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B A doubt in calculating torque

  1. Oct 11, 2016 #1
    When we say torque about a point what axis do we mean,how can rotational motion happen without an axis?
     
  2. jcsd
  3. Oct 11, 2016 #2

    russ_watters

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    The axis has to be specified or calculated.
     
  4. Oct 11, 2016 #3
    Torque involves two physical variables the Force and the radius vector drawn from the fixed point to the force ( point of application of the force)
    Does the definition of torque , the vector product of the two defines the direction of an instantaneous axis of rotation transferred to the fixed point?

    The effect of torque is motion (rotational /spin motion). Let us try to understand the effect .....i.e. the rate of change of rotational /angular momentum as equivalent to net external torque.
    so do you think a pre-fixed axis is necessary to analyze the orbital/spin motion. or a fixed point (may be center of mass of the system) can serve the purpose.
     
  5. Oct 12, 2016 #4

    Thank you for replying sir,when we say torque about a point,do we assume the axis is perpendicular to the plane passing through that point?and we study torque and angular momentum to analyse the rotational motion of a body,what is the use of calculating torque and angular momentum about a point when body can't rotate about a point.
     
  6. Oct 12, 2016 #5
    But when we say,torque about a point we don't assume any axis sir.
     
  7. Oct 12, 2016 #6
    But when we say,torque about a point we don't assume any axis sir.
     
  8. Oct 12, 2016 #7

    CWatters

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    Yes we do. That's why we say "about a point".

    A child on a seesaw (teetertotter) creates a torque about the pivot point of the seesaw. They create no torque about their seat.
     
  9. Oct 12, 2016 #8

    CWatters

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    Yes.

    Why do you think a body cannot rotate about a point?
     
  10. Oct 12, 2016 #9

    jbriggs444

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    When discussing torque "about a point", we are working in three dimensions. The torque will not be a scalar quantity which is merely positive or negative (clockwise or counter clockwise). It will be a [pseudo-]vector quantity with direction as well.

    The direction of the torque pseudo-vector is perpendicular to both the applied force and to the radius vector. It is determined by the vector cross-product of the two.
     
  11. Oct 12, 2016 #10

    russ_watters

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    Taking this broader; references are arbitrarily chosen for your convenience. You don't even need there to be any rotation at all to choose an axis/point of rotation and you can use whatever location helps your analysis.

    Consider a static analysis of a simple two-dimensional bridge. To analyze it, you sum the torques around one of the supports. And for a more complicated real bridge, you'll analyze the torques around various points along the span of the bridge to ensure the bridge's bending resistance is strong enough.
     
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