you throw a coin straight up, how many forces act on it midair? The textbook answer is “one”: after it leaves your hand, the only force on the coin is gravity.63 The popular answer, however, is “two”: the force of the throw keeps sending it up, and the force of gravity keeps dragging it down. Popular with whom? Virtually everyone—physics students included.64 At the beginning of the semester, only 12% of college students in introductory mechanics get the coin problem right. At the end of the semester, 72% still get it wrong. After students learn how to handle complex homework and exam problems,
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