A student pushes a heavy box across a rough floor at a steady speed. The student does work on the box, but the kinetic energy of the box does not increase. Explain why the kinetic energy of the box stays the same and describe what happens to the energy transferred by the student.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student pushes a heavy box across a rough floor at a steady speed. Despite the student doing work, the box does not speed up.
Model answer (5 marks)
The student applies a horizontal force on the box. On the rough floor a kinetic friction force acts in the opposite direction. The magnitude of the friction force equals the applied force, so the forces are balanced and the net force is zero.
With no net force the box moves at constant speed, meaning its kinetic energy,
K= frac12mv^2, remains unchanged.
The work done by the student, W=F·d, is therefore not stored as kinetic energy. It is dissipated as heat by the frictional interaction between the box and the floor, spreading the energy into the surroundings and making it unavailable for useful work.
With no net force the box moves at constant speed, meaning its kinetic energy,
K= frac12mv^2, remains unchanged.
The work done by the student, W=F·d, is therefore not stored as kinetic energy. It is dissipated as heat by the frictional interaction between the box and the floor, spreading the energy into the surroundings and making it unavailable for useful work.
Examiner tips
- Show the force balance first – friction equals applied force. Explain that zero net force gives constant speed → no change in kinetic energy. State that the student’s work becomes heat via friction. Use correct terminology: kinetic energy, work, friction, heat.
- common_mistakes
- :
- Forgetting to mention the frictional force. Assuming the work becomes potential energy instead of heat. Using “energy is lost” without explaining dissipation.
Mark scheme (5 marks)
- There is a resistive force (friction) acting on the box opposing its motion
- The pushing force equals the resistive force / forces are balanced, so there is no resultant force
- Because there is no resultant force, the speed does not change and so kinetic energy stays the same
- The work done by the student is transferred to the surroundings as heat / thermal energy due to friction
- This is dissipation of energy / the energy spreads out into the surroundings and is no longer useful
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →