IB Physics HL A.4 Rigid body mechanics questions

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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 st=3.0\ \text{s}

0123456-8-6-4-20246t / sω / rad s-1
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Answer: C. At t=3.0 st=3.0\ \text{s} the graph gives ω=+2 rad s−1\omega=+2\ \text{rad s}^{-1} so the wheel rotates anticlockwise. The segment has a negative slope, so α<0\alpha<0 because ω\omega and α\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.

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