In a dramatic lecture demonstration, a physics professor blows hard across the t

Question

In a dramatic lecture demonstration, a physics professor blows hard across the top of a copper penny that is at rest on a level desk. By doing this at the right speed, he can get the penny to accelerate vertically, into the airstream, and then deflect it into a tray, as shown in the figure. Assuming the diameter of a penny is 1.90 cm and its mass is 2.80 g, what is the minimum air speed needed to lift the penny off the tabletop? Assume the air under the penny remains at rest.

FIGURE: http://session.masteringphysics.com/problemAsset/1131899/1/Figure_9.42.jpg

***I've put this problem up on Cramster before and the person gave me the answer of 12.7 which Mastering Physics said was close but not quite right. I only have 1 guess left before I get the answer wrong and I really need to get this one right! Thank you so much!

Explanation / Answer

penny mass = 2.80 g = 2.80 x 10-3 kg penny weight = 2.80 x 10-3(g) = 2.80 x 10-3(9.81) = 2.75 x 10-2 N penny area = [pD²]/4 = [p(2.1 x 10-2)²]/4 = 3.46 x 10-4 m² The pressure difference required to lift penny = 2.75 x 10-2/3.46 x 10-4 = 7.93 x 10-1 = 79.3 N/m² V²/2 x density of air = pressure difference (solve for V to get answer) density of air = 1.20 kg/m³ (standard) (V²/2)*(1.20) = 0.60V² = 79.3 V² = 79.3/0.60 = 132.167 V = 11.496 m/s

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