Radiation pressure of a light bulb

1. Jan 18, 2012

aytrees

1. The problem statement, all variables and given/known data
Estimate the radiation pressure due to a 100 W bulb at a distance of 8.4 cm from the center of the bulb.

2. Relevant equations
Power= Work/Time W=Force(distance)

Pressure = Force/Area = Poynting vector/speed of light

3. The attempt at a solution
I found the cross sectional area by doing 4∏ r^2, so I have that. Then, seeing as I was given power and distance, I tried to figure out the Force in terms of Power and distance, but in doing that, I got the formula F= Power (Time)/distance, and I wasn't given the time. Is there some formula that I'm missing?

2. Jan 18, 2012

lightgrav

distance/Time is velocity ... so your "(Time)/distance" = 1/c .
This means that your F= Power/c ... Pressure = Intensity/c .

Note that this is only true if the 100W of light is absorbed
(light being reflected would double the pressure)