physics energy

• Work


  The work done on an object is equivalent to the change in energy of that object
• Work


  1. Applied force causes displacement over distance

  2. Work is done
• Work


  W = Fs where the displacement is parallel to the force
• Work is done when an applied force causes the position of an object to change
• The work done on an object is equivalent to the energy transfer that takes place when a force causes the position of an object to change (displacement)
• Work


  • W = 125 x 3.20 = 400 J
• Work is equivalent to change in energy
• No work is done in the vertical direction because the vertical displacement is zero
• Forms of energy


  • Kinetic
  • Elastic
  • Gravitational potential
  • Rotational kinetic
  • Heat
  • Electrical
• Heat is not a form of energy but a flow of energy between two objects due to a temperature difference
• Energy transfer


  Work done (W) is equal to the change in energy (∆E) of an object or system
• Energy is transferred whenever work is done on or by an object (or system)
• The energy content of an object or system decreases when it does work, and increases when work is done
• Energy is transformed whenever energy changes forms
• Kinetic energy


  Speed increases when work is done on a moving object
• Gravitational potential energy


  Energy an object has due to its position in a gravitational field
• Work is done in changing the vertical position of an object in a gravitational field
• Work is done in moving an object away from the surface of the Earth
• An object does work as it moves towards the surface of the Earth
• The work done (W) is equal to the change in the gravitational potential energy (Ep) of the object
• Conservation of energy


  Energy is conserved when transferred from one object to another in an isolated system
• Solving problems using the conservation of energy

  1. Identify the types of energy involved

  2. Apply the law of conservation of energy

  3. Calculate changes in energy
• Law of Conservation of Energy


  Energy is never created or destroyed, it can only be transferred into different types or transferred between objects
• Example: A ball falling under gravity

  • Object has gravitational potential energy but no kinetic energy when held at a height
  • Gravitational potential energy decreases as it falls
  • Gravitational potential energy is transformed into kinetic energy

  
• Change in gravitational potential energy (∆Ep)


  Equal to the change in kinetic energy (∆Ek) of the object, if friction is ignored

  
• Ep1 + Ek1


  Equal to Ep2 + Ek2
• Mechanical energy


  The sum of the potential and kinetic energies of a macroscopic object
• The mechanical energy of an object or system is conserved in any physical process
• ∆Ek + ∆Ep


  Equal to 0
• The speed of a ball can be determined using the Law of conservation of mechanical energy

  
• The speed of the skier is unlikely to be as high as the calculated value
• Energy losses in systems involving energy transfers

  • Energy is lost during a mechanical process
  • Moving objects are opposed by restrictive forces (such as friction) that act in the opposite direction to the motion
  • Friction includes air resistance and sliding friction
  • A moving object does work in overcoming the restrictive forces which results in less energy being transformed into useful work
  • No mechanical process is 100% efficient in transforming energy into useful forms

  
• Efficiency is less than 100% due to energy losses
• Wasted energy is the energy that is not transformed into useful work
• Worked example: Calculating the useful work and efficiency of a forklift truck


  1. Calculate the useful work done (Ep = mgh)

  2. Calculate the efficiency (useful energy / total energy)
• The efficiency of the forklift truck is 49%