A student pushes a heavy crate across a warehouse floor at a constant speed. Explain the energy transfers that take place and why the total energy of the system is conserved, even though the crate eventually comes to rest when the student stops pushing.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A warehouse worker applies a steady force to push a heavy wooden crate across a concrete floor. The crate moves at constant speed while being pushed, then slows to a stop the moment the worker stops pushing.
Model answer (5 marks)
1. The worker does work on the crate because a horizontal force acts over a distance.
2. The work is stored as kinetic energy, so the crate moves at constant speed.
3. Friction between crate and floor removes kinetic energy and converts it into thermal energy.
4. When the worker stops, the kinetic energy has been fully dissipated as heat in the crate, floor and air.
5. Energy is conserved because the total energy of the system (crate + worker + surroundings) remains constant; it is merely transferred and transformed, never created or destroyed.
2. The work is stored as kinetic energy, so the crate moves at constant speed.
3. Friction between crate and floor removes kinetic energy and converts it into thermal energy.
4. When the worker stops, the kinetic energy has been fully dissipated as heat in the crate, floor and air.
5. Energy is conserved because the total energy of the system (crate + worker + surroundings) remains constant; it is merely transferred and transformed, never created or destroyed.
Examiner tips
- Use the word ‘work’ and ‘friction’ explicitly. Show the energy flow: work → kinetic → thermal. Mention the system includes surroundings to justify conservation. Keep each point short and to the point.
Common mistakes
- Confusing work with force or ignoring friction. Claiming energy is lost instead of transformed. Forgetting to include the surroundings in the energy balance.
Mark scheme (5 marks)
- Work is done on the crate because a force causes it to move through a distance
- The useful energy transfer is kinetic energy (the crate moves)
- Friction between the crate and the floor causes a waste energy transfer to thermal energy / heat
- When the crate stops, all the kinetic energy has been dissipated / transferred to thermal energy of the surroundings
- Energy is conserved because it is transferred, stored or dissipated but never created or destroyed — the total energy of the closed system remains constant
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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