IB Physics HL A.4 Rigid body mechanics questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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Multiple choice Written

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Question 1

Multiple choiceMedium1 mark

A wheel rotates about a fixed axis. The graph shows its angular velocity; anticlockwise is defined as the positive direction. Which pair describes the wheel’s motion at $t=3.0\ \text{s}$

Show answer and explanation

Answer: C. At $t=3.0\ \text{s}$ the graph gives $\omega=+2\ \text{rad s}^{-1}$ so the wheel rotates anticlockwise. The segment has a negative slope, so $\alpha<0$ because $\omega$ and $\alpha$ have opposite signs, the angular speed is decreasing.

A: identifies the rotation direction correctly but assumes negative angular acceleration means increasing speed.
B: reverses the stated sign convention: positive angular velocity is anticlockwise.
D: reads the angular velocity as negative; at this time it is positive, while the graph’s slope is negative.
Remember: angular speed decreases when angular velocity and angular acceleration have opposite signs.

Question 2

Paper 2Medium4 marks

A horizontal turntable rotates about a fixed vertical axis. Its moment of inertia, excluding two identical small masses, is $0.40\ \mathrm{kg\,m^2}$ The masses are moved inward by radial forces through the axis. The bearing exerts negligible torque about the vertical axis. Treat the masses as point particles and the arms as having negligible mass.

Figure 1 shows the initial and final positions of the masses.

(a)

Determine the angular speed of the turntable after the masses have moved inward.

[3]

(b)

Explain why the angular momentum of the turntable–masses system about the vertical axis remains constant while the masses move.

[1]

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