When you whirl a small rubber stopper on a cord in a vertical circle, you find a critical speed at the top for which the tension in the cord is zero. At this speed. the force of gravity on the object 1S itselfisufficient to supply the necessary centripetal force. How slowly can you swing a 15-g stopper like this so that It will just follow a circle with a radius of m?

  √(9.8/m) radians per second

Explanation:

The centripetal acceleration required for a radius of m is ...

  a = mω²

so, the angular speed required is ...

  ω = √(a/m) =  √(g/m) . . . . . where g is the acceleration due to gravity

  ω = √(9.8/m) . . . radians per second

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The speed is independent of the mass. For a 1 m radius, the speed is about 29.9 RPM.

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hope this : )