A wind-up toy robot is wound up tightly and then released on a rough carpet. As it moves, the robot gradually slows down and stops. Explain the energy transfers that take place from the moment the robot is released until it stops completely.

Eduqas A-Level Physics — 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 wound‑up spring stores elastic potential energy. When the robot is released this potential energy is converted into kinetic energy of the robot’s motion. As the robot moves over the rough carpet, friction between the robot and the carpet dissipates energy; the kinetic energy is transferred to thermal energy, heating the robot and the carpet. The energy is conserved – the total energy at the end (thermal + any remaining kinetic) equals the elastic potential energy that was stored in the spring at the start.

Examiner tips

  • Mention the spring’s elastic potential energy first, then the conversion to kinetic energy, then friction and heat, and finish with conservation of energy.
  • Use the exact terms: elastic potential energy, kinetic energy, friction, thermal energy, conservation of energy.
  • Show the sequence clearly – start, middle, end – to demonstrate understanding of the transfer process.

Common mistakes

  • Forgetting to mention the spring’s stored energy or the frictional heat transfer.
  • Using vague terms like ‘energy loss’ instead of specifying friction and thermal energy.
  • Claiming that energy is destroyed rather than conserved.”

Mark scheme (5 marks)

  1. The wound-up spring stores elastic potential energy
  2. As the robot moves, elastic potential energy is transferred to kinetic energy
  3. Friction acts between the robot and the carpet, causing a waste energy transfer
  4. Kinetic energy is transferred to thermal energy (heat) in the surroundings / robot and carpet warm up
  5. Energy is conserved throughout — the total energy at the end equals the elastic potential energy at the start, consistent with conservation of energy

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