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