How to calculate the intensity of light, given area and mass

AI Thread Summary
To calculate the intensity of light needed to support a fireproof paper sheet measuring 20.8 cm by 28.8 cm and weighing 1.70 g, the area is determined to be 0.0599 m². The intensity (I) can be calculated using the formula I = P/A, but the power (P) is initially unknown. The discussion highlights that since the paper absorbs all the light, this fact is crucial for understanding how momentum conservation applies to the problem. Ultimately, the participants conclude that acknowledging the absorption of light is key to solving the intensity requirement. The conversation emphasizes the relationship between light intensity, area, and the physical properties of the paper.
Michael Moynihan
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Homework Statement



You want to support a sheet of fireproof paper horizontally, using only a vertical upward beam of light spread uniformly over the sheet. There is no other light on this paper. The sheet measures 20.8 cm by 28.8 cm and has a mass of 1.70 g .
(a) If the paper is black and hence absorbs all the light that hits it, what must be the intensity of the light beam?[/B]

Homework Equations


I=P/A
A=w*h

The Attempt at a Solution


A=0.0599m^2
I do not know power, so I do not know the intensity. What information does saying that the paper absorbs all the light that hits it actually give me?
 
Physics news on Phys.org
Each photon can have momentum, as well as having energy. I'm on my phone, but you can search for momentum photons to get related information.
 
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Michael Moynihan said:
What information does saying that the paper absorbs all the light that hits it actually give me?
It affects the momentum conservation equation.
 
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Thanks guys! Got it!
 

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