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Homework Help: Photons and mometum

  1. Oct 11, 2013 #1
    1. The problem statement, all variables and given/known data
    In space near earth, about 3.84*10^21 photons are incident per square meter. On average, the momentum of a photon is 1.3*10^-27. Assume we have a 1205 kg spaceship, and a square sail that is 26.3m wide.
    How fast could the ship be travelling after 21 days?

    2. Relevant equations

    3. The attempt at a solution
    I started out by finding the force the photons would have on 1 m^2 and then multplied that by 26.3 to get the force on the sail
    [itex] (1.30*10^-27)(*3.84*10^+21)*(26.3^2) [/itex]
    and got .00345 N
    Since F=ma, I divided that by 1205 to get 2.865*10^-6
    I then took 21 days and changed it to 1814400 s. So i multiplied 2.865*10^-6 by1814400 to get 5.19914 m/s. Did i go wrong somewhere cause the computer says im wrong.
  2. jcsd
  3. Oct 11, 2013 #2


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    Is any information given about whether or not the photons are reflected back by the sail?
  4. Oct 11, 2013 #3

    Andrew Mason

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    Your method is right. It may be a matter of significant figures. Try to work out the solution using symbols and plug in numbers at the end. It makes it easier to follow:

    Fsail = dpship/dt= ndpph/dt = σAdpph/dt where σ is the number of photons per unit area

    mΔv = FΔt = ΔtσAdpph/dt

    Last edited: Oct 11, 2013
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