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I Is there a true upward force on a rotating object?

  1. Dec 10, 2016 #1
    I understand that the torque on a gyrating object is defined as the force vector cross multiplied by the lever arm position vector, which produces a resultant vector that is normal to both of the original vectors. However, when an object (let's say a disk) is rotating about an axis counterclockwise, there is no actual acceleration felt by the object upward along the axis in the direction of the torque vector. Is this because cross multiplication is needed to multiply the two vectors and get a vector, and as such, the actual direction of the cross product vector does not accurately reflect the actual direction of the torque?
  2. jcsd
  3. Dec 10, 2016 #2


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    It reflects the direction of the torque, not the force, and the direction of the net torque when greater than 0 is in the direction of the angular acceleration, perpendiculTto the force and position vectors. The direction represents the axis of rotation. It s often convenient to indicate the direction as clockwise or counterclockwise , but when addng torques about different axes, the resultant torque is found by summing the individual torque vectors vectorialy.
  4. Dec 10, 2016 #3
    You didn't answer my question. I was asking why the object didn't experience movement in the direction of the torque vector (which is along the axis due to it being a cross product between the force and lever arm). Is the torque vector only pointing upward because of the mathematical convenience of using a cross product?
  5. Dec 10, 2016 #4
    To clarify the question,

    What does the torque vector itself tell you? What happens in the direction that the torque vector is pointing in (upward along the axis)? Is there an acceleration in the direction of the torque?
  6. Dec 11, 2016 #5

    Vanadium 50

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    Because it's the torque, not the force.
  7. Dec 11, 2016 #6
    What does the torque do in that direction?

    Torque is defined as the "tendency" for a force to cause the object to turn at a certain distance from the center of gravity. However, why does this tendency manifest itself as an upward vector?

    How do you use the torque vector itself?

    Is there an intuitive reason for the torque vector's direction?
    Last edited: Dec 11, 2016
  8. Dec 11, 2016 #7
    Since the torque vector is normal to both vectors, you can find the plane of rotation with the torque vector function coefficients.

    Based on the direction of the vector, you can find the direction of rotation.

    The magnitude of the torque tells you the "amount" of rotation.

    However, this means that the vector itself isn't mathematically useful.
    Last edited: Dec 11, 2016
  9. Dec 11, 2016 #8
    It is mathematically useful, because you can add torque vectors together to get the net torque, in the same way that you can add force vectors together to get the net force. Where you seem to be getting hung up is that there's no actual motion in the direction of this vector. That's because the vector isn't meant to signify a direction of motion; it signifies the axis of rotation and the magnitude of the angular quantity under consideration. That is perhaps a bit more abstract than force vectors (at least in terms of visualizing them), but they seem just as mathematically useful to me.
  10. Dec 11, 2016 #9
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