Describe and calculate the changes in energy involved when a system is…

The idea

When a force does work on a system, energy is transferred into or out of that system's stores. Lifting a 2 kg mass by 3 m with a force equal to its weight (about 20 N) does about 60 J of work, increasing the gravitational potential energy store by the same amount. Pushing a block along a rough floor transfers energy from a kinetic store to a thermal store through friction, so the block slows as the surfaces warm. The work done equals the energy change: W = F × s, and this can be equated to changes such as m × g × h for a raised mass. Describe the store that loses energy, the store that gains it and the force responsible, then calculate the change using the appropriate equation and units.

What earns the marks

• Identifies the energy store that decreases and the store that increases when a force does work on the system.
• States that the work done by the force equals the change in energy of the system.
• Calculates the energy change using W = F × s with force in newtons and distance in metres, giving the answer in joules.
• Describes a named example, such as a raised mass gaining gravitational potential energy or friction transferring energy to a thermal store.
• Equates work done to a specific energy change, for example m × g × h for a mass lifted through a height.

Common mistakes and how to put them right

• Claims energy is used up or destroyed.

  Put it right: State that energy is transferred between stores and conserved overall.

• Names only one store without saying which loses and which gains energy.

  Put it right: Identify both sides of the transfer.

• Multiplies force by distance without converting units.

  Put it right: Express force in newtons and distance in metres first.

• Treats work done as unrelated to the energy change.

  Put it right: Equate the work done to the change in the relevant store.

Exam tips

• Name the force doing the work and the two stores involved before calculating.
• Show the equation and substitution with units, then state the energy change in joules.
• Link the numerical answer back to a described change, such as a rise in gravitational potential energy.