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Homework Help: Forces of a wrecking ball against a wall at rest.

  1. Oct 16, 2012 #1
    Hello everyone, this is my first time posting to these forums, and my first time posting any online help regarding math. As you can imagine, this is a bit awkward and embarrassing for me. My textbook has no examples of the type of problem below and hours of googling has failed me. Hopefully someone would be kind enough to help me with my dilemma!

    Question Description
    A wrecking ball that has a mass of 200kg is resting against a wall. It hangs from a cable from the top of a crane that is touching the wall. The cable makes an angle of 9° with the wall. Ignore friction between the wall and ball.

    Questions To Be Answered
    1. Apply newtons second law, but do not substitute the letters for numbers.
    2. What is the magnitude and direction of the force of the wrecking ball on the wall?

    As always I attempted to draw a free body diagram but became instantly stuck. From the picture I attached, you can see the two FBD I drew. In the first one I encountered a force which I could not label. I felt like it should be classified as weight, but that would give me two weight forces in differing directions.

    http://db.tt/v3KiVQvS [Broken]
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 16, 2012 #2


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    welcome to pf!

    hello physicalConst! welcome to pf! :smile:

    your top diagram (with four arrows), i don't understand at all :confused:

    your bottom diagram is correct (with only 3 forces), except of course that weight is always vertical :wink:
  4. Oct 16, 2012 #3
    Thanks for your response tiny-tim!

    When I made the first diagram I was also confused, I couldn't see how it could be possibly right, but I though I missing something about the fundamentals of forces. So I drew up that second diagram, and although I knew that weight should be vertical, the whole pushing against the wall threw me. I think I over analyzed the whole problem.
  5. Oct 17, 2012 #4


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    hi physicalConst! :smile:

    (just got up :zzz:)
    yeah! :biggrin:

    don't do that … mechanics really is usually quite simple!

    weight is always vertical, reaction forces on a frictionless surface are always normal, and tension is always along the cable or string :wink:

    (and if you can't think of a reason for a force … it isn't there!!)​
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