A.4.6 (HL)—Moment of inertia
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Understand Moment of Inertia
HL onlyRotational inertia
Moment of inertia measures resistance to angular acceleration about an axis. It depends on total mass and how far that mass is distributed from the axis.
Compare distributions
For the same mass and outer radius, more mass farther from the axis gives larger I. A ring therefore has greater rotational inertia than a disk of the same mass and radius.
Common trap
Moment of inertia is not determined by mass alone; always specify the rotation axis and distribution.
A.4.6 Exam Analysis
HL onlyAssessment in practice
2 marksHow it is assessed
The evidence asks which of a disk and ring reaches the bottom first, rewarding comparison of their rotational inertia and energy allocation.
Command terms
Explain
What earns marks
Compare the moment of inertia for each mass distribution about the same axis. The object with smaller I gains angular speed or reaches the bottom sooner under the same available energy and rolling constraints.
Watch for
Comparing only total mass and radius while ignoring that the ring places more mass farther from the axis.
Representative question
Question 1
[Maximum number: 3]
The disk and a ring, with the same mass and radius, are released from the top of the slope at the same time. Explain, without numerical calculation, which one will reach the bottom of the inclined plane first.
ring has a larger moment of inertia/ mass further from the axis of rotation ring will have smaller angular acceleration
OR
higher portion of/more energy is stored in rotational KE for ring
Reverse argument allowed in terms of the disk.
ring arrives last
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