Bullet and Pendulum Problem

1. The problem statement, all variables and given/known data

diagram: http://goo.gl/uPl0

Part A
A bullet of mass 0.0175 kg and speed vb passes completely through a pendulum bob of mass 1.3 kg. The bullet emerges with a speed 0.5vb. The pendulum bob is suspended by a stiff rod of length 1.1 m and negligible mass. The acceleration of gravity is 9.8 m/s2. What is the critical value v, such that when vb > v , the pendulum bob will barely swing through a complete vertical circle?

Part B

Suppose that the pendulum bob is suspended from a light flexible cord instead of a stiff rod.
Now what is the minimum value of v such that the pendulum bob will swing through a
complete vertical circle?

2. The attempt at a solution

I was able to get part A by using conservation of momentum and conservation of energy and ended up with 975.61 m/s for v.

However, I am stuck on part B and do not know how to proceed. I know it has something to do with the tension of the cord, but am not exactly how to factor that in.

Any help would be much appreciated. Thanks!
help anyone?
never mind, i got it! i figured out that at the top of the pendulum, centripetal force had to be equal to the force of gravity because there is no tension to support the bob.

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