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Math Help - How do I calculate length of bridge on this one?

  1. #1
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    How do I calculate length of bridge on this one?

    A hiker who weights 985 N is strolling thru the woods and crosses a small horizontal bridge. The bridge is uniform, weights 3610N and rests on two concrete supports, one at each end. He stops 1/5th of the way along the bridge. What is the magnitude of the force that a concrete support exerts on the bridge First at the near end and at the for end???
    How do I do this with no length of the bridge.
    I am lost on this one.

    Can someone help please.

    I know he goes 1/5th, but how does that give me a distance??
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  2. #2
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    You need relative distance, not actual distance. The actual length has nothing to do with it, except perhaps it would change the bridge's newtons.
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  3. #3
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    Quote Originally Posted by bradycat View Post
    A hiker who weights 985 N is strolling thru the woods and crosses a small horizontal bridge. The bridge is uniform, weights 3610N and rests on two concrete supports, one at each end. He stops 1/5th of the way along the bridge. What is the magnitude of the force that a concrete support exerts on the bridge First at the near end and at the for end???
    How do I do this with no length of the bridge.
    I am lost on this one.

    Can someone help please.

    I know he goes 1/5th, but how does that give me a distance??
    let L = length of bridge

    \sum F_y = 0

    \sum \tau = 0

    L will cancel in your calculations
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  4. #4
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    Have not been shown how to do that?
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  5. #5
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    How do I do torque if I have no distance. I am not sure what you mean? I looked thru my book and show nothing.
    Can you explain a bit more please.
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  6. #6
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    Quote Originally Posted by bradycat View Post
    How do I do torque if I have no distance. I am not sure what you mean? I looked thru my book and show nothing.
    Can you explain a bit more please.
    let F_1 = upward support force on the near end of the bridge

    F_2 = upward support force on the far end of the bridge

    forces up = forces down

    F_1 + F_2 = 3610 + 985



    let the near end be the pivot point ...

    torque clockwise = torque counter-clockwise

    \displaystyle 985 \cdot \frac{L}{5} + 3610 \cdot \frac{L}{2} = F_2 \cdot L

    note that each term has L = bridge length as a factor ... like I said previously, they will all cancel.

    all set up ... solve the system.
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  7. #7
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    Hi,
    I am still in the fog. I do appreciate the help. I am really lost on this one.
    Thanks for helping, I will write down what you gave.
    I kinda understand, but I don't. One of those things, tonight nothing has been clicking like it should, but this one was a good one.
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