A roller coaster car starts from rest at the top of a tall hill and rolls down to the bottom, reaching its highest speed just before the brakes are applied. Explain the energy transfers that take place as the car travels from the top of the hill to the moment it is brought to a complete stop by the brakes, naming all relevant energy stores involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. At the top the car possesses gravitational potential energy (PE = mgh) because of its height and mass.
2. As it rolls down the hill, this PE is converted into kinetic energy (KE = ½mv²), giving the car speed.
3. During the descent some of the PE is lost to friction and air resistance, which transfers energy into thermal energy of the surroundings.
4. When the brakes are applied, the car’s KE is converted into thermal energy in the brake pads and the surrounding air.
5. Throughout the whole process the total energy remains constant – energy is only transferred between potential, kinetic and thermal stores, never created or destroyed.
2. As it rolls down the hill, this PE is converted into kinetic energy (KE = ½mv²), giving the car speed.
3. During the descent some of the PE is lost to friction and air resistance, which transfers energy into thermal energy of the surroundings.
4. When the brakes are applied, the car’s KE is converted into thermal energy in the brake pads and the surrounding air.
5. Throughout the whole process the total energy remains constant – energy is only transferred between potential, kinetic and thermal stores, never created or destroyed.
Examiner tips
- Use the exact terms – gravitational potential, kinetic, thermal energy. Show the sequence of transfers clearly. Mention friction/air resistance as a loss. End with conservation of energy statement.
Common mistakes
- Confusing kinetic with potential energy. Forgetting to mention friction/air resistance. Saying energy is ‘destroyed’ instead of ‘transferred to thermal energy’.
Mark scheme (5 marks)
- The car has gravitational potential energy at the top of the hill (because it is raised / has height and mass)
- As the car rolls down, gravitational potential energy is transferred to kinetic energy
- Some energy is also dissipated / transferred to the thermal energy store of the surroundings due to friction / air resistance during the descent
- When the brakes are applied, kinetic energy is transferred to the thermal energy store of the brakes / surroundings
- Energy is conserved throughout — it is never created or destroyed, only transferred between stores (law of conservation of energy)
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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