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A disc starts from rest and rotates about its central axis with constant angular acceleration. At one instant its angular speed is 12 rad/s, and after a further angular displacement of 80 rad its angular speed becomes 28 rad/s. How long (in seconds) does it take to undergo this 80 rad displacement?

1. 2 s
2. 4 s
3. 5 s
4. 6 s

Correct answer: 4 s

Solution

From 28² = 12² + 2*alpha*80, alpha = 4 rad/s²; then t = (28 - 12)/4 = 4 s.

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