Concepts Of Physics MCQ Edition [Volume 1]PhysicsRotational Mechanics
A simple pendulum of length l is displaced to form an angle heta with the vertical. Calculate the magnitude of the torque exerted by the weight w of the bob about the point of suspension, and state when this torque is zero.
Options
- Awl , when the bob is at the lowest point
- Bwl , when the bob is at an extreme position
- Cwl , when the bob is at the lowest point
- Dwl , when the bob is at an extreme position
Correct answer
C. wl , when the bob is at the lowest point
Step-by-step solution
The torque about the point of suspension is given by the cross product of the position vector r and the force F . Here, the force is the weight w acting vertically downwards, and the magnitude of the position vector is the length of the pendulum l . The angle between the position vector and the force vector is . The magnitude of the torque is = | r F | = l w . The torque is zero when = 0 , which implies = 0 , giving = 0 . This condition corresponds to the bob being at its lowest point.