# Tension in string and light rod in vertical circle

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?

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.