Distance for a full circle = 2*Pi*R
Distance for a certain angle = ((2*Pi*R)/360)*angle .. R being distance(object.x,object.y,pivot.x,pivot.y) ... on the moment of collision.
Speed = distance / time
It must travel at 20 pixels / tick
So for 1 tick .. ((2*Pi*R)/360)*angle should be = 20
((2*Pi*R)/360)*angle = 20 ... how much angle to rotate per tick ? (R is known)
((2*Pi*R)/360) / 20 = 1/angle ... or angle = 20 / ((2*Pi*R)/360)
Hope this stands, not that big of a math brain. Dont forget to 'dt'.