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

Question about torque and center of mass

  1. Dec 20, 2014 #1

    AGC

    User Avatar

    I am a beginner and this is my first post.

    We know torque ## \vec T = \vec r × \vec F ## Now, should r be fixed in magnitude? I mean can I calculate torque only if the particle is rotating at a fixed distance around a fixed axis? I am having this idea because torque is explained (from where I read) using an analogy with doors.

    Second question. I read that generally a system of forces acting on a rigid body can not be reduced to a single force equal to the vector sum of the forces. Then I get confused because in case of weight, we replace gravitational forces on each particle of a rigid body with a resultant force through the center of mass.
    Will someone please explain?
     
  2. jcsd
  3. Dec 20, 2014 #2
    about your first question , your question is how can we deal with torque when r is variable 'not constant in magnitude' ?
     
    Last edited: Dec 20, 2014
  4. Dec 20, 2014 #3
    About your second question , [As I know] we treat the rigid body as a particle-like object so that the gravitational force will act only on its center of mass.
     
  5. Dec 20, 2014 #4

    AGC

    User Avatar

    I think my first question was not clear enough. I asked if the concept of torque is applicable only if the particle (or body) is rotating about an axis? And should it rotate in a fixed radius? Sorry if the question is of very intro level.
     
  6. Dec 20, 2014 #5
    No , [As far I know] The concept of torque can be applied if the rigid body is rotating about either a fixed point or a fixed axis , where the point 'similarly to the axis of rotation ' is allocated on an axis vertical to the xy-plane.
     
  7. Dec 20, 2014 #6
    See this vedio:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Question about torque and center of mass
Loading...