A small toy car is wound up by compressing a spring inside it. When released on a flat surface, the car moves forward and gradually slows to a stop. Explain the energy transfers that take place from the moment the spring is released until the car stops moving.

WJEC A-Level Physics (Wales) — 1.3 Energy concepts · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The compressed spring stores elastic potential energy.
When the spring is released, that elastic potential energy is converted into kinetic energy of the car.
As the car moves, friction between the wheels/axle and the surface acts, converting some of the kinetic energy into heat.
The kinetic energy is therefore transferred to thermal energy in the surroundings and is dissipated.
Throughout the process the total energy is conserved; it is only transferred or dissipated, never created or destroyed.

Examiner tips

  • Use the exact terms: elastic potential energy, kinetic energy, thermal/heat energy, friction.
  • Show the sequence of transfers clearly – spring → kinetic → heat.
  • Mention conservation of energy to demonstrate full understanding.
  • Keep each point brief and to the point.

Mark scheme (5 marks)

  1. The compressed spring stores elastic potential energy
  2. As the spring releases, elastic potential energy is transferred to kinetic energy of the moving car
  3. As the car moves, friction acts between the wheels/axle and the surface, causing a waste energy transfer
  4. Kinetic energy is transferred to thermal/heat energy in the surroundings (dissipated)
  5. Energy is conserved throughout — the total energy in the closed system remains the same, it is only transferred/dissipated, not created or destroyed

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