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How Much Does A Shadow Weigh?

More than I thought (well less).

submitted by: uatme
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How Much Does A Shadow Weigh?. More than I thought (well less).
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Hits: 10202 | Favorites: 4 | Emailed: 0 | Rating: 3.3 | Category: Science | Date: 10/13/2012
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Male, 30-39, Canada
 5597 Posts
Monday, October 15, 2012 8:04:15 AM
So, to answer pumba62's question, yes; if the light was below you, you would weigh less as it would be slightly lifting you up.

This is technically true, light does exert a force, but not at the same speed as the light, rather, with the same momentum as the light. The momentum (p) of a single photon of light is equal its energy divided by the speed of light (E = c p). So how much momentum does a beam of light carry? Well, suppose we shine a 100 Watt spotlight. We are producing 100 Joules of light energy per second. So every second we are giving that light a momentum of (100J)/(300,000,000m/s) = .00000033 kg m/s.

NASA actually has plans for prototypes of a "solar sail" to push space probes out of the solar system using no propulsion other than that provided by reflected sunlight.

Male, 18-29, Europe
 164 Posts
Saturday, October 13, 2012 3:43:22 PM

Male, 18-29, Western US
 555 Posts
Saturday, October 13, 2012 2:41:45 PM
Cool post. Enjoyed it.

Male, 18-29, Eastern US
 2549 Posts
Saturday, October 13, 2012 1:18:38 PM
Yo' mama`s so fat, her shadow weighs 42 pounds!

Male, 40-49, Canada
 4089 Posts
Saturday, October 13, 2012 12:31:38 PM
The light would have to be concentrated, because if it hit you anywhere else, in any other direction, it would apply force there as well, right?"

Partially true. The reflected light would have lost a large amount of its energy, after hitting the surface it reflected off of. Consequently, the energy pushing down would be more than the energy pushing up. So, it would still be pushing you down.

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