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Math Help - Mechanics moments question =)

  1. #1
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    Mechanics moments question =)

    Basically i get the answer but i don't understand it lol:
    A mirror of mass 24 is supported by two screws 0.8m apart. One of the screws breaks so someone applies a force F to it, 1.2m below the screws to keep it in the same position.
    Find the force required.
    I'll draw a picture =)


    Oh and there's a mass on here which i forgot to draw sorry. Acting from center with force 24g.

    Ok, so my working went like this:
    1.2F = 0.4 x 24g
    And i got the correct answer of 78.4N
    Then i managed to confuse myself.
    I took moments at the broken screw..but why is one distance taken horizontal (0.4, half of 0.8) whilst the other is vertical (1.2)...shouldn't you take both verticle or both horizontal? i.e (1.2 and 0.6) or (0.8 and 0.4)?
    Thanks =)
    Hopefully i explained that ok.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by AshleyT View Post
    Basically i get the answer but i don't understand it lol:
    A mirror of mass 24 is supported by two screws 0.8m apart. One of the screws breaks so someone applies a force F to it, 1.2m below the screws to keep it in the same position.
    Find the force required.
    I'll draw a picture =)


    Oh and there's a mass on here which i forgot to draw sorry. Acting from center with force 24g.

    Ok, so my working went like this:
    1.2F = 0.4 x 24g
    And i got the correct answer of 78.4N
    Then i managed to confuse myself.
    I took moments at the broken screw..but why is one distance taken horizontal (0.4, half of 0.8) whilst the other is vertical (1.2)...shouldn't you take both verticle or both horizontal? i.e (1.2 and 0.6) or (0.8 and 0.4)?
    Thanks =)
    Hopefully i explained that ok.
    You are taking moments about the un-broken screw. The moment is the perpendicular distance between the line of action of the force and the point about which you are taking moments times the force (also taking acount of the sense of the moment).

    CB
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  3. #3
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    Quote Originally Posted by CaptainBlack View Post
    You are taking moments about the un-broken screw. The moment is the perpendicular distance between the line of action of the force and the point about which you are taking moments times the force (also taking acount of the sense of the moment).

    CB
    Oooooo!
    Im so stupid!
    Thank-you very much!
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