A ball is tied to the end of a cable of negligible mass. The ball is spun in a circle with a radius of 2m making 7 revolutions every 10 sec. What is the magnitude of the acceleration of the ball?

I know:
Circumference = 2 pi r
so
12.57m

Tangential velocity = Circumference revolution/ sec
so
8.79m

Centripetal acceleration = w ^2 r
so
????

(What goes in for w? Is it 2 pi revolutions/sec? Or pi revolutions/sec? Does it depend on the problem?) HELP SOON