When a bob is attached to a string and whirled in a vertical circle tension in the string becomes zero for some specific velocity at specific angle from vertical but when bob is attached to light rod then why does not tension in the light rod becomes zero for any case?


1 Answer 1


The tension in the rod does become zero, at exactly the same point as for the string. The difference is that the rod does not buckle and go limp like the string when the tension decreases even further and becomes -ve. It continues moving in a circle, keeping the bob exactly the same distance from the pivot. That is because the rod is rigid and the string is not. The string can only prevent the bob from moving further away from the pivot, it cannot stop it from moving closer.

When a pendulum reaches the end of its swing the only force pulling the bob away from the pivot point, and causing the tension in the string, is the component of the bob's weight. When the end of the pendulum swing reaches $90^{\circ}$ this component becomes zero, so the tension becomes zero.

If the pendulum swings any higher than $90^{\circ}$ the component of weight now acts toward the bob, causing compression of the string. Strings fold up when you try to compress them, so the pendulum bob leaves the circle it was swinging on, and moves like a projectile until the string becomes taut again.

However, this does not happen if the pendulum bob is moving fast enough and its speed $v$ never reaches zero when it goes up above $90^{\circ}$. Then the pendulum swings in a full circle and keeps going in the same direction into the next circle.

The difference with a rod is that, because it is rigid, it doesn't fold up or buckle when compressed by a force. So the bob continues to move in a circular arc after the tension in the rod has changed to compression.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.