Frame of reference, would have to be formulated like: Only the water “rotates” with a certain angular velocity, its surface is a plane. The deviation from a plane increases with the deviation from this particular state of motion. The state of rest produces also a paraboloid. Again the rotation of the Pail is immaterial. Newton’s Pail experiment brings out very clearly what is meant by “suitable” Frame of reference. We can describe nature and we can formulate its law using whatever Frame of reference we choose.