Work and power

• If the force acts in the opposite direction to the movement

  The object will lose energy (dissipated to the surroundings usually by heating)
• The amount of energy transferred (in joules) is equal to the work done (also in joules)
• If an object is lifted up, energy will be transferred to its gravitational store
• If a force acts in the direction that an object is moving

  The object will gain energy (usually to its kinetic energy store)
• Work


  Done when an object is moved over a distance by a force applied in the direction of its displacement
• When work is done on an object, energy is transferred
• Energy in the gravitational potential store of an object is defined as the energy an object has due to its height in a gravitational field
• Worked example

  1. Step 1: List the known quantities

  2. Step 2: Write out the equation relating work, force and distance

  3. Step 3: Calculate the work done on the car by the brakes
• Examples of Work

  • Work done on a ball when lifted to a height
  • Work done when a bird flies through the air
• If a force is applied to an object but doesn’t result in any movement, no work is done
• Calculating Work Done

  The amount of work done is related to the size of the force and the distance moved by the object in the direction of the force
• Air resistance (drag) does work against the bird as it flies through the air
• Force does work on the object when it is moved
• Contents of the topic

  • 4.2.1 Work Done
  • 4.2.2 Gravitational Potential Energy
  • 4.2.3 Kinetic Energy
  • 4.2.4 Work, GPE & KE
  • 4.2.5 Power
  • 4.2.6 Energy Resources
  • 4.2.7 Comparing Energy Resources
• Calculating Work Done

  W = f × d
• Work is done when a force is used to move an object
• If an object falls, energy will be transferred away from its gravitational store
• Kinetic energy


  The amount of energy an object has as a result of its mass and speed
• Worked example

  Calculate the increase in energy transferred to the gravitational potential store of a man climbing stairs
• Energy is transferred to the mass's gravitational store as it is lifted above the ground
• Gravitational field strength

  • The gravitational field strength (g) on Earth is approximately 10 N/kg
  • The gravitational field strength on the Moon is less than on Earth
  • The gravitational field strength on gas giants like Jupiter and Saturn is more than on Earth
• Graphs show the linear relationship between GPE and height
• Exam Tip: Gravitational field strength value is provided in exam papers
• Worked example

  Calculate the kinetic energy stored in a vehicle moving at a certain speed
• Kinetic energy equation variables

  • KE = kinetic energy (J)
  • m = mass (kg)
  • v = speed (m/s)
• Values for g in the Solar System

  • Earth
  • Moon
  • Gas giants
• Gravitational potential energy equation

  GPE = m × g × h
• Gravitational potential energy equation variables

  • GPE = gravitational potential energy (J)
  • m = mass (kg)
  • g = gravitational field strength (N/kg)
  • h = height (m)
• Exam Tip: Double-check that the speed is squared in kinetic energy calculations
• Kinetic energy equation

  KE = 1/2 × m × v^2
• Gravitational potential energy


  The energy an object has due to its height in a gravitational field
• In reality, there is no such thing as a perfect energy transfer
• This is a consequence of conservation of energy
• The rate of this energy transfer, or the rate of work done, is called power
• Energy is transferred back and forth between these two stores as the pendulum swings
• All of the energy in the kinetic store of the pendulum is transferred mechanically into its gravitational potential store
• When the question tells you to ignore the effects of resistance (i.e., wasted energy transfers) this is a clue that may need to use energy equivalency to find the missing quantity needed for your calculation
• When performing calculations using the kinetic energy equation, always double-check that you have squared the speed. Forgetting to do this is the most common mistake that students make
• KEtotal

  GPEtotal
• If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time