A skier starts from rest at the top of a snow-covered slope and slides to the bottom. At the bottom, the skier is moving more slowly than would be expected if no energy were lost. Explain why the skier's speed at the bottom is lower than expected, and describe what has happened to the 'missing' energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A skier starts from rest at the top of a snow-covered slope. At the bottom of the slope, measurements show the skier is travelling more slowly than predicted from the change in gravitational potential energy alone.
Model answer (4 marks)
A resistive force, such as friction between the skis and the snow and air resistance, acts on the skier while she slides down.
The skier does negative work against this resistive force, so part of the gravitational potential energy is used to overcome the resistance.
The energy lost to the resistive force is converted into thermal energy, heating the skis, the snow and the air.
Because only part of the initial gravitational potential energy becomes kinetic energy, the skier’s kinetic energy – and therefore her speed – at the bottom is lower than the value predicted if no energy were lost.
The skier does negative work against this resistive force, so part of the gravitational potential energy is used to overcome the resistance.
The energy lost to the resistive force is converted into thermal energy, heating the skis, the snow and the air.
Because only part of the initial gravitational potential energy becomes kinetic energy, the skier’s kinetic energy – and therefore her speed – at the bottom is lower than the value predicted if no energy were lost.
Examiner tips
- Use the terms ‘resistive force’, ‘negative work’ and ‘thermal energy’ – these are the exact words the mark scheme rewards.
- Show the chain: force → work → energy transfer → lower kinetic energy.
- Keep the answer short and to the point – 4 marks only.
- Mention both friction and air resistance to cover all resistive forces.
Common mistakes
- Forgetting to mention that the energy is converted to heat rather than simply ‘lost’.
- Using vague phrases like ‘energy is wasted’ instead of ‘converted to thermal energy’.
- Not linking the resistive force to the reduction in kinetic energy explicitly.
Mark scheme (4 marks)
- A resistive force (friction / air resistance) acts on the skier as they move down the slope
- The skier does work against the resistive force
- Energy is dissipated / transferred to the surroundings as thermal (heat) energy
- Because not all the gravitational potential energy is transferred to kinetic energy, the skier has less kinetic energy / a lower speed at the bottom
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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