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AP · AP C:M · multiple choice

AP Physics C: Mechanics · Question 4

AP Physics C: Mechanics · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
demanding
Marks
1
Topics
1
Answer
Complete
A uniform thin rod with its axis of rotation through one endaxis, perpendicular to the pageuniform rod, mass ML
Fig. 4.1A uniform thin rod is drawn horizontally across the figure and labelled as having mass M. At its left-hand end a small circle with a dot at its centre marks the axis of rotation, which is perpendicular both to the rod and to the page; a short dashed leader connects that symbol to the words "axis, perpendicular to the page". A dimension line beneath the rod, with a tick at each end, runs from the axis to the far end of the rod and is labelled L.
multiple choice1 mark

A uniform thin rod of mass M and length L rotates about an axis perpendicular to the rod through one end. What is its moment of inertia about that axis?

  1. A

    ML²/12

  2. B

    ML²/6

  3. C

    ML²/3

  4. D

    ML²

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Answer overviewCorrect answer: C — ML²/3
01

Answer and reasoning

1 mark
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. 1

    The correct choice is C: ML²/3

  2. 2

    A is the moment of inertia about the centre. Applying the parallel-axis theorem, I = ML²/12 + M(L/2)² = ML²/3, and a candidate who quotes A has answered a different question.

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