scguy Posted June 3, 2005 Share Posted June 3, 2005 This question is out of my study guide but i am not sure if it can even be answered as it does not say how long the rod is. G= 10ms^2 Q: A uniform rod of mass 20kg is hinged at its lower end and supported at 60deg to the verical by a horizontal string attached to its upper end. The upper end of the rod carries a load of 40kg. Find the tension in the string. Now, surely if i dont know the length of the rod then it could be 1000m long or 1mm which would give very different moments about the axis at the hinged end and hence different tensions in the string. Either i am completely missing the point here or this has to be a typo. Link to comment Share on other sites More sharing options...
swansont Posted June 3, 2005 Share Posted June 3, 2005 I think you'll find the length doesn't matter. The two torques (from the tension and from the weight) scale with the length, so it should cancel out in the final result. Link to comment Share on other sites More sharing options...
scguy Posted June 4, 2005 Author Share Posted June 4, 2005 I dont know why i didnt just assign an arbitrary value to the length, the further from the horizontal the more leverage the supporting wire gains and also the ratio still remains constant, i feel so stupid now Link to comment Share on other sites More sharing options...
ydoaPs Posted June 4, 2005 Share Posted June 4, 2005 why would you assign a value? Link to comment Share on other sites More sharing options...
swansont Posted June 5, 2005 Share Posted June 5, 2005 why would you assign a value? You might assign it an arbitrary length L, which would then cancel out of the equations you set up. But I've noticed some students actually want to assign and calculate numerical values in intermediate steps rather than manipulate the variables. It's a really bad habit to get into. Link to comment Share on other sites More sharing options...
ydoaPs Posted June 5, 2005 Share Posted June 5, 2005 i would have just used L and let it cancel itself out. Link to comment Share on other sites More sharing options...
swansont Posted June 5, 2005 Share Posted June 5, 2005 i would have just used L and let it cancel itself out. And that would be the right way to do it. Link to comment Share on other sites More sharing options...
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