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

Homework Help: Compare torque from force applied at different points

  1. Apr 5, 2007 #1
    1. The problem statement, all variables and given/known data
    Two equal forces are applied to a door. The first force is applied at the midpoint of the door; the second force is applied at the doorknob. Both forces are applied perpendicular to the door. Which force exerts the greater torque?

    A) The first force at the midpoint of the door.

    B) The second force at the doorknob.

    C) Both forces exert equal non-zero torques on the door.

    D) Both forces exert zero torque on the door.

    3. The attempt at a solution
    I think it's B. Would this be correct?

  2. jcsd
  3. Apr 5, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Indeed, you are correct. The torque is given by the cross product of the force vector and the position vector of the applied force. Since the force is applied orthogonal to the door this simplifies to T = F*r.
  4. Apr 5, 2007 #3
    or as archimedes once said, give me a place to stand and a long enough lever, ....
  5. Apr 5, 2007 #4
    Thank you! :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook