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Homework Help: Having problems with forces in a static problem

  1. Jul 20, 2012 #1
    I have this situation:


    I know that tension and friction act poining at left. Weight is obviously downward but then I need a force pointing at right for the rod to be static, and the only force left is the normal force but why should it be pointing at left? Isn't it vertical?

  2. jcsd
  3. Jul 20, 2012 #2
    why friction is pointing to left if you want equilibrium.
  4. Jul 20, 2012 #3
    No. Normal force has a component to the right. Friction is pointing to left. I think I undeerstand why normal force has a component and it's not vertical only

  5. Jul 20, 2012 #4
    just be careful.normal force is always perpendicular to the direction of motion of point of contact
  6. Jul 20, 2012 #5
    Sorry, I don't get it. What is wrong? Thanks!
  7. Jul 20, 2012 #6
    normal force will be perpendicular to surface ,it does not have left component or right at least in this case.
  8. Jul 20, 2012 #7
    Well then friction should be pointing to left, what are you saying? If friction pointed to right then net torque wouldn't be zero and it would rotate! But if friction pointed to left it would have a translational motion. I need a force pointing to right
  9. Jul 20, 2012 #8
    you will have to take moment about center of mass .normal reaction is opposing that torque.
    edit-also if you take moment about point of contact then tension and weight would be counteracting
    Last edited: Jul 20, 2012
  10. Jul 20, 2012 #9
    Ahhhhh now I see it. Normal force has a moment! Thank you very much andrien!
  11. Jul 20, 2012 #10
    Yeah, that's actually whaat I had to do to calculate the tension force given the fact that I know the weight. After that I can calculate the friction coefficient which doesn't depend on anything, just on the angle theta. It's 0.29 for 60ยบ aproximately
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