IB · A · multiple choice
Space, time and motion · Question 5
Space, time and motion · Original GioPhysics question with a detailed, mark-by-mark answer guide.
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- Topics
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multiple choice1 mark
A disc of moment of inertia I rotates freely about a vertical axis at angular speed ω. A second, stationary disc of moment of inertia 2I is dropped onto it and the two rotate together. What is the new angular speed?
- A
ω/3
- B
ω/2
- C
2ω/3
- D
3ω
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Answer overviewCorrect answer: A — ω/3
01
1 markAnswer and reasoning
How to approach it
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 A: ω/3
- 2
No external torque acts about the axis, so Iω = (I + 2I)ω′ and ω′ = ω/3. B is the answer of a candidate who assumes the second disc is identical to the first.
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