A 0.5 kg toy car is placed at the top of a 30° incline that is 1.2 m high. It is released from rest and rolls down to the bottom, where it compresses a spring of constant 200 N m⁻¹ until it comes to rest. Describe the energy changes that occur from the moment the car is released until it stops, and explain how the energy is stored before and after the compression of the spring.

AQA GCSE Physics (8463) — 4.1.1 Energy changes in a system, and the ways energy is stored before and after such changes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. The toy car has gravitational potential energy at the top: U_g = mgh = 0.5 kg × 9.8 m s⁻² × 1.2 m = 5.88 J.
2. As it rolls down the incline, this potential energy is converted into kinetic energy (both translational and rotational) of the car.
3. When the car reaches the bottom, its kinetic energy is transferred to the spring, which stores the energy as elastic potential energy: U_s = ½kx².
4. At the instant the car stops, all the initial gravitational potential energy has been stored as elastic potential energy in the spring, so ½kx² = 5.88 J.

Examiner tips

  • Use the exact formulae (U_g = mgh, U_s = ½kx²) and include the numerical calculation for the initial potential energy.
  • Show the energy conversion chain: potential → kinetic → elastic potential, and state that the final state is all energy in the spring.

Common mistakes

  • Calculating potential energy with the wrong height (using the incline length instead of vertical height).
  • Forgetting to include the spring constant or the ½ factor in the elastic potential energy formula.

Mark scheme (4 marks)

  1. Identify the initial potential energy of the car at the top of the incline.
  2. State that the car’s potential energy is converted into kinetic energy as it rolls down.
  3. Explain that when the car reaches the bottom, its kinetic energy is transferred to the spring as elastic potential energy.
  4. Describe the final state where all the initial potential energy is stored as elastic potential energy in the spring.

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