Topic 8 - Energy: Forces Doing Work

• State the law of energy conservation

  Energy cannot be created or destroyed (it can only be transferred into different forms).
• Describe the energy changes involved when a ball is thrown upwards and then returns to its starting position. Ignore air resistance


  ● Upwards: KE is converted to GPE

  ● Peak: Maximum GPE, zero KE

  ● Downwards: GPE is converted to KE
• State any changes in the total energy of a ball that is kicked, assuming that no external forces act


  The total energy of the system remains constant due to the conservation of energy.
• Describe the energy changes that occur in a filament light-bulb.

  ● Electrical energy is transferred into light and heat energy

  ● Light is a useful energy form, heat is waste energy
• Describe the energy transfers for a bungee jumper.

  ● When falling, the GPE is converted to KE of jumper

  ● As the cord tightens, KE is converted and stored as Elastic Potential Energy (EPE)

  ● At lowest point, the jumper's initial GPE equals the EPE stored in the cord
• Explain why a bungee jumper slows down once the cord begins to stretch.

  ● Kinetic energy decreases since it is converted to elastic potential energy

  ● Since KE is proportional to (velocity)², as KE decreases, so does velocity.
• Describe the energy changes in a power station

  - heat energy released in the reaction heats water to make steam

  - steam moves the turbine, converting kinetic energy

  - kinetic energy turns the generator

  - electrical energy as the generator generates electricity
• In what ways can the energy of a system be changed?

  ● Through work done by forces

  ● Through electrical input

  ● Through heating
• How can energy be changed through work done by forces?

  A force which moves an object (kinetic energy) causes work to be done over a distance.
• What equation is used to measure the work done on an object?

  work done (J) = force (N) x distance (m)

  W = Fd
• Why do wasteful mechanical processes cause a rise in the temperature of the surroundings?

  They cause energy to be transferred by heating the surroundings.
• What is the equation for kinetic energy? Give SI units for all quantities involved


  E = ½ m v2

  Energy = ½ x Mass x (Velocity)²

  Energy (J), Mass(kg), Velocity(m/s)
• What is the equation for gravitational potential energy? Give SI units for all quantities involved


  E = mgh

  Energy = Mass x Gravitational Field Strength x Height

  Energy (J), Mass(kg), Gravitational Field Strength (N/kg), Height (m)
• What is the definition of 'power'?

  The rate at which energy is transferred (or rate at which work is done).
• State two equations for power. Give SI units for all quantities involved.


  power = energy transferred/time

  power = work done/time

  Energy (J), Work Done (J), Time (s)
• What is the unit for power?

  Watts (W)
• What is one Watt equivalent to?

  1 joule per second (J/s)
• What does a power rating of 10W mean for an electrical device?

  It converts 10 Joules of energy every second.
• Two motors lift the same mass through the same height. Motor A does this in half the time of Motor B. Which dissipates the most power?

  Motor A.

  The energy transferred is the same but the time taken is less (P=E/t).
• What is efficiency?


  The proportion of energy that is converted usefully, rather than wasted/dissipated.
• How do you calculate efficiency?

  Useful energy output / total energy input
• What are the units of efficiency?

  Efficiency is a ratio so has no units, but it can be expressed as a percentage.