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Homework Help: Gravitational attraction of plumbline

  1. Oct 5, 2015 #1
    1. The problem statement, all variables and given/known data
    By what angle, in seconds of arc, will a plumbline be pulled out of its normal vertical direction by the gravitation attraction of a 10-ton that parks 20 ft away? Do you think that this effect could be detected?

    2. Relevant equations

    I think (G*m1*m2)/r^2

    3. The attempt at a solution

    I first converted 10 tons to Newtons which i got to be around 88964 Newtons (from some conversion website).

    then i divided 88964 by 9.8 to get 9077.96 kg for the truck.

    20 ft becomes 6.096 meters.

    Plugging this into the equation, I get

    (G * (m(plumb) * 9077.96 kg))/(6.096 m)^2

    I dont know how to find the mass of the plumbline.
  2. jcsd
  3. Oct 5, 2015 #2


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    Hi TS,

    Converting tons to new tons (sorry) isn't working properly. Dimensions don't match. In my region (I presume SI, but the ton doesn't fit in there) 1 ton is 1000 kg. What about your tons? Are they tonnes or tons (long, short, UK, US, himalaya, etc... -- only you can tell) ?

    And you don't need the mass of the plumb. Write out a few equations that help you find the angle they are asking for. Who knows this mass might divide out....
  4. Oct 5, 2015 #3
    The title of this thread does not conform to PF rules and guidelines. Please provide a new title.

  5. Oct 5, 2015 #4


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    Chet is right of course. Except for that and my advice in post #2, yet another tip: make a drawing! Or even two: one free body diagram for a plumb (i.e. ⊥) plumb and one where the plumb is attracted towards a parked truck. Exaggerate the angle (:wink:) and then work out the post#2 tip.

    [edit] and I wonder why you do worry about the mass of the plumbline, but not about its length !? :rolleyes: Or am I making it worse now?)
  6. Oct 5, 2015 #5


    Staff: Mentor

  7. Oct 6, 2015 #6
    Thanks I did not know how to change the name of the thread. (I still don't despite attempting to look for the answer).
  8. Oct 6, 2015 #7
    Thanks for all the help BvU.

    I have been trying to model this like a pendulum, however I have been trying for sometime and it seems that I am doing something wrong.

    Is this the right approach or is this completely wrong?

    Furthermore, my thinking is that the Tension in the X direction (back towards the vertical) would be equal to the Force of Gravity from the truck.

  9. Oct 7, 2015 #8


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    A swinging plumb is no good :smile: . Better look at the equilibrium situation. Easier.

    looks good to me. Tension is force. Force is mass times acceleration. Plumb mass is the same in both cases-- that's why it cancels. Is that enough of a "hint" ?

    Can you post your free body diagram ?
  10. Oct 7, 2015 #9
    Here is a handdrawn version. I hope you can read it, but if not, please tell me. I then proceeded to have Tx = Fg. Thanks for the help
    . IMG_0003.jpg
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