Portlandjib.com
 

With enough counter-weight, you can lift a 140lb load

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Without a counter-weight the crane will tip over

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a group of 7th grade girls built this bridge

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this crane can lift 100 lbs

a working crane

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    A suspension bridge.


 

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this model of the Hawthorne bridge can be raised and lowered just like the real bridge

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deck truss with a 250 lb live load

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  A trebuchet, with a falling counter-weight