When the quarter jumps into a cup (and when it does not)

Abstract

While Bernoulli's equation is one of the most frequently mentioned topics in Physics literature and other means of dissemination, it is also one of the least understood. Oddly enough, in the wonderful book "Turning the world inside out" [1], Robert Ehrlich proposes a demonstration that consists of blowing a quarter dollar coin into a cup, incorrectly explained using Bernoulli's equation. In the present work, we have adapted the demonstration to show situations in which the coin jumps into the cup and others in which it does not, proving that the explanation based on Bernoulli's is flawed. Our demonstration is useful to tackle the common misconception, stemming from the incorrect use of Bernoulli's equation, that higher velocity invariably means lower pressure.