A uniform rod of mass 2 kg and length 1.41 m is hinged at its top end and allowed to swing in the vertical plane of the paper. It is initially stationary in a vertical position. A clay ball of mass 0.25 kg is propelled towards its lower end at a speed v. The ball sticks to the end of the rod on impact. What ball speed v is needed for the rod and the clay ball to rise to an angle of 90 degrees before stopping and falling back?

Question

A uniform rod of mass 2 kg and length 1.41 m is hinged at its top end and allowed to swing in the vertical plane of the paper.  It is initially stationary in a vertical position.  A clay ball of mass 0.25 kg is propelled towards its lower end at a speed v. The ball sticks to the end of the rod on impact. What ball speed v is needed for the rod and the clay ball to rise to an angle of 90 degrees before stopping and falling back?

I first used the conservation of angular momentum (mvr = Iw) to get that v = 5.17w.

Then I used the conservation of energgy theorem (0.5Iw^2(initial) + mgh(initial) = 0.5Iw^2(final) + mgh(final)). However I get my answer wrong. I cannot understand why there is gravitational potential energy at the initial position because the center of mass has not moved yet. And does the center of mass not change when the clay ball sticks to the rod.

Please help?

Thank you.