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Clockwise or counterclockwise?

  1. Oct 16, 2007 #1
    Hi! I just have a really quick question. I've just started on torque, and I'm confused about clockwise vs. counterclockwise. Like, how DO you know when something is clockwise or counterclockwise? I'm having trouble distinguishing this in pretty much every situation that I come across...

    Thank you very much! :)
     
  2. jcsd
  3. Oct 16, 2007 #2

    Doc Al

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    Find the pivot. Draw a line from the pivot to the point of application of the force. If the force acts to one side of that line, the torque will be clockwise; if it acts to the other side, the torque will be counterclockwise. Pick a real example if you want more.
     
  4. Oct 16, 2007 #3

    D H

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    I too have a problem with the terms "clockwise" and "counterclockwise", as well as with the rule "lefty loosy, righty tighty".

    Imagine a clock encased in glass. The hands move clockwise when viewed from the front of the clock but they move counterclockwise when viewed from the rear. In other words, to apply these terms you have to know where viewer is located.
     
  5. Oct 16, 2007 #4

    Dale

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    I agree. Forget clockwise and counterclockwise, just learn how to consistently and correctly apply the right-hand rule. Then you will always get everything in the correct direction regardless if you are in front or behind the "clock".
     
  6. Oct 16, 2007 #5
    Um...I sort of get what you guys are saying. But at the same time I'm not too sure. As suggested, I posted a question where I'm having trouble deciphering between clockwise or counterclockwise. I already figured out the numerical answers; I just don't know whether to label the answers as clockwise or counterclockwise. I hope the link works.

    [​IMG]

    Calculate the net torque (magnitude and direction) on the beam about the following axes.

    (a) an axis through O, perpendicular to the page.
    clockwise or counterclockwise?

    (b) an axis through C, perpendicular to the page.
    clockwise or counterclockwise?

    The torque here is net torque, so I don't understand how to apply the right-hand rule here. Or maybe I'm not understanding the right-hand rule properly.
     
  7. Oct 16, 2007 #6

    Doc Al

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    While D H and DaleSpam make valid points, for your purposes just choose the viewpoint of a person looking into the page--like you are! (Note that they specify that the axis is perpendicular to the page.)

    Before you worry about the direction of the net torque, you first have to calculate the net torque. Do that by adding up all the individual torques.

    For example, for part (a) the axis is O. So what's the torque about that point due to each of the three forces that act on the object? Use the rule I stated in my first post to determine whether a given torque is clockwise or counterclockwise. If you call counterclockwise torques positive and clockwise torques negative, you can just add them up.

    Give it a try.
     
  8. Oct 16, 2007 #7

    robphy

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    Excuse me for asking... but have you seen an analog clock? (With the rise of digital timepieces, I've been wondering when folks will have trouble with clockwise and counterclockwise.)

    I agree with DaleSpam that one should really learn the "right-hand rule".
     
  9. Oct 16, 2007 #8
    Answer the question this way:
    Which direction is the dancer turning? Clockwise, or counter-clockwise?

    Dancer

    And if she changes directions while you're looking at her, then you're really in trouble.

    Ok; but seriously, if you've done the calculations you should be able to tell whic direction the bar will be turning around whichever axis is being described. If you can see which way the bar is turning, then take that same bar and turning motion and put it on the face of a clock with hands. Does the bar turn in the same direction that the clock hands are turning? If so, then it is clock. If it is turning in the opposite direction, then it is counter-clock. Wise huh?

    For example, if with the axis through O you see the right side of the bar moving upwards, then the bar is moving counter-clockwise.
     
  10. Oct 16, 2007 #9
    I've calculated the net torque already, and I've found them both to be positive. Does that mean that they are both counterclockwise?

    And yes, I do know what clockwise and counterclockwise mean when applied to an analog clock. What I'm having trouble with is applying that to physics because everything is denoted by arrows, so therefore if someone like me who has never learned the physical definitions of clockwise and counterclockwise were to distinguish between the two with just basic reasoning from life, he or she would be confused because the arrows can, in an essence, be identified as either clockwise or counterclockwise due to the sheer fact that the arrows aren't moving.

    The right-hand rule as stated in my book is as follows:

    1. Point the fingers of your right hand in the direction of r.
    2. Curl your fingers toward the direction of vector F.
    3. Your thumb then points approximately in the direction of the torque, in this case out of the page.

    So according to the right-hand rule, I am supposed to basically position my hand so that these maneuvers are possible? And even so, which direction would be clockwise and which direction would be counterclockwise? With my thumb toward myself or toward the page?

    Thanks for the answers so far, everyone.
     
  11. Oct 16, 2007 #10

    Doc Al

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    It depends on how you did the calculation. Did you give each counterclockwise torque a positive sign when you added the individual torques to find the net torque?

    Did you try using the "method" that I described in my first post? You can also just ask yourself: If that force were the only force acting, which way would the force make the object turn?

    Frankly, I would not bother with the right hand rule for this kind of problem. It's much simpler than that. But if you do use it, then if your thumb ends up pointing out of the page, the torque was counterclockwise; if it points into the page, it was clockwise.

    I recommend the following. Describe what you got for the individual torques when you found the net torque in part (a). How did you determine if they were clockwise or counterclockwise?
     
  12. Oct 16, 2007 #11

    robphy

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  13. Oct 16, 2007 #12
    yoshi-chan7 - It is not simply whether or not the force is positive or negative, it also has to do with where the force is being applied. For example. If I am holding one end of a long bar on a balance, and my friend is holding the other end of the bar on the other side of the balance either one of us could use a positive force (pushing up); but depending on which one of us is pushing it up will determine which side goes up and which side goes down. If you were looking at my friend and I with me on the right and him on the left and he pushed his end up, then you would see the bar turning in a counter-clockwise direction. However, if I pushed MY end up, then you would see the bar turning in a clockwise direction.

    What? No one thought that dancer was cool? It tells you if you're left-brained or right-brained.
     
  14. Oct 17, 2007 #13
    Thanks so much, everyone. I think I understand it now. I went over all the replies and yup, it's all good. :D

    By the way, I found out that I am very strongly right-brained from the dancer.
     
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