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!

Homework Help: Really weird physics problem, can you help me?

  1. May 27, 2012 #1
    Suppose that, while lying on a beach near the equator watching the Sun set over a calm ocean, you start a stopwatch just as the top of the Sun disappears. You then stand, elevating your eyes by a height H = 1.70 m, and stop the watch when the top of the Sun again disappears. If the elapsed time is t = 11.1 s, what is the radius r of Earth?
  2. jcsd
  3. May 27, 2012 #2
    Use the celestial sphere to solve the problem.
  4. May 27, 2012 #3


    User Avatar
    Science Advisor
    Homework Helper

    Hi TrevorRyan! :wink:

    Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:

    (you'd better start with a really good diagram!)
  5. May 27, 2012 #4
    Yes, an accurate diagram would really benifit
  6. May 27, 2012 #5
    Looks like a trigonometry problem. You are given two sides and the angle between them.
  7. May 27, 2012 #6
    How so? You can figure the angle that the earth rotates in the problem, but you aren't given the sides that enclose that angle.
  8. May 27, 2012 #7
    I'd like to see a better description on that first sentence too.
  9. May 27, 2012 #8
    One side is the radius the other is the radius plus 1.70m.
  10. May 27, 2012 #9
    Those sides aren't given, the radius of the earth is an unknown in this problem. If you try to apply the law of cosines, you get 2 unknowns and only one equation.
  11. May 27, 2012 #10
    At the tangent to the surface is a right angle.
    Last edited: May 27, 2012
  12. May 27, 2012 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Moderator's note

    At this point, please let TrevorRyan respond with an attempt toward working out the problem, in accordance with our forum guidelines on Homework Help. Please do not post further hints and help until we here from the OP.
  13. May 28, 2012 #12
    I always get to the this equation, R+1.7cos0.04625=R, and Have no idea what to do from there.
  14. May 28, 2012 #13


    User Avatar
    Homework Helper

    Add parentheses at the proper places? Your equation means that 1.7 cos0.04625=0 which is obviously wrong.

    For better understanding, I attach the missing figure. One sees the sunset when the ray arriving to the eye from the sun is tangent to the Earth.


    Attached Files:

  15. May 28, 2012 #14
    wow im retarded lol. Thx ehild, now i get it
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook