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Determining whether torque is positive or negative via counter-clockwise (+)...

Discussion in 'MCAT Study Question Q&A' started by Zona Pellucida, Dec 20, 2008.

  1. Zona Pellucida

    7+ Year Member

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    ..and clockwise (-)

    So I've been making my way through NOVA's MCAT Book and have been doing pretty well until I hit the Torque chapter. I SUCK at torque.

    I understand Torque = r*F*sin()

    I don't understand how to determine whether rotation will be clockwise or counter-clockwise and I suck at problems with multiple torques going on. I was bad at this in Physics I this past semester also but somehow got by with an A minus because I made some stuff up that was close enough haha.

    Is there an easy way to think about torques that I am not understanding? Or perhaps a good interactive website/video that could help me out?

    Also, is torque a main theme seen on the MCAT?

    Thanks
     
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  3. aquariuscharm

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    A good rule of thumb to remember is: "Clocks are negative," meaning that if the torque is in the clockwise direction, it is a negative torque. Also, I believe the direction of the torque is determined by the direction of the force acting on the moment arm.

    I'm going off pure memory here, but I think you can just add torques to find a net torque.

    I don't know if this helps, but you can think of torques in terms of a seesaw. The distance from the fulcrum to the weight is the moment arm and the weight is the force. A net torque of 0 = a balanced seesaw.

    I have no experience with the MCAT (I'm taking it April 18th).
     
  4. Zona Pellucida

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    Thanks that helped a bit but I'm still confused on determining which direction a force is going... I understand clockwise and counter-clockwise direction easily, I just don't know how to determine which way a given force is "going?"
     
  5. cbrons

    cbrons Ratatoskr! *Roar*
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    Use this trick: Put your fingers in the direction of "r." Then curl them in the direction of F and your thumb will point in the direction of the torque vector.
     
  6. Ailleurs

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    Um, I drew a really rough sketch in paint. I *hope* it helps a *little bit.*

    If not, just PM me :p

    It's been like 2-3 weeks since I last looked at physics stuff, so if any of this wrong, let me know. I hope not!

    And, I think the EK physics book does an okay review of torque and direction, although I thought it was a bit WAY CONDENSED when I was using the EK book to study for my physics class.

    If you want more complicated examples, like a hanging board held by a 45degree string and figuring out all the Torques and direction, let me know. I can make up a complicated example for you, one which was a practice problem for my physics test. It was a really good problem to test conceptual understanding so, I'll have to go through my papers to see if I still have it though :p

    [​IMG]

    edit: GAH! Why is it so SMALL?! The original is bigger so that the font isn't miniscule... And also, the blue is so darn BRIGHT. The blue in the original is a more saner and less retina burning blue. But somehow imageshack doesn't like my drawing regardless of me uploading it 500000x :(
    [​IMG]
     
    #5 Ailleurs, Dec 26, 2008
    Last edited: Dec 26, 2008
  7. w0lf137

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    Nice drawing. For the 2nd problem, I think its Ft=(3/2)g(m+(1/2)mb). Divide both sides by 2/3, it is like multiplying by 3/2. Also gj remembering the fan has weight too.
     
  8. Zona Pellucida

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    Thanks everyone - will look this over !
     
  9. Ailleurs

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    Woops, yeah! I forgot to flip it to 3/2. Thanks for clearing the mistake :D *is too lazy to fix it though* sorry zona T_T
     
  10. shahak49

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    But it ony tells me out of page or into page....
    Still don't get it...............

     
    #9 shahak49, Jun 29, 2009
    Last edited: Jun 29, 2009
  11. Seraph 84

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    I've always defined counterclockwise torque positive and clockwise torque to be negative.
     
  12. shahak49

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    How do I know if it's clockwise or counterclock? (ya, clock=- and counter=+). But, how do u find the direction?
     
  13. Seraph 84

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    Draw a picture of your system. If you have, say, two people on a seesaw, with a 50kg person on the left and a 100kg person on the right. The 50 kg person on the left will act to produce counterclockwise, or positive, torque. The 100 kg person on the right will produce clockwise, or negative torque. The direction of the torque on your free body diagram is simply the intuitive direction that the bodies would want to rotate or "push" the lever arm.

    Don't worry too much about using the right-hand rule for torques; I never used it, only really used it for electromagnetism. If you really want to use it, then go ahead; a torque vector out of the page is positive torque, while a vector into the page is negative torque.
     
    #12 Seraph 84, Jun 29, 2009
    Last edited: Jun 29, 2009
  14. EyEnStein 07

    EyEnStein 07 Senior ɸ Member
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    Just some quick advice....to determine try to evaluate the diagram or draw one. if something is balanced on a wall (for example a store shed or sign) try to understand which way it will rotate if a cable or something is removed. That should give you a better idea of which way the torque should be. MOST IMPORTANTLY, torques are always taken from a specific POINT.....just because its CCW at one point of rotation does NOT mean it will be CCW at another point.
     
  15. al112987

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    I've never learned any sort of clockwise or counterclockwise convention for torque being positive or negative, I've always just been taught to just pick one direction as positive and assign the other as negative, in the end it doesn't change your answer.
     
  16. G1SG2

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    I've always just based it on my reference point. For example, if we have two blocks on a board, and I chose the center of the board to be my reference point, everything to the right of the board would rotate clockwise (negative), and everything to the left would rotate counterclockise (positive). Once I set them equal to each other for balancing, it really didn't matter anymore. Or maybe I'm just a dud, who knows :cool:
     
  17. shahak49

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    Thanks everyone for responses.
     
  18. maybemed2013

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    Bump. I need some help in determining if it counterclockwise or clockwise.
     
  19. anon4895

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    I'm not sure what the question is. If the torque is trying to produce a ccw rotation it's positive, it's trying to produce a cw rotation it's negative. You can define your coordinate system however you want and this means cw could be positive while ccw is negative. I'd recommend not doing this since you may get confused. It'll mean you'll have to start defining other things against convention and you may not realize and/or get confused. Most simple torque problems probably won't run into that problem though I guess.

    Also if for some reason you can't figure out which way the torque is trying to turn the object you can take the cross product. If you know how to take r cross F then just do that, it's pretty fool proof.

    I know most of you have only taken intro physics, so idk if you have experience with cross products or not.



    Yeah but only works if you use your right hand. If you use your left hand you'll be pointing in another direction.
     
  20. ikillmcatz

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    Is this just like a memorization thing? I don't know the logic behind it...I just know the rule....:mad: I spent forever trying to understand WHY clockwise was negative and counterclockwise was positive yesterday and got so annoyed.
     
  21. anon4895

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    I'm pretty sure it's just consistency. There's a way to multiply vectors together to give you another vector. This is what you're doing when you find torque. So when something rotates in the xy plane the torque vector points either into or out of the page along the z axis. Following the rules of this vector multiplication clockwise torques produce a torque vector into the page (negative) and counterclockwise torques produce a torque vector out of the page (positive). The z-axis just happens to be defined like that just like the x-axis is positive going right instead of left.
     

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