IB · A · multiple choice
Space, time and motion · Question 2
Space, time and motion · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- demanding
- Marks
- 1
- Topics
- 1
- Answer
- Complete
A book rests on a table. Which pair of forces is a Newton's third law pair?
- A
the weight of the book and the normal force of the table on the book
- B
the weight of the book and the force of the book on the table
- C
the normal force of the table on the book and the force of the book on the table
- D
the weight of the book and the weight of the table
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Answer and reasoning
Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.
- 1
The correct choice is C: the normal force of the table on the book and the force of the book on the table
- 2
A is chosen by most candidates because the two forces are equal and opposite — but they act on the same body and are of different types, so they are a balanced pair, not a third law pair. A third law pair acts on two different bodies and is always the same kind of force.
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