AP · AP C:M · multiple choice
AP Physics C: Mechanics · Question 5
AP Physics C: Mechanics · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- demanding
- Marks
- 1
- Topics
- 1
- Answer
- Complete
A figure skater spinning with her arms extended pulls her arms in close to her body. Which of the following is true?
- A
Her angular momentum and her kinetic energy both stay the same.
- B
Her angular momentum stays the same and her kinetic energy increases.
- C
Her angular momentum increases and her kinetic energy stays the same.
- D
Her angular momentum and her kinetic energy both increase.
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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 B: Her angular momentum stays the same and her kinetic energy increases.
- 2
A is the intuitive answer and is wrong about the energy. With L = Iω constant and I decreasing, K = L²/(2I) must rise — the skater does work pulling her arms inward against the centripetal requirement, and that work appears as kinetic energy.
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