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Light Fundamentals (3) problems

  1. May 21, 2009 #1
    1. Light takes 1.28 s to travel from the moon to Earth. What is the distance between them?

    2. d= vt

    3. d = vt
    = 3 * 108(1.28)
    = 3.84 * 108m

    1.The sun is 1.5 * 108 km from Earth. How long does it take for its light to reach us?

    2. d= vt

    3. d = vt
    t = d/v
    = 1.5 * 108/ 3 * 108
    = .5s
    Did i do these 2 problems right? I'm not sure because it seems to easy.

    b]1.Ole Romer found that the maximum increased delay in the appearance of lo from one orbit to the next was 14s.
    a. How far does light travel in 14s?
    b. Each orbit of lo is 42.5 h. Earth traveled the distance calculated above in 42.5h. Find the speed of Earth in km/s.
    c. See if your answer for part b is reasonable. Calculate Earth's speed in orbit using the orbital radius, 1.5 * 108km, and the period, one year. [/b]

    2. a/b. d= vt
    c. ?

    3. a) d = vt
    = 3 * 108/14
    = .4.2 * 109m

    b) d= vt
    v = d/t
    = 4.2 * 109/42.5
    = 98823529.41(3600)
    = 3.56 * 1011km/s
    c) I do not know if I should use Kepler's Third Law or d= vt to solve this problem.

    I have a question for the equation E = P/4pid2. I know that the P represents the luminous flux, but can I put in m/s instead or does it work for lm only and what does lm and lx stand for? :confused:

    Thank You!! :smile:
  2. jcsd
  3. May 22, 2009 #2


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    Homework Helper

    The first two are right ;-) They are as easy as they look.

    For the third problem, part (a) you got right. Part (b) is mostly right but you need to be more careful with the units. If you're confused, one way to do it is to convert the distance to kilometers and the time to seconds before you divide. In part (c), why would you use Kepler's third law? It relates the orbital radius (a.k.a. semimajor axis) to the orbital period, but you already have both of those numbers. There's no need for you to calculate either of them.
  4. May 22, 2009 #3


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    Science Advisor
    Gold Member

    The second problem is wrong. Don't forget your units.
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