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    Hey, I have no idea how to do this:

    A lorry of weight 40000N crosses a 20m long bridge of weight 100000N. What are the forces supplied by each bridge support when it is 5m from one support?


    Thanks
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    Total force 'down' must equal total force up

    20/5 = 4 therefore 4 supports.


    40000 + 100000 = 140000

    140000/4 = 35000 each support.
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    Think of the principle of moments.
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    (Original post by Hancock orbital)
    Total force 'down' must equal total force up

    20/5 = 4 therefore 4 supports.


    40000 + 100000 = 140000

    140000/4 = 35000 each support.
    I think you misunderstood, unless I did :p:

    Two supports, one at each end. The truck is 5 m from one end.

    OP, think torque. Net torque and net force must be zero for the system to remain in equilibrium.
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    (Original post by D-Day)
    I think you misunderstood, unless I did :p:

    Two supports, one at each end. The truck is 5 m from one end.

    OP, think torque. Net torque and net force must be zero for the system to remain in equilibrium.
    yeah - four supports does seem strange for a 20m bridge. duh.
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    I'm not sure if this is right but i got 80,000N and 87,000N.....
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    Got to add up to 140000 - so no its not right
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    Hmm....

    Not very sure how to do it then...

    i did 40kNx5m=200KNm+ (100knx10m)= 1200kNm= anticlockwise momentum

    clockwise= 20mx= 1200kNm
    clockwise=1200kNm/20m=60,000N

    then i did the 140,000-60,000= 80,000N

    and then i did something similar to the other side and got 87,000N....


    So then is the answer 60,000N and 80,000N, if it has to add up to 140kN?
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    HELP ~ What have i done wrong?
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    Net torque is zero. Assume that the pivot point is on one of the supports. This eliminates an unknown.

    (40,000 N)(5 m)+(100,000 N)(10 m)=(F2)(20 m)

    Now move the pivot point to the other support.

    (100,000 N)(10 m)+(40,000 N)(15 m)=(F1)(20 m)
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    Well so....
    F2=600,000N
    F1=800,000N

    Then is that the answer?
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    Check your work. Those are off by a factor of ten. Should be 80,000 and 60,000.
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    Ok, thanks alot
 
 
 
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