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: Calculating earth's speed using radius and speed of light, etc.

  1. Mar 22, 2009 #1
    Ole Roemer found that the average increased delay in the disappearance of Io from one orbit around Jupiter to the next is 14 s.
    (a) How far does light travel in 14 s?
    1 m

    (b) Each orbit of Io takes 42.5 h. Earth travels the distance calculated in part (a) in 42.5 h. Find the speed of Earth in km/s.
    2 km/s

    (c) Check to make sure that your answer for part (b) is reasonable. Calculate Earth's speed in orbit using the orbital radius, 1.5 108 km, and the period, one year.

    speed of light = 3x10^8

    For part (a) i got 4.2e9, which was correct. d=(3x10^8)(14)
    For part (b) I got 27.45 km/s which was correct. 4.2e9m = 4.2e6km
    Part (c) I dont know...
  2. jcsd
  3. Mar 22, 2009 #2


    User Avatar
    Homework Helper

    The Earth goes around the sun in a circle. Find the distance around the circle. The time taken to go around, in seconds. Use v = d/t to find the speed.
  4. Mar 23, 2009 #3
    Well I figure, radius is half a diameter, right?
    So i multiplied the radius (1.5 x 10^8) by two.
    then i found the number of seconds in a year, which is 31,536,000.
    I even tried just doing (1.5x10^8)/# seconds in a year
    and it wa s still wrong.

    **Because it won't let me edit the first post....**
    orbital radius, 1.5 108 km = 1.5x10^8
    Last edited: Mar 23, 2009
  5. Mar 23, 2009 #4


    User Avatar
    Homework Helper

    Isn't the circumference of a circle = 2πr ?
  6. Mar 23, 2009 #5


    User Avatar
    Science Advisor

    Light doesn't travel around the circumference of a circle- travels across the diameter.
  7. Mar 23, 2009 #6
    I used LowlyPion's equation and plugged in the radius, and then divided by the number of seconds.
    It marked it correct. :D
    Thanks so much!!
  8. Mar 23, 2009 #7


    User Avatar
    Homework Helper

    That may well be, but the question is asking for Earth's speed, and the circumference/period. Earth's speed is relevant for explaining the 14 s interval, and represents an exceedingly small arc of earth's orbit right?
  9. Apr 24, 2009 #8
    hey it is the angular velocity you need to consider i.e omega,not linear velocity
    then v=d/t cannot be used .
  10. Apr 24, 2009 #9


    User Avatar
    Homework Helper

    Just curious would you have the θ for 14 sec divided by a year load of seconds?

    And could you identify the error difference between the arc of θ, and the chord of θ?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook