P2.2 Kinetic Energy of Objects Speeding Up or Slowing Down

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P2.2.1 Forces and Energy

When a force causes an object to move through a distance work is done. Energy is transferred from one form to another when work is done (the amount of energy transferred is equal to the amount of work done)

W = Fd

  • W = work done in J
  • F = force applied to the object in N
  • d = distance moved in the direction of the force in m
    • Must be measured in the same direction as the force is acting
  • Thrust of a vehicle works by transferring the energy stored in fuel to kinetic energy
  • Resistive forces work by transferring kinetic energy of a moving object to heat

P = E/t

  • P= power in W - the amount of work done or energy transferred in a given time
  • E = energy transferred or work done (remember they are equal) in J 
  • t = time taken in s
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P2.2.1 KE and GPE

Gravitational potential energy is the energy that an object has because of its position in a gravitational field

GPE or Ep = mgh

  • GPE or Ep = gravitational potential energy in J
  • m = mass of object in kg
  • g = gravitational field strength in N/kg 
  • h = change in height or height above a given level in m

Kinetic energy is the energy an object has when it is moving

Ek = 1/2 m v(squared)

  • Ek = kinetic energy in J
  • m = mass of object in kg
  • v = speed of the object in m/s
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P2.2.2 Momentum

p = mv

  • p = momentum of the object in kg m/s
  • m = mass of object in kg
  • v = velocity of object in m/s

In a closed system, where no objects or energy are added or taken away, the total momentum before an event (a collision or explosion) is equal to the total momentum after the event. This is conservation of momentum 

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