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Homework Help: Equations and how to calculate moments and couples?

  1. Oct 28, 2012 #1
    1. The problem statement, all variables and given/known data

    A flagpole of mass 25kg and length 2.5m is held in a horizontal position by a cable at 30° from the end of the pole. The centre of gravity is 1.5m from the fixed end.


    1. the tension in the cable.

    2. the vertical component of the force at the fixed end of the pole.

    2. Relevant equations

    moment = force x perpendicular distance from a stated point

    3. The attempt at a solution

    Well I was away for the lesson in which we covered moments, couples and torques etc and I really don't understand. I manage to calculate the length of the cable which I got to be 2.89m but I'm pretty sure this is irrelevent. Can somebody possibly point me in the right direction as where to start?
  2. jcsd
  3. Oct 28, 2012 #2


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    Draw a force diagram. Since the pole isn't moving...

    1) The moments sum to zero.
    2) The vertical forces sum to zero.
    3) The horizontal forces sum to zero.

    So you can write simultaneous equations and hopefully solve them.

    Have a go, post your diagram and initial equations.
  4. Oct 28, 2012 #3
    I don't even know where to begin with a force diagram. Sorry to sound stupid but I was away for this lesson and have only just moved to the physics class. How would I go about drawing a force diagram? I can do the maths involved I just don't really understand the physics aspect of everything
  5. Oct 28, 2012 #4
    Draw the horizontal flag pole and the cable from one end of the pole to the wall. Both the flag pole and the cable are fixed to the wall. So you now have a triangle with a 30 degree angle between the pole and the cable at the end that is not fixed to the wall.
    Then convert the mass of the flagpole to its weight in Newtons. Mark on this weight acting at 1.5m from the fixed end (the wall) pointing downwards. This force is balanced by the vertical component of the tension in the cable. You can now set up the clockwise moment of the weight times its distance from the fixed pivot is equal and opposite to the anti clockwise moment of the vertical tension (your unknown) times its distance from the fixed pivot. This is the principle of moments for an object in equilibrium
  6. Oct 28, 2012 #5
    So have you learn't about resolving forces into horizontal and vertical? Have you learnt about centre of gravity? and have you learnt about the principle of moments. These are the concepts you would need to solve this problem
  7. Oct 28, 2012 #6


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    If you only missed one lesson you should know something about force diagrams as this problem is unlikely to be set the first time force diagrams are introduced in class.
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