1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Force by table leg

  1. Apr 29, 2015 #1
    1. The problem statement, all variables and given/known data
    https://courses.edx.org/static/content-mit-mrev~2013_Summer/problems/MIT/rayyan/check_points/Pictures/BK85.png [Broken]

    A horizontal uniform bar of mass 220 kg and length L = 3.0 m is placed on two supports, labeled A and B, located as shown in the diagram. A block of mass 30 kg is placed on the right end of the bar.

    Find the normal forces F_A and F_B exerted on the bar by the supports.

    2. Relevant equations
    I'm sure we'll use torque, T = rFsinθ. We'll probably use center of mass, Σmx / M. Because the bar is uniform, though, we know the center of mass will be the mid-point of the bar. Torque will be determined by the gravity acting on the center of mass, which has equation F = mg. Since the object is static the sum of all torques will be 0.

    3. The attempt at a solution
    I treat the point B as if it were a pivot point, in which case the distance to the center of mass is 0.5m to the left. Then the torque applied is T1 = 0.5 * 220g = 110g. The block has distance 1m to the right of the pivot and so exerts a torque T2 = -1*30g in the opposite direction. The normal force due to support A will then satisfy 110g-30g+N=0 and this implies N = -80g.

    However, this answer turns out to be wrong. Any idea what my mistake is?
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Apr 29, 2015 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Are you defining N to be the force at A or the torque it exerts about B?
     
  4. Apr 29, 2015 #3
    Oh wow, can't believe how simple my mistake was. Thanks for the hint, haruspex!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Force by table leg
  1. Force table vector (Replies: 3)

Loading...