GCSE P2 Chapter 3

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  • Created by: emma998
  • Created on: 07-03-14 16:07

Energy and work

  • Work(Nm) = force(N) X distance(m)
  • When a force moves an object energy is transfered and work is done
  • If the object is not moved, no work is done
  • When work is done moving an object energy is transfered to the object
  • Work done to overcome friction is usually transfered to the surroundings by heating. 
  • When the brakes are applied to a vehicle:
    • Friction between the brake pads and the wheel discs oppose the motion of the wheel.
    • Kinetic energy of the vehicle is transfered into energy that heats the brake pads and the wheel discs aswell as the surroundings
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Gravitational potential energy

  • Gravitational potential energy is the stored energy an object has due to its position in Earth's gravitational field
  • The gravitational field strength of Earth is about 10N
  • The higher an object is in Earth's gravitational field strength the greater the gravitational potential energy the object has
  • When an object is moved vertically upwards it gains gravitational potential energy equal to the work done on it by the lifting force
  • This energy is then released into kinetic energy if the object falls back to the surface of the Earth
  • Gravitational potential energy(J) = Mass(Kg) X gravitational field strength(N) X Height(m)
  • Power is the rate of transfer of energy
  • Power(w) = Energy(J) / time(s)
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Kinetic energy

  • Kinetic energy(J) = 1/2 X mass(kg) X velocity*(m/s)
  • Kinetic energy is the energy an object due to its motion
  • All moving objects have kinetic energy
  • The greater the mass and the greater the velocity of an object the greater the kinetic energy
  • Elastic potential enrgy of an elastic object is transfered into kinetic energy when the force deforming its shape is removed
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  • Momentum(Kgm/s) = Mass(Kg) X Velocity(m/s)
  • All moving objects have momentum
  • The greater the mass and the greater the velocity the greater the momentum
  • Conservation of momentum: The total momentum before an interaction is the same as after the interaction (change in momentum equals zero)
  • Interaction of two objects could be a collision or an explosion
  • After an interaction the objects may move off togther or in opposite directions
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  • Momentum has both size and direction
  • In an explosion one momentum is positive the other is negative
  • Because momentum before and after an interaction must be same and before an explosion momentum of objects = zero, momentum of both objects after explosion must be equal and opposite (one + one -)

Firing a bullet from a gun is an example of an explosion:

  • Momentum before explosion is zero
  • After explosion bullet flies off in one direction
  • Gun recoils with equal momentum in opposite direction

Momentum = Mass X Velocity so:

  • Bullet has a very low mass therefore velocity is very large
  • Gun has same size momentum so because much larger mass, velocity much lower
  • Both have same size momentum
  • Direction of momentum of bullet is positive, direction of momentum of gun is negative
  • Overall momentum after explosion is zero so overall momentum is unchanged
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Impact forces

  • When a force acts on an object that is able to move, the object's momentum changes
  • For a paticular change in momentum, the longer the time taken for the change, the smaller the impact force that acts
  • In a collision the momentum of an object often becomes zero during the impact - object comes to rest
  • Impact forces are increased if the time taken is decreased
  • The shorter the impact time the larger the impact force
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Car safety

  • Modern cars contain a number of safety featues designed to reduce impact forces if a collision were to occur
  • Side impact bars and crumple zones are designed to fold in slowly, increasing the impact time and reducing the impact forces in a crash
  • Seat belts and air bags are also designed to increase impact time by:
    • Seat belts stretch slightly increasing impact time reducing impact force on passengers and driver. Also spreads froce equally across the chest
    • Air bags also increase impact time and spread force across body. If drivers head hits an airbag momentum changes slowly reducing impact force
  • After a crash measurements can be taken from the scene and the law of conservation of momentum applied to work out speeds of the vehicles before collision
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