A student pushes a heavy crate across a rough floor. The crate moves at a constant velocity. Explain why the crate moves at constant velocity and describe what happens to the energy that is transferred when the student does work against the resistive force.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student pushes a heavy crate across a rough floor at constant velocity. The student exerts a horizontal force of 80 N on the crate.
Model answer (5 marks)
1. The crate moves at constant velocity because the resultant force on it is zero.
2. The horizontal force the student applies (80 N) is balanced by an equal and opposite resistive force from the rough floor.
3. The student therefore does work against this resistive (frictional) force.
4. The work done is converted into thermal energy, which is transferred to the surroundings as heat.
5. Energy is conserved: the energy supplied by the student equals the energy dissipated as heat in the surroundings.
2. The horizontal force the student applies (80 N) is balanced by an equal and opposite resistive force from the rough floor.
3. The student therefore does work against this resistive (frictional) force.
4. The work done is converted into thermal energy, which is transferred to the surroundings as heat.
5. Energy is conserved: the energy supplied by the student equals the energy dissipated as heat in the surroundings.
Examiner tips
- Use the word "resultant" to show understanding of forces. Mention that the resistive force equals the applied force. Explain that work against friction becomes heat. State that total energy is conserved.
Common mistakes
- Confusing constant velocity with zero speed. Forgetting to mention that the resistive force equals the applied force. Saying the energy disappears instead of being transferred as heat.
Mark scheme (5 marks)
- Constant velocity means the resultant force on the crate is zero
- The student's pushing force equals the resistive force (friction) in magnitude
- Work is done against the resistive force (friction)
- Energy is dissipated / transferred to the surroundings as heat (thermal energy)
- The total energy is conserved — the energy transferred by the student equals the energy dissipated to the surroundings
Key terms in this question
resistive force · constant velocity
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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