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Force exerted by photons

  1. Dec 7, 2006 #1
    1. The problem statement, all variables and given/known data
    It is desired to move a small 50 kg space vehicle by a lamp which emits 100 watts of blue light(lambda= 4700 Angstrom). If the vehicle is in free space, what will its acceleration?

    2. Relevant equations

    3. The attempt at a solution
    The solution as given in my book is as follows:

    Here we take the particle nature of light. Since the emitted light carries momentum the change in momentum per second will be responsible for exerting force on the vehicle.

    Suppose the lamp emits N photons per second. Then,
    Nhf = Power of lamp
    Where h = Planck’s constant, f = frequency of light
    On simplification we get,
    N= 2.4 x 10^20
    Each photon will have a momentum given by,
    p = h/lambda
    = 1.4 x 10^(-27) N – sec
    The total force on the vehicle as per Newton’s 2nd law is,
    F = d/dt(Np)
    = p(dn/dt)
    = 3.4 x 10^(-7) N
    acceleration = F/mass
    = 6.8 x 10^(-9) m/sec^2
    I understood till the momentum calculation. I didn’t understand the force calculation. I calculated it in the following way:
    The change in momentum of each photon after striking the space vehicle per second= 2p
    change in momentum of N photons per second = 2pN
    F = dp/dt
    = 2pN
    = 3.32 x 10^(-6) N

    Could somebody please guide me?
  2. jcsd
  3. Dec 7, 2006 #2


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    Gold Member

    Well what you have done is tried to calculate the momentum change of the photon when what you want is the momentum imparted to the ship by the photons.
  4. Dec 7, 2006 #3


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    Homework Helper

    I don't see how you got your number from computing 2pN. If you use their values of p and N, you should get twice their result. They apparently assumed the photons are being absorbed, while you are assuming they are being reflected, so your force is twice as much as theirs. That will give you a factor of 2 difference between your result and theirs.
  5. Dec 7, 2006 #4
    You are right. I never thought about it. Thanks.
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