What force must provide the centripetal acceleration of the block?

[arildno]

Just a final note:
The problem you've just solved is a rather rough approximation, since you haven't taken into account the actual rope length!
If you did do that, you should be able to figure out that the centripetal acceleration is
\frac{v^{2}}{R+L\sin\theta}
where L is the rope length.
When L<<R, the approximation is very good..

 

[Winner]

Hmm, well the length of the rope is signficantly smaller than the radius, so all is good