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Measuring torque

  1. Apr 12, 2009 #1
    Hello,
    How can I measure how much torque I'm exerting on a lever?
    Can it be done with a torque wrench? Or do I need ?
    Thanks
     
  2. jcsd
  3. Apr 12, 2009 #2
    A torque is a rotational force about a fixed point so you don’t really exert a torque on a lever, you exert a torque with a lever. Though I suppose there may be a continuous set of torques along its length.

    If you can use a torque wrench as your lever to exert your force about a fixed point then hey-presto, job done.

    Otherwise you need something that can measure a linear force, such a spring balance, with which you can pull or push your lever. Provided you use your measuring device at a right-angle to your lever, the torque about your fixed point will be the force exerted times the distance along the lever between the rotational centre and the point at which your measuring device is attached.

    If it happens to be spring balance marked in Newtons and you measure the radius in metres, your answer will be in Newtons times metres (Nm).
     
  4. Apr 15, 2009 #3
    Can a spring balance be in "lbs"? Torque = F x r
    How would I calculate the Torque in the case that the force applied to the lever is perpendicular to the direction of gravity? e.g. force applied to a door
    Is it correct if I multiply the lbs to just the length?
    Thank you for your help.
     
    Last edited: Apr 15, 2009
  5. Apr 16, 2009 #4
    Can a spring balance be in "lbs"? Torque = F x r
    How would I calculate the Torque in the case that the force applied to the lever is perpendicular to the direction of gravity? e.g. force applied to a door
    Is it correct if I multiply the lbs to just the length?
    Thank you for your help.

    The SI units for torque are Newton-metres. A spring balance could be in lbs in which case it would be conventional to measure the distance in feet (foot-pounds). You can use any unit of force times any unit of length (lifting capacity of an European swallow time the length of your forearm, for example) but then your measurement become less meaningful to other people.

    A spring balance measures force, it only coincidentally weighs things if you dangle them – then its measuring the force of gravitational attraction between the object and the Earth.

    If your spring balance is horizontal, with no force applied, it will read 0, if you hook one end of it to your door handle and use it to shut the door it will register the force required to shut the door. If you multiply that by the distance between the handle and the hinge, you have the torque applied to the hinge by the handle, through the door.

    Not going to get into vectors.
     
  6. Apr 16, 2009 #5
    How were the conversions derived for: N-m, ft-lb?
    If ft-lb is just the length (ft) multiply force (lbs), and N-m is length (m) multiply force (N), how did the conversion 1 ft-lb = 1.356 N come about?
     
  7. Apr 17, 2009 #6
    A mass of one pound is equal to 0.4536 kg. A force of one pound is equal to 0.4536 kg x 9.81 ms^-2. You now have a conversion factor for the force. One foot is equal to 0.3048 m. Multiply the two factors together, and hey presto!
     
  8. Apr 17, 2009 #7
    Torque is a linear force about a point.
     
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