One of the most fun demonstrations in a freshman mechanics class is the levitation of a ball in a steady air stream even when the jet is directed at an angle. This and other demonstrations1 are often used to argue for the validity of Bernoulli's principle. As cautioned by some authors,2–4 however, it is important to avoid making sweeping statements such as “high speed implies lower pressure” with respect to interpreting the popular demonstrations. In this paper I present a demonstration that can be used in conjunction with the discussion of Bernoulli's principle to encourage students to consider assumptions carefully. Specifically, it shows that a correlation of high speed with lower fluid pressure is not true in general.
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2007
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