Energy stores and systems: an object projected upwards

The idea

When an object is projected upwards, the system is the object and the Earth. At the point of release, the object has a kinetic store because it is moving. As it rises, it slows, so the kinetic store decreases while the gravitational potential store of the object–Earth system increases. The transfer is mechanical work done against gravity. At the highest point, the kinetic store is momentarily zero and the gravitational potential store is maximum. As it falls back, the reverse happens: gravitational potential store decreases and kinetic store increases. If air resistance is ignored, the energy transferred equals the gain in gravitational potential store; with air resistance, some energy is dissipated to the thermal store of the surroundings.

What earns the marks

• Names the object–Earth system and identifies the kinetic store and gravitational potential store as the stores that change.
• States that as the object rises, kinetic store decreases and gravitational potential store increases, transferred mechanically by gravity doing work.
• Recognises that at the highest point the kinetic store is zero and the gravitational potential store is maximum.
• Describes the reverse transfer on the way down, with gravitational potential store decreasing and kinetic store increasing.
• Explains that with air resistance some energy is dissipated to the thermal store of the surroundings, so the energy returned as kinetic is less than the initial kinetic store.
• Uses conservation of energy to relate the decrease in one store to the increase in the other when dissipation is negligible.

Common mistakes and how to put them right

• Saying the object 'has no energy' at the highest point.

  Put it right: The kinetic store is zero but the gravitational potential store is at its maximum.

• Treating gravitational potential energy as a property of the object alone.

  Put it right: It is a store of the object–Earth system because it depends on the separation between them.

• Ignoring air resistance entirely when the question mentions it.

  Put it right: State that work done against air resistance dissipates energy to the thermal store of the surroundings.

Exam tips

• Describe the motion in stages: release, rising, highest point, falling, and state the stores at each stage.
• Use the phrase 'transferred mechanically by gravity doing work' to show the pathway clearly.
• If air resistance is mentioned, add a sentence about dissipation to the thermal store of the surroundings rather than saying energy is lost.