A shopping trolley is pushed along a flat supermarket floor at a constant velocity. The person pushing the trolley then applies a greater forward force. Explain what happens to the motion of the trolley immediately after the greater force is applied, and describe what eventually happens to the trolley's velocity if the person maintains this greater force.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A shopping trolley moves at a constant velocity across a flat supermarket floor when a constant forward force is applied. The person pushing the trolley then increases the forward force.
Model answer (5 marks)
At constant velocity the forward force equals the frictional (resistive) force, so the resultant force is zero.
When a greater forward force is applied the forward force exceeds the frictional force, giving a non‑zero resultant force in the forward direction.
This resultant force causes the trolley to accelerate, i.e. its velocity increases.
As the trolley speeds up, the frictional force increases (because it depends on normal force and speed).
Eventually the frictional force will equal the new greater forward force, the resultant force becomes zero again, and the trolley reaches a new higher constant velocity.
When a greater forward force is applied the forward force exceeds the frictional force, giving a non‑zero resultant force in the forward direction.
This resultant force causes the trolley to accelerate, i.e. its velocity increases.
As the trolley speeds up, the frictional force increases (because it depends on normal force and speed).
Eventually the frictional force will equal the new greater forward force, the resultant force becomes zero again, and the trolley reaches a new higher constant velocity.
Examiner tips
- Use the word "resultant" to show the net force. Show the sequence: greater force → acceleration → increased friction → new equilibrium. Mention that the increase in friction is due to higher speed (or normal force if relevant).
Common mistakes
- Confusing friction with air resistance. Saying the trolley immediately stops instead of accelerates. Forgetting to explain that friction increases with speed and leads to a new constant velocity.
Mark scheme (5 marks)
- At constant velocity, the forward force is equal to (balanced by) the friction/resistive force, giving a zero resultant force.
- When the greater force is applied, the forward force is now greater than the friction/resistive force, so there is a resultant force in the forward direction.
- The resultant force causes the trolley to accelerate (speed up / increase in velocity).
- As the trolley speeds up, the friction / resistive force acting on it increases.
- Eventually the friction/resistive force equals the new greater forward force, giving a zero resultant force, so the trolley reaches a new higher constant velocity.
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 →
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