The problem is that I'm doing what seems to be perfectly reasonable algebra but I'm getting a result that makes no sense.(adsbygoogle = window.adsbygoogle || []).push({});

Let's say a photon travels from the left and hits a stationary solar sail which is perpendicular to the light. There is perfect reflection and a change in momenta for both objects. For momentum we have:

p_iphoton + p_iSS = p_fphoton + p_fSS =>

h/c*nu1 + m*(0)^2 = h/c*nu2 + mv

h/c*(nu1-nu2) = mv

energy:

KE_iphoton + KE_iSS = KE_fphoton + KE_fSS

h*nu1 + 0 = h*nu2 + mv^2

h*(nu1-nu2) = mv^2 = c*mv => v = c

(KE_iphoton is the photon's initial kinetic energy, p_iphoton is its initial momentum, KE_fphoton is the final KE, etc.; h = planck's constant, c = speed of light, m = mass of solar sail, v = final velocity of solar sail, nu1 = initial frequency of photon, nu2 = final frequency of photon.)

Obviously, the final speed of the solar sail isn't the speed of light. What did I do wrong?

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# Homework Help: Photon hitting a solar sail

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