1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light moving in an shrine

  1. Feb 18, 2013 #1
    1. The problem statement, all variables and given/known data
    The walls of an ancient shringe are perpendicular to the four cardinal compass directions. On the first day of spring, light from the rising sun enters a rectangular windows in the eastern wall. The light traverses 2.37m horizontally to shine perpendicularly on the wall opposite the window. A tourist observes the patch of light moving across this western wall.

    Part A:
    The speed the illuminated rectangle move is 1.7×10^(-4)m/s

    Part B:
    The tourist holds a small, square mirror flat against the western wall at one corner of the rectangle of light. The mirror reflects light back to a spot on the eastern wall close beside the window. The speed of the smaller square of light moves across the wall at 3.45×10^(-4)m/s

    Part C:
    In what direction does this smaller square of light on the eastern wall move.

    2. Relevant equations
    v(1)=rω
    v(R)=2V(1)

    3. The attempt at a solution
    I was able to do the part A and part b when I got to part c I said 15 degrees upward but I was told I need to put more information so I am not sure about this answer.
     
  2. jcsd
  3. Feb 19, 2013 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    hi deltafee! :smile:
    yes, but 15° upward to the north, or to the south? :wink:
     
  4. Feb 19, 2013 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The reflection on the wall will not move upward, and I'm curious as to how you calculated 15 degrees. Won't it depend on the latitude?
    As tiny-tim suggests, the question might (also?) be asking whether the reflection moves to the north or the south. However, there's not enough information to answer this unless you assume the shrine is in a particular hemisphere. Again, it comes down to the latitude.
     
  5. Feb 20, 2013 #4
    Here's a clue
    ..." traverses 2.37m horizontally to shine perpendicularly on the wall" ...
    Which means the sun's reflection will move downwards as it rises upwards?
     
  6. Feb 20, 2013 #5

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I fail to see what the perpendicularity has to do with it.
     
  7. Feb 20, 2013 #6
    In order to talk about direction you need a reference direction. In this case it is a line perpendicular to the wall. The sun will rise upwards from this direction, agreed a bit tilted over towards the north or south depending on the latitude (unless it is on the equator), but with the given info this will barely be visible. On the spring equinox it will also rise due east.
     
  8. Feb 20, 2013 #7

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The direction of movement would be the same even if the light ray were not exactly perpendicular to the wall. The point of mentioning it at all in the statement of the problem is merely to reassure the reader the building is roughly a box, not some peculiar shape.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook