I need a formula for an angle with which there is no frictional force during circular motion. No data. No hint. My teacher gave us such task on the latest test. She said that it is easy we just need to think. Nobody solved it. I don't know now if we are such fools or she is a poor teacher that gives exercises on tests without prior solving similar ones. To improve YOUR mood: she teaches on IB Diploma and she hasn't said anything this year in English and she spelled frictional force on test: "ficktional". Also we do mainly theory on lessons while tests are only tough exercises. Nevertheless any ideas? Is it commonly used formula? Can You show how to get to it?