Two square reflectors, each 1.85 cm on a side and of mass 4.25 g, are located at

Question

Two square reflectors, each 1.85 cm on a side and of mass 4.25 g, are located at opposite ends of a thin, extremely light, 1.00-m rod that can rotate without friction and in a vacuum about an axle perpendicular to it through its center (see figure below). These reflectors are small enough to be treated as point masses in moment-of-inertia calculations. Both reflectors are illuminated on one face by a sinusoidal light wave having an electric field of amplitude 1.50 N/C that falls uniformly on both surfaces and always strikes them perpendicular to the plane of their surfaces. One reflector is covered with a perfectly absorbing coating, and the other is covered with a perfectly reflecting coating. What is the angular acceleration of this device?
     rad/s2

Two square reflectors, each 1.85 cm on a side and of mass 4.25 g, are located at opposite ends of a thin, extremely light, 1.00-m rod that can rotate without friction and in a vacuum about an axle perpendicular to it through its center (see figure below). These reflectors are small enough to be treated as point masses in moment-of-inertia calculations. Both reflectors are illuminated on one face by a sinusoidal light wave having an electric field of amplitude 1.50 N/C that falls uniformly on both surfaces and always strikes them perpendicular to the plane of their surfaces. One reflector is covered with a perfectly absorbing coating, and the other is covered with a perfectly reflecting coating. What is the angular acceleration of this device?

Explanation / Answer

2. Relevant equations
Near as I can tell:
Emax = cBmax
Prad for Perfect absorbtion = Intensity/c
Prad for Perfect reflection = 2*Intensity/c
Prad = F/Area
Intensity = (Emax * Bmax)/(2?o)
Moment of Inertia for Rod = (mL2)/12
Parallel Axis Theorem: Icm=I + md2
Angular acceleration ? = ?2r
Angular frequency ?2 = mgd/Inertia


3. The attempt at a solution
I calculated Bmax and used 4?x10^-7 for ?o to get the Intensity of the light. Considering the light is hitting both cubes, both receive a Radiation Pressure corresponding to whether they absorb the waves or reflect them. I assume the forces on the areas will cancel to some degree, leaving a Radiation Pressure of I/c on the right cube (that one looks like it is supposed to be the reflecting cube). Trying to yield an acceleration by using the equation a = I(A)/mc (since pressure is force/area), where m is the mass of both cubes gives me horribly large and horribly wrong results.

The moment of inertia for a rod is normally (mL2)/12, but the question claims the rod is "extremely light", which I seem to notice usually means the rod has negligable mass. I resort to the Parallel Axis theorem to solve for the moment of inertia of this magic device, which would mean Icm= 2md2, since both cubes are of equal masses and equal distance from the pivot point of the rod. Substituting in for mgd/Inertia to get ?2 and multiplying by r to get ? yields incorrect results.

I suppose what I'm asking is whether I'm on the right track (which doesn't seem likely, considering my results being so unrealistic) and what I should consider when facing an equation where "m" is needed, as I'm a bit confused as to whether I should be using the mass of both cubes or the mass of only one.

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