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Homework Help: Unable to visualize the concept of Torque.

  1. Jul 26, 2007 #1
    Hi guys,

    I find it quite difficult to visualize torque.
    The picture I use as reference could be found here.

    I have had this disk with spindle thing & had tried rotating it.
    But I did not feel anything in the direction of torque.(red arrow)
    What is torque ?
    Is it just a direction like North & South ?
    When I open & close a door, what goes up & what comes down ?

    [Other things like angular-velocity could be easily observed, say, for eg, marking dots at different radii on the disc]

    I will be glad if somebody can ignite me to think in the right direction, by explaining it in layman's terms.
    (Equations & derivations are available everywhere)


    Attached Files:

  2. jcsd
  3. Jul 26, 2007 #2
    Hi. My response is going to seem weird to you, but here goes.

    You will not feel anything in the direction of the red arrow. The problem is that vector algebra, the current language of choice for introductory physics, does not adequately describe quantities that come from vector cross products. There is a language, called geometric algebra, that is far superior to vector algebra in that it more correctly describes what you WILL observe in this problem. Specifically, you will observe that the applied torque causes the wheel to rotate one way or the other.

    Note that in the diagram, the force vector F and the position vector r form a plane. The diagram leads you to think that "something" happens in a direction perpendicular to this plane. This contradicts what you actually observe, namely the wheel's rotation. Depending on whether the wheel was initially rotating and in which direction, the wheel will visibly spin faster or spin slower. The spinning takes place in the same plane made by the force and position vectors. This is more intuitive than what the diagram leads you to (incorrectly) think.

    Basically, torque is a force applied so as to cause an object to change its state of rotation.

    The problem isn't you. The problem is how introductory physics is presented.
  4. Jul 27, 2007 #3
    yea I had the same problem understanding torque, until I learned about the cross product in Calculus III. The way that torque has a "direction" is from the right-hand rule. You will not be able to experience torque's direction in real life. The magnitude (value) of torque implies how strong the rotation is. When you open and close a door, nothing goes up or down. The torque vector, however, is either up or down depending on whether the door is turned clockwise or counterclockwise.

    heafnerj explains it well
  5. Aug 15, 2007 #4
    I am glad that you both understand my situation quite well :).

    Can I then confirm, then that, though a vector has both magnitude & direction, it may not manifest itself in reality ?
  6. Aug 15, 2007 #5


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    I don't believe it is correct to talk about torques this way. The direction of the torque indicates the axis about which the torque is applied. Since it is a vector quantity, it obeys the laws of addition of vectors just like forces. Let's say for example you apply 2 force vectors equal in magnitude through a point on an object, which each force oriented 90 degrees to each other. The resultant force is along a 45 degree angle. Now let's say you apply 2 torque vectors equal in magnitude about the center of that object, one oriented to give a rotation about the x axis, the other oriented to give a rotation about the y axis. The resultant direction of the torgue is then along an axis 45 degrees to the axes. This is the rotation axis of the combined torques, and is quite real.
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