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Calculating the torque angle in a free-body diagram

  1. Nov 24, 2015 #1
    I am seriously confused still about how to calculate the torque angle. Here is a sample question on my recent test that I got wrong. I just want to know exactly how you find the torque angle. Thanks
     

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  3. Nov 25, 2015 #2

    sophiecentaur

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    Personally, I have never used that term but I presume that angle is the one you use to calculate the perpendicular distance between the line of action of the force and the fulcrum (= the nearest point on the line of action to the fulcrum). I notice you haven't drawn a construction line from the Tension vector to the pivot and the red ring tells you that you are doing the wrong trig calc at that stage. I would have drawn in that line when 'taking moments' because the trigonometry becomes more straightforward. You seem to have done that for the weight vector (the horizontal dashed line) and got the right answer for the torque there. (I'm assuming the marker is correct!)
     
  4. Nov 25, 2015 #3
    Thanks for the reply! Yes, the red marker indicates the correct answer. When he was showing us the correct answer in class, he supposedly subtracted 180 degrees from the angle between the tension force and the rod. However,at the same time, that makes me ponder how I got -90 degrees as my torque angle for my weight forces. So the whole idea of calculating torque is still unknown to me.
     
  5. Nov 25, 2015 #4

    sophiecentaur

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    Force times perpendicular distance is the moment about a fulcrum. My last post pointed that out and that you need to calculate that distance. How can it be Unknown? It is defined all over the place and in your text book(?). Have you looked anywhere else than PF? Keep looking until you find something that makes sense to you.
    I am not sure f how you are actually approaching this problem but calculating torque requires two forces and a distance. I can't think that it could help to calculate the torques independently if it isn't necessary (i.e. in that structure of yours) so taking moments about just one point (you know the term Moments?) takes care of all that.

    PS where does your "torque angle" come into this? Explain exactly what you are doing here and I may see where our different terms can be brought together.
     
  6. Nov 25, 2015 #5
    Yeah, I understand how the term moment is used interchangeably with torque and etc. I am primarily talking about this particular force diagram in the attachment of the OP.
     
  7. Nov 25, 2015 #6

    sophiecentaur

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    OK, so you have the force and you can easily calculate the perpendicular distance. Just drop a perpendicular from the pivot to the line of the string and find its length. I can't see why that should be a problem if you know a bit of trig. There are angles marked everywhere so it is easy to work out which (sin,cos,tan) to use for any angle you choose.
    That particular diagram is presented in a very friendly way - no tricks there.
     
  8. Nov 25, 2015 #7

    CWatters

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    The top string is the only one that might need some thought..

    Torque.jpg
     
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