Modern coin vending machines use a confi guration of magnets to help sort appropriate currency from fake or foreign coins. When a coin is dropped through the slot, it passes within a few millimeters of the magnets as it falls. Light sensitive sensors detect the passage of the coin as it breaks a beam at the top and bottom of the magnet assembly. How might the magnets help reject fake or foreign coins? How would the time of passage of a zinc coin compare with that of a coin made with copper in the presence of the magnetic fi eld? (Modern pennies are copper-plated zinc, and modern dimes and quarters have a layer of copper sandwiched between the face and reverse.)