A roller coaster cart starts from rest at the top of a hill and travels down to the bottom. Explain the energy transfers that occur as the cart moves from the top of the hill to the bottom, and explain why the kinetic energy of the cart at the bottom is always less than the gravitational potential energy it had at the top.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A roller coaster cart of mass 500 kg starts from rest at the top of a hill that is 20 m above the ground. The cart rolls down to ground level. The track is not smooth.
Model answer (5 marks)
The cart has gravitational potential energy (GPE) at the top of the hill.
As it moves down, GPE is converted into kinetic energy.
Friction between the cart and the track, and air resistance, act on the cart.
Work is done against these forces, transferring part of the energy into thermal energy in the surroundings, track and cart.
Because some of the initial GPE is lost as thermal energy, the kinetic energy at the bottom is always less than the original GPE.
As it moves down, GPE is converted into kinetic energy.
Friction between the cart and the track, and air resistance, act on the cart.
Work is done against these forces, transferring part of the energy into thermal energy in the surroundings, track and cart.
Because some of the initial GPE is lost as thermal energy, the kinetic energy at the bottom is always less than the original GPE.
Examiner tips
- Use the phrase ‘converted into’ to link GPE and kinetic energy. Mention both friction and air resistance as non‑conservative forces. Show the energy loss as work done against these forces. Explain that the remaining energy is thermal, not kinetic.
Common mistakes
- Forgetting to mention friction or air resistance. Saying the kinetic energy equals the initial GPE. Using vague terms like ‘energy loss’ without specifying thermal energy.
Mark scheme (5 marks)
- The cart has gravitational potential energy (GPE) at the top of the hill
- As the cart moves down, gravitational potential energy is transferred to kinetic energy
- Friction (between cart and track) / air resistance acts on the cart as it moves
- Work is done against friction / air resistance, transferring energy to the thermal energy store of the surroundings / track / cart
- Because some GPE is transferred to thermal energy (not to kinetic energy), the kinetic energy at the bottom is less than the original GPE
Key terms in this question
gravitational potential energy · kinetic energy
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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