Explain why a book resting on a table remains stationary, using Newton's laws of motion. Include in your answer an identification of the action–reaction pair of forces acting in this situation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The book’s weight, a gravitational force of magnitude mg acting downwards, is exerted by the Earth on the book. The table exerts a normal contact force of equal magnitude mg acting upwards on the book. The two forces are equal and opposite, so the net force on the book is zero. By Newton’s first law a body with zero net force remains at rest, so the book stays stationary.
The action–reaction pair is: the Earth pulls the book downward (weight) and the book pulls the Earth upward with an equal and opposite gravitational force.
The action–reaction pair is: the Earth pulls the book downward (weight) and the book pulls the Earth upward with an equal and opposite gravitational force.
Examiner tips
- State the weight and normal force with correct directions. Show they are equal and opposite. Explain that the net force is zero, invoking Newton’s first law. Identify the action–reaction pair correctly.
Common mistakes
- Confusing the normal force with the weight or giving the wrong direction. Forgetting to mention that the forces act on different bodies. Using the wrong law (e.g. Newton’s second law) instead of the first law for equilibrium.
Mark scheme (4 marks)
- The weight of the book (gravitational force on the book by the Earth) acts downwards on the book.
- The table exerts a normal contact (reaction) force on the book directed upwards, equal in magnitude to the book's weight.
- Since the net/resultant force on the book is zero, by Newton's first law the book remains stationary (in equilibrium).
- The Newton's third law action–reaction pair is: the Earth pulls the book downward (weight) AND the book pulls the Earth upward with an equal and opposite gravitational force — these two forces act on different objects.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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