# How does the weight of the pendulum's bob cause a torque? I know that a force will cause a torque if the force has a perpendicular component to the distance from the pivot point. However, in the figure above, I can't seem to visualize how the weight of this bob would create a torque about the pendulum's suspension point?

EDIT: I know how to calculate the torque mathematically but I can't seem to understand the mechanism through which the toque is applied on the suspension point... does the string have a role in it?

• Can you expand a bit on why you can't see there is a torque. If you extend the $m\vec{g}$ vector upwards it meets the horizontal line at a distance $L\cos 60$ so the force times the perpendicular distance is obviously not zero. Were you wondering how this torque is actually physically created? – John Rennie Oct 2 '14 at 9:40
• @John Rennie: Sorry, I couldn't phrase my question properly. I knew how to calculate the torque as in any normal question but I can't seem to understand "how this torque is actually physically created?" – Eliza Oct 2 '14 at 11:34
• Comment to the question (v2): Are you asking to the role of a flexible massless string vs a rigid massless rod? – Qmechanic Oct 2 '14 at 12:49
• @Qmechanic: Yes – Eliza Oct 2 '14 at 13:36

The weight vector, $W=mg$, can be split up into two components, $mg\sin\theta$ and $mg\cos\theta$, where $\theta = 30$ in your case. The $mg\sin\theta$ is the force vector which would generate a torque, $\tau = \bf{L}\times{}mg\sin\theta$. 