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Homework Help: Special Relativity - Relativistic Dynamics

  1. Mar 16, 2012 #1
    1. The problem statement, all variables and given/known data
    A.P. French 6.8
    A thrust-beam space vehicle works bearing a sort of sail which feels the push of a strong steady laser light beam directed at it from Earth. If the sail is perfectly reflected, calculate the mass of light required to accelerate a vehicle of rest mass [itex]m_0[/itex] up to a fixed value of [itex]\gamma[/itex].

    2. Relevant equations
    I usually define [itex]c=1[/itex] for convenience.
    [itex]m'=m_o \gamma[/itex]
    [itex]p = m_0v[/itex]
    [itex]E = m_0c^2 = m'c^2+ q = E' + q[/itex] q is the energy of the photon(s) emitted
    [itex]p=0=m'v - q/c = p' - q/c[/itex]
    [itex]cp' = q[/itex]
    3. The attempt at a solution
    Since the sail is perfectly reflective I view as if the vehicle is emitting photons. Since it is accelerated to [itex]\gamma[/itex] we get [itex]v = \gamma[/itex] so [itex]p' = m'v = m_0 \gamma ^2[/itex]. Also [itex]q = m_0 \gamma ^2[/itex]

    Honestly, I have little idea what I'm doing. I'm following French's book (Emission of photons p.177) and I keep running into dead ends. Any help would be appreciated.
    Last edited: Mar 16, 2012
  2. jcsd
  3. Mar 16, 2012 #2


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    Science Advisor
    Homework Helper

    Hi Jonmundsson! :smile:
    Hint: if a photon has "mass" m, by how much does its momentum change? :wink:
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