A book is placed on a table and remains stationary. A student pushes the book horizontally across the table with a constant force. The book accelerates at first, then moves at a constant velocity, and finally comes to rest after the student stops pushing. Explain the forces acting on the book during each of these three stages of its motion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
During acceleration the student’s horizontal pushing force exceeds the kinetic friction force, giving a net forward force that accelerates the book.
During constant velocity the pushing force equals the kinetic friction force, so the horizontal forces balance and the net force is zero.
After the student stops pushing, only kinetic friction acts horizontally, producing a net backward force that decelerates the book until it stops.
During constant velocity the pushing force equals the kinetic friction force, so the horizontal forces balance and the net force is zero.
After the student stops pushing, only kinetic friction acts horizontally, producing a net backward force that decelerates the book until it stops.
Examiner tips
- Use the word ‘exceeds’ or ‘equals’ to show comparison of forces; mention ‘net force’ and its direction. State the type of friction (kinetic) when it is the only horizontal force. Show the sequence of stages clearly to match the 5 marks.
Common mistakes
- Confusing static with kinetic friction; using static friction after the student stops pushing. Saying the book moves at constant velocity because there is no friction. Forgetting to mention the direction of the net force during acceleration.
Mark scheme (5 marks)
- During acceleration: the student's pushing force is greater than the friction force, so there is a resultant force in the direction of motion
- During constant velocity: the pushing force equals the friction force, so there is no resultant force (forces are balanced)
- After the student stops pushing: friction is the only horizontal force, so there is a resultant force opposing motion, causing the book to decelerate and stop
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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