1. Nov 18, 2005

### jarfungus

Hello! First time on this site, so I hope I do this right. I have a homework question that I could use some help on:

(From Physics for Scientists and Engineers, Third Ed. Fishbane, Gasiorowicz, Thornton)

Chapter 34 #39.
Suppose that you want to use the radiation pressure from a beam of light to suspend a piece of paper in a horizontal position; the paper has an area of 50 cm^2 and a mass of 0.20 g. Assume that their is no problem with the balance, that the paper is dark and absorbs the beam fully, and that the entire beam can be used to hold the paper against the pull of gravity. How many watts must the light produce? Given your answer, what do you think will happen to the paper?

Since power=c*u*A, I can easily calculate the amount, right? The problem comes when solving for u. I know that u=1/u0*B^2. But how can I find the magnitude of the magnetic field with the information given? Would it be right to manipulate the formulas so that B=E^2/(sqrt(1-c^2))? (Then what is E?) ...

2. Nov 18, 2005

### jarfungus

I got it:)

Nevermind!! I think I figured it out. I forgot the u=F/A, and the force is m*g....so I am all set:)
Thanks for all the viewing.