A student pushes a heavy box along a rough floor at a constant speed. The student applies a horizontal force of 80 N and pushes the box a distance of 5 m. Explain why the work done against friction equals the work done by the student, and state what has happened to this energy at the end of the push.
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. The student applies a horizontal force of 80 N and the box moves a horizontal distance of 5 m.
Model answer (5 marks)
The work done by the student is calculated from the horizontal force and the distance moved: W = F × d = 80 N × 5 m = 400 J.
Because the box moves at constant speed, the driving force equals the friction force – the forces are balanced.
The friction force acts over the same distance of 5 m, so the work done against friction is also 400 J.
Thus the work done by the student equals the work done against friction.
The 400 J of mechanical energy is converted into thermal energy in the box and the floor, heating the surroundings.
Because the box moves at constant speed, the driving force equals the friction force – the forces are balanced.
The friction force acts over the same distance of 5 m, so the work done against friction is also 400 J.
Thus the work done by the student equals the work done against friction.
The 400 J of mechanical energy is converted into thermal energy in the box and the floor, heating the surroundings.
Examiner tips
- Show the calculation W = 80 N × 5 m = 400 J; include units. State that constant speed means forces are balanced. Explain that equal forces over equal distance give equal work. Mention the energy is dissipated as heat in the box and floor.
Common mistakes
- Forgetting to include units in the work calculation. Assuming the work against friction is negative instead of positive. Not recognising that the energy is transferred to thermal energy rather than remaining as kinetic energy.
Mark scheme (5 marks)
- Work done is calculated as force multiplied by distance (in the direction of the force) / W = F × d
- The box moves at constant speed, so the driving force equals the friction force (forces are balanced)
- Because the two forces are equal and act over the same distance, the work done by the student equals the work done against friction
- The work done by the student is 400 J (80 N × 5 m)
- The energy has been transferred to the thermal energy store of the box and the floor (due to friction) / the surroundings are heated
Key terms in this question
work done · friction · constant speed · force
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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