A student pushes a stationary trolley along a smooth, flat surface. After the push, the trolley moves at a constant velocity. The student claims that because the trolley is now moving at constant velocity, no force acted on it during the push. Explain why the student is wrong, and describe what happened to the trolley's motion and energy during the push.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A stationary trolley is pushed briefly by a student on a smooth, flat surface. After the push, the trolley moves at constant velocity.
Model answer (5 marks)
A force must have acted on the trolley during the push because its velocity changed from zero to a non‑zero value, i.e. it accelerated.
Work was done by the student’s force as the trolley moved a distance while the force was applied.
The work transferred energy to the trolley, increasing its kinetic energy from 0 to … J.
After the push the surface is smooth, so no frictional force acts; the net force is zero and the trolley keeps a constant velocity.
Because no energy is lost to friction, the kinetic energy remains constant once the push stops.
Work was done by the student’s force as the trolley moved a distance while the force was applied.
The work transferred energy to the trolley, increasing its kinetic energy from 0 to … J.
After the push the surface is smooth, so no frictional force acts; the net force is zero and the trolley keeps a constant velocity.
Because no energy is lost to friction, the kinetic energy remains constant once the push stops.
Examiner tips
- Show the change in velocity to justify the force; include the word ‘accelerated’.
- Explain that work = force × distance and that this work becomes kinetic energy.
- Mention the absence of friction after the push to justify constant velocity.
Common mistakes
- Claiming no force acted because velocity is constant after the push; ignores the acceleration phase.
- Forgetting to state that work is transferred as kinetic energy.
Mark scheme (5 marks)
- A force must have acted on the trolley during the push because the trolley changed from being stationary to moving (i.e. its velocity changed / it accelerated)
- Work was done on the trolley by the pushing force (because a force moved the trolley through a distance)
- The work done transferred energy to the trolley as kinetic energy
- After the push ends, the surface is smooth so there is no resultant force / no friction, which is why the trolley continues at constant velocity
- The kinetic energy of the trolley stays constant after the push because no energy is transferred away (smooth surface means no friction / no waste energy transfer)
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
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- Decode the mark scheme abbreviations →
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