What about the component of Newton's Second Law in the radial direction?
Recall that the bob doesn't move in the radial direction since
. By Newton's Second Law the tension T in
the rod, the radial component of the force due to gravity, and the
radial acceleration of the mass are in balance:

, m, T, and g in this
little quiz. Recall the diagrams on the Math Model Concepts page.
What is Fr?
It can be shown that the radial acceleration is given by

Now substitute these expressions for ar and Fr into the equation Fr=mar and solve for T.
What is the tension in the rod?