A heavy wooden crate is pushed across a rough concrete floor at a constant velocity by a warehouse worker. Explain why the worker must continue to apply a force to keep the crate moving at constant velocity, and describe what would happen to the motion of the crate if the worker suddenly doubled the pushing force.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A warehouse worker pushes a heavy wooden crate across a rough concrete floor. The crate moves at a constant velocity.
Model answer (4 marks)
The worker must keep pushing because the rough floor exerts a frictional force that opposes the motion. At constant velocity the pushing force equals the frictional force, so the net force is zero and the crate moves steadily.
If the worker suddenly doubled the pushing force, the pushing force would exceed the frictional force. A non‑zero resultant force would act in the direction of the push, causing the crate to accelerate and increase its speed.
If the worker suddenly doubled the pushing force, the pushing force would exceed the frictional force. A non‑zero resultant force would act in the direction of the push, causing the crate to accelerate and increase its speed.
Examiner tips
- Use the words ‘friction’, ‘resistive’, ‘equal and opposite’, ‘net force = 0’, ‘resultant force’, ‘accelerate’
- Show the balance of forces at constant speed and the imbalance when the force is doubled
- Mention that acceleration is in the direction of the larger force
Common mistakes
- Saying the crate will stop because the worker is pushing; ignoring friction
- Confusing ‘constant velocity’ with ‘constant force’
- Not stating that the crate will accelerate when the pushing force exceeds friction
Mark scheme (4 marks)
- Friction acts as a resistive contact force opposing the motion of the crate
- At constant velocity the pushing force and friction are equal / balanced, giving a resultant force of zero
- If the worker doubles the pushing force, there is now a resultant force in the direction of motion (the pushing force is greater than friction)
- The resultant force causes the crate to accelerate / speed up in the direction of the push
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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