Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Extremely Simple Torque Calc

  1. Mar 31, 2015 #1
    I'm auditing someone's work and it seems I've found an error.

    We have a very simple scenario where a circular steel shaft lies on sandstone. What is the torque required to turn the shaft (the shaft is in a static position)?

    Torque = weight of shaft × radius of shaft × friction of steel on sandstone

    It seems that he has divided the total by 2 which doesn't make sense to me. Then again my physics days are way behind me.

    If someone could confirm that I'm crazy/lucid that would be great.
  2. jcsd
  3. Mar 31, 2015 #2


    User Avatar
    Homework Helper
    Gold Member

    Yes i think you are right , all you need to do is to counter the torque of friction which seems to be equal to what you state. er to be more precise torque=weight x radius x friction coefficient

    But if by shaft you mean a solid shaft where the mass is distributed in the whole surface of shaft and not in an outer ring of small thickness then the torque of friction is different.
    Last edited by a moderator: Apr 3, 2015
  4. Mar 31, 2015 #3
    Two torque components.
    (1) torque to overcome friction of body at contact point T1 = (force)(radius). Force = (weight)(coeff of friction). (??...perhaps)
    (2) torque to accelerate the mass of the bar from rest T2 = (mass moment of inertia)(angular acceleration)
  5. Apr 9, 2015 #4

    Randy Beikmann

    User Avatar
    Gold Member

    Is the shaft supported at the driven end? If only one end of the shaft rests on the sandstone, then the normal force causing friction is only half the shaft's weight. That may be why he divided by 2.
  6. Apr 10, 2015 #5

    jack action

    User Avatar
    Science Advisor
    Gold Member

    Are you sure he did not use the shaft diameter in his calculation instead of the radius? In that case, you have to divide the diameter of the shaft by 2 to get its radius.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook