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Homework Help: Need help on another electromagnetic problem

  1. Mar 14, 2005 #1
    A uniform circular disk of mass 27.0 g and radius 37.0 cm hangs vertically from a fixed, frictionless, horizontal hinge at a point on its circumference. A horizontal beam of electromagnetic radiation with intensity is incident on the disk in a direction perpendicular to its surface. The disk is perfectly absorbing, and the resulting radiation pressure makes the disk rotate. Find the angle through which the disk rotates as it reaches its new equilibrium position. (Assume that the radiation is always perpendicular to the surface of the disk.)

    I used the formula P = S/c, where P is the pressure and S is the Poynting vector and c is the speed of light. I made P = (2*S*cos(theta)^2)/c and solved for theta, but the answer was incorrect. I kind of felt this wasn't the right way of doing it because I didn't use the mass or radius in the problem. I need some help...any help would be greatly appreciated. Thanks! :)
  2. jcsd
  3. Mar 14, 2005 #2


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    One needs to determine the net force of the light. Apply the force at the center of mass.

    The center of mass rotates upward to a new elevation and the mg[/itex]\Delta h[/itex], The light pressure (force) balances the lateral (horizontal) component of the gravity force. Assume small angle.
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