The Trajectory of a Steel Ball Passing Close to a Magnet

If a comet or a spacecraft passes close to a planet, the orbit will be deflected by the gravitational pull of the planet. The object may even be captured in a circular or elliptical path around the planet if the gravitational pull is large enough.1,2 A similar result arises if a charged particle passes close to another charged particle of opposite sign. The deflection angle can be calculated theoretically but it is a relatively advanced topic and is not usually presented in introductory mechanics or atomic physics classes. Nevertheless, the basic physics can be understood from ``back-of-the-envelope" calculations and is illustrated in the present paper by an experiment using a steel ball projected past a magnet.