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Homework Help: The thrust from a flashlight?

  1. Oct 11, 2009 #1
    What would be the thrust from a 1W light output flashlight, assuming light is not scatered at all.
    I'm not sure but if it depends on the wavelength, assume it only emits 500nm light.

    and if I were to put a perfect mirror right infront of this flashlight would the mirror recieve the same amout of force?
    please breifely explain how it's calculated. (the equation and the Plank's constant?)
  2. jcsd
  3. Oct 12, 2009 #2


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    Science Advisor

    Well, light has momentum, so you can just use the standard conservation laws and the equation for light's momentum to calculate any "thrust", or reaction force, you wish. As far as mirrors are concerned, remember momentum is a vector.
  4. Oct 12, 2009 #3


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    Staff: Mentor

    Welcome to the PF, Jacob. Per the PF Rules link at the top of the page, we shouldn't offer you help until you post the relevant equations and show some work/effort on your part. That's how we try to help you learn how to learn, eh?
  5. Oct 14, 2009 #4
    well I can garentee you I'll never be asking about any homeworks. I only ask for personal interest. That's the reason for my simple numbers, because it's just a question i randomly made up.
    it's not like i'm trying to make anyone to work for me or anything like that.

    so you see if I didn't even try, and you tell me the answer, i don't benifit at all. and I know the answer is going to be so small it won't make any sense in this application.
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