Is air resistance present in uniform circular motion?
In this case air resistance is going to have two components acting on your rubber stopper. One component is going to act in the tangential direction and we would call it Drag. Assuming you are trying to calculate the centripetal force acting on the stopper, this drag force will not have any effect as it is orthogonal to the centripetal force. Assuming the rubber stopper is not symetrical, it is possible that there will be another component of air resistance which we will call Lift that will have an effect on your calculated value for the centripetal force. However it is probably negligable.
