A horizontally oriented tube is rotating with angular velocity ω about one end of the tube which is open. The tube has ideal liquid filled inside it as shown in the attachment. Why does there have to be a pressure difference between two points in the ideal liquid at the same horizontal level? Doesn't pressure only change with depth and remains constant for all points on the same horiozontal level for a stationary liquid? Does the pressure difference exist so as to create an unbalanced force which accounts for the centripetal force on the liquid? But centripetal force exists only when a particle is executing circular motion. And when the liquid would have just been poured in the tube, it would slided back towards the closed end of the tube due to centrifugal force. And in the process of doing so (sliding back), there wasn't any centripetal force. So why then all of a sudden does the liquid halt at the end of the tube and starts executing circular motion all of a sudden when it was not doing so anywhere before halting? Why doesn't the liquid keep moving backward because of the centrifugal force and burst open the closed end of the tube?