A student pushes a heavy box along a rough floor at a constant speed. Explain why the student must keep applying a force to maintain the constant speed, and describe what happens to the energy transferred by the student during this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student pushes a heavy box along a rough floor at a constant speed across a warehouse. The student notices they are getting warm and can hear the box sliding along the floor.
Model answer (4 marks)
The student must keep applying a force because friction between the box and the rough floor opposes the motion. To keep the speed constant the applied force must equal the frictional force, so the student does work over the distance the box moves.
The kinetic energy of the box does not change because its speed is constant. The energy supplied by the student is therefore not stored as kinetic energy but is dissipated as heat in the box, floor and the student’s muscles, and as a small amount of sound energy.
The kinetic energy of the box does not change because its speed is constant. The energy supplied by the student is therefore not stored as kinetic energy but is dissipated as heat in the box, floor and the student’s muscles, and as a small amount of sound energy.
Examiner tips
- Show the friction force opposing motion and that the applied force equals it to keep speed constant. Explain that kinetic energy remains constant, so the work done is converted to heat and sound. Use terms: work, friction, kinetic energy, dissipated energy.
Common mistakes
- Saying the student’s force is unnecessary or that the box gains kinetic energy. Confusing the direction of friction with the direction of the applied force. Forgetting to mention heat or sound as the form of dissipated energy.
Mark scheme (4 marks)
- Friction acts between the box and the floor (opposing the motion)
- The student must do work / apply a force over a distance to overcome friction
- The useful energy transfer / kinetic energy of the box does not increase (because speed is constant)
- The energy transferred by the student is dissipated / wasted as heat (and sound), which is a waste energy transfer
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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