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: Speed of Light and Galileo

  1. Feb 26, 2006 #1
    I'm having trouble with a proble I'm trying to complete. The Problem states:

    Galileo attempted to measure the speed of light by measuring the time elapsed between his opening a lantern and his seeing the light return from his assistant's lantern. The experiment is illustrated in Figure 25-24. What distance, d, must separate Galileo and his assistant in order for the human reaction time, t = 0.2 s, to introduce no more than a 18% error in the speed of light?

    The only way I can think to do the problem is with, time = distance/speed of light. Time would equal 0.1 s and speed of light minus 18% error (I think its minus, because the error would make the velocity seem slower) would be 2.46E8 m/s. d= 0.1s(2.46E8 m/s) = 2.46E7 m (the wrong answer). Any suggestions? Thanks
  2. jcsd
  3. Feb 27, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The measured speed would be [itex]c_{m} = d/t[/itex] where t = total elapsed time which includes the reaction time of .2 sec.

    The actual speed of the light would be [itex]c = d/(t-t_r)[/itex]

    So the question asks: what value of d will make the difference [itex]c-c_m[/itex] less than or equal to .18c? That means that [itex]c_m \ge .82c [/itex].

    Use the relationship: [itex]t - t_r = d/c[/itex] and [itex]t = d/c + t_r[/itex]

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook