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March 3, 2015

March 3, 2015

Posted by **Chao** on Saturday, May 7, 2011 at 11:29pm.

Assuming the diameter of a penny is 1.70 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.

- Physic -
**drwls**, Sunday, May 8, 2011 at 8:26amRequire that the Bernoulli-effect pressure difference above and below the penny balance the weight.

(1/2)(rho)V^2*pi*D^2/4 = M g

M is the penny's mass and rho is the density of air, about 1.3 kg/m^3.

Solve for V.

V = sqrt[8*M*g/(pi*D^2*rho)]

= 13.6 m/s

(about 30.5 mph)

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