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!

Finding mass of Moon with Pendulum Bob

  1. Mar 23, 2013 #1
    1. The problem statement, all variables and given/known data

    It is possible to determine the mass of a planet or large moon by using a ‘pendulum bob’ horizontal circular motion experiment. A cosmonaut on the moon finds that a 25 cm long ‘pendulum bob’ moving in uniform horizontal circular motion makes an angle of 22° to the vertical and moves with a period of 2.3 s. What is the mass of the moon? (The radiusof the moon is 1.5 х 105 m)

    2. Relevant equations

    ??

    3. The attempt at a solution

    0.25sin(22) = 0.094 (radius)

    v = (2∏r) / 2.3 = 0.257 ms-1
     
  2. jcsd
  3. Mar 23, 2013 #2
    Start with forces acting on the bob.
     
  4. Mar 23, 2013 #3
    so I tried ac = 0.2572 / 0.094 = 0.7 ms-2

    tan(90-22) = g / 0.7
    g = 1.12

    1.12 = (6.67 x 10-11)M / (1.5 x 105)2

    and I got M = 3.78 x 1020 which is wrong.


    I'm not sure how to go with this one
     
  5. Mar 23, 2013 #4
    You could use the pendulum equation.

    Since 22.3 degree is small enough, use the small angle approx, sin θ ≈ θ so you could derive from the pendulum equation T ≈ 2π√(l/g). So measure the period of the pendulum and find g of the moon. Since g = Gm/r2, calculate m since you know r.
     
  6. Mar 23, 2013 #5
    Where does tan(90 - 22) = g / 0.7 come from?
     
  7. Mar 24, 2013 #6
    you have 22 degrees as the top angle and the Ac in a right angled triangle. 90-22 = 58. Is that right, I'm not sure.

    tan(58) = g / 0.7
     
  8. Mar 24, 2013 #7
    First of all, 90 - 22 = 68, not 58. Even then, I do not understand where your equation comes from. What equations do you get from the free body diagram?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Finding mass of Moon with Pendulum Bob
  1. Bob of mass (Replies: 19)

  2. Pendulum bobs (Replies: 23)

Loading...