

  • All moving objects have kinetic energy - greater mass = faster speed of object = more kinetic energy
  • Ek = 0.5 x m x v²
    • Ek - kinetic energy - J
    • m - mass - kg
    • v - speed - m/s
  • Elastic - regains shape after stretched or squashed - when work is done on elastic object to stretch or squash it, energy transferred is stored as elastic potential energy - when it returns to original shape, energy is released
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P2.3.4 - Momentum

  • All moving objects have momentum - greater mass = greater velocity = greater momentum
  • p = m x v
    • p - momentum - kg m/s
    • m - mass - kg
    • v - velocity - m/s
  • Objects interacting - momentum before interation = momentum after (provided no external forces act on it) - law of conservation of momentum
  • Can be described as - total change in momentum = 0
  • Interaction could be collision or explosion - after collision objects may move off together or move apart
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P2.3.5 - Explosions

  • Momentum has both size and direction - in calculations, momentum in one direction is positive and negative in the other
  • Two objects at rest - momentum = 0, explosions - objects move apart with equal and opposite momentum - one momentum is positive, the other negative - total = 0
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P2.3.6 - Impact Forces

  • Force acts on moving object or object able to move, momentum changes - for particular change in momentum: longer change takes to happen = smaller force acts
  • Collision - momentum of object often = 0 during impact: object comes to rest - short impact time = large forces on object - as impact time increases, forces decrease
  • Crumple zones in cars - designed to fold in collision - increases impact time: reduces force on car and people in it
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P2.3.7 - Car Safety

  • Modern cars contain safety features designed to reduce forces in occupants of car
  • Side impact bars and crumple zones - fold up in collision to increase impact time and reduce forces acting
  • Seat belts and air bags - spread forces on body across larger area - if drivers head hits air bag it changes momentum slowly - force on head less than it would be if it changed momentum quickly by hitting steering wheel
  • Seat belts - stops wearer being flung forward if car suddenly stops - stretches slightly to increase imapct time and reduce force
  • After a car crash, police use measurements from scene and change in momentum to calculate vehicles speed before collision
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