Bakliwal Tutorials - IIT

Ok, firstly, what does arc(cosTHETA) mean??
Here's the question.......

Q: A thin weightless, rigid rod with a point mass at one end is deflected by a small angle ALPHA from its mean position and then released (like a pendulum).

At the moment when the rod forms an angle BETA with the vertical, the ball undergoes a completely elastic collision with an inclined wall.

Determine the ratio T*/T of the period of oscillations of this pendulum to the period of a simlpe pendulum of same length.

I was quite close to the answer, but got it wrong.