Question

A disc of moment of inertia \( 3.5 \, \text{kg m}^2 \) is rotating at \( 30 \, \text{rad/s} \). Torque to stop it in 5 seconds is:

• A. \( 84 \, \text{Nm} \)
• B. \( 42 \, \text{Nm} \)
• C. \( 10.5 \, \text{Nm} \)
• D. \( 21 \, \text{Nm} \)

Hint:

To compute torque when stopping rotation: \( \tau = I \cdot \frac{\omega}{t} \)

Answer 1

The Correct Option is D

Use: \( \tau = I \alpha \), where \( \alpha = \frac{\omega}{t} = \frac{30}{5} = 6 \, \text{rad/s}^2 \)
\[ \tau = 3.5 \cdot 6 = 21 \, \text{Nm} \]