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Question
A simple pendulum of length l is pulled aside to make an angle θ with the vertical. Find the magnitude of the torque of the weight ω of the bob about the point of suspension. When is the torque zero?
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Solution
Distance between the line of force and point of suspension, `r = l sinθ`
\[\text{Torque, }\overrightarrow{\tau} = \overrightarrow{F} \times \overrightarrow{r} \]
\[ \Rightarrow \tau = wr \sin\theta = wl\sin\theta\]
Here, w is the weight of the bob.
The torque will be zero when the force acting on the body passes through the point of suspension, i.e., at the lowest point of suspension.
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