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Homework Help: Distance question

  1. Apr 12, 2010 #1
    1. The problem statement, all variables and given/known data

    If the distacne of the star from earth is 4.2 light years away , how long does it take for light from the star to reach earth ?

    2. Relevant equations


    3. The attempt at a solution

    4.2 light years = 3 x 10^8 x 3600 x 365.25 x 4.2 m/s

    Is my conversion correct ?
  2. jcsd
  3. Apr 12, 2010 #2


    User Avatar
    Science Advisor

    But why are you converting "light years" to meters per second?

    The definition of "light year" is "the distance light travels on one year".

    Light will travel 4.2 light years in 4.2 years!
  4. Apr 12, 2010 #3
    because the speed of the light is 3 x 10^8 m/s , its in meters per second and i want to make sure that the unit matches ,

    it cant be s=vt , 4.2=(3 x 10^8) t ?
  5. Apr 12, 2010 #4


    Staff: Mentor

    The question is a "gimme." To repeat what HallsOfIvy said, it takes 4.2 years for light to travel 4.2 light years.
  6. Apr 12, 2010 #5
    grrr , dumb me .. thanks
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