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.

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.

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  1. Require 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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