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Math Help - Mechanics question

  1. #1
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    Mechanics question

    Hey i am stuck on this question i have found in a past paper can someone please help me solve this asap as i have a mechanics exam tomorrow and i need to be able to do all topics as well as possible:

    A steel girder AB has a weight 210N. It is held in equilibrium in a horizontal position by two vertical cables. One cable is attatched to the end A. The other cable is attatched to the point C on the girder, where AC = 90cm. The girder is modeled as a uniform rod, and the cables as light inextensible strings.

    Given that the tension in the cable at C is twice the tension in the cable at A, show that AB = 120cm.


    Thanks to anyone who helps.
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  2. #2
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    Quote Originally Posted by Big_Joe View Post
    Hey i am stuck on this question i have found in a past paper can someone please help me solve this asap as i have a mechanics exam tomorrow and i need to be able to do all topics as well as possible:

    A steel girder AB has a weight 210N. It is held in equilibrium in a horizontal position by two vertical cables. One cable is attatched to the end A. The other cable is attatched to the point C on the girder, where AC = 90cm. The girder is modeled as a uniform rod, and the cables as light inextensible strings.

    Given that the tension in the cable at C is twice the tension in the cable at A, show that AB = 120cm.


    Thanks to anyone who helps.
    Did you draw a diagram?

    From mine you can see where A, B and C with relation to T1 and T2. From the question you can deduce T_2 = 2T_1

    Resolve vertically about point C and let from C to A be the positive direction:

    mg - x = 2 \times 90 \rightarrow 210-x=180

    x = 30

    From the diagram we can see that AB = AC + CB = 90+30 = 120cm

    Edit1: oops my diagram is wrong - I'll upload the correct one soon
    Edit 2: done
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