Any time we have a circular motion problem for some object, we need to think in terms of using our generic mv^2/R for the net centripetal force. As always, draw the force diagram. But this time, also note where the center of the circle is, and draw in the radius line. Use the picture to set up the math! Anything that points along the radius towards the center is a positive centripetal force, and anything that points away from the center is a negative centripetal force (i.e. centrifugal force). Check out both horizontal and vertical examples.