What is the work done on a block as its radius decreases on a horizontal table?

[wolly6973]

A small block of mass 0.91 kg slides without friction on a horizontal table. Initially it moves in a circle of radius r0 = 0.63 m with a speed 1.5 m/s. It is held in its path by a string that passes through a small hole at the center of the circle. The string is then pulled down a distance of r0 - r1 = 0.12 m, leaving it at a radius of r1 = 0.51 m. It is pulled so slowly that the object continues to move in a circle of continually decreasing radius.
How much work was done by the force to change the radius from 0.63 m to 0.51 m?

I need help with setting this one up. I don't know what to do. Thanks.

 

[Office_Shredder]

The added force is incredibly small, so you can assume the centripetal acceleration is the same at the end as it was in the beginning. So calculate the new velocity based on that

 

[wolly6973]

So i get the final velocity as 1.667m/s, but I need acceleration to calculate work, and I don't have the time.