A 8.0-m massless rod is loosely pinned to a frictionless pivot at 0. A 4.0-kg ball is attached to the other end of the rod. The ball is held at A, where the rod makes a 30° angle above the horizontal, and is released. The ball-rod assembly then swings freely in a vertical circle between A and B. In Fig. 7.1a, the tension in the rod when the ball passes through the lowest point at C is closest to:

The answer is 98N but I dont know how to get it.

Well you first need to find potential energy.

PE= mgh = 4 kg (9.81 m/s^2) (1+sin 30)

Then use conservation of energy to get kinetic energy

PE=KE=1/2 m v^2

Solve for v

Once you have v, you can get tension

Tension = m g + (m v^2)/L

I'm not getting an answer consistent with 98N, but maybe I goofed...

This post was edited by Azrad on Apr 8 2013 06:29am