A2L Item 177

• Description: Identify the angular acceleration of a disk free to move and unwind.
• Goal: Problem solving with rotational dynamics
• Source: UMPERG-ctqpe135.5
• Keywords: Angular Acceleration, Mechanics, Rotational Dynamics, Torque

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The question for students:

A disk, with radius 0.25 m and mass 4 kg, lies on a smooth horizontal table. A string wound about the disk is pulled with a force of 8N. What is the angular acceleration of the disk about its center?

1. 0
2. 64 rad/s²
3. 8 rad/s²
4. 4 rad/s²
5. 12 rad/s²
6. None of the above.
7. Cannot be determined

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Commentary for teachers:

Answer

(6) The correct value of α_cm is 16 rad/s². Students have the erroneous concept of ‘conservation of force’. Many think that since the disk moves, the full force cannot contribute to the torque about the center of the disk.

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