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Homework Help: Statics: Incline plane an friction

  1. Dec 4, 2005 #1
    Ive been working on this problem for a little while now and basically, I dont know where to go. I have found the friction angle of 7 degrees but I really have no idea where to go next. My book doesnt have any examples like the problem. The problem looks alot like a basic physics problem so I pulled out my old physics work to see if there was an equation for it, no luck there. Anyways, any help would be much appreciated. Thanks

    I scanned the problem from my book. I also circled it (sorry about the marks on there, my eraser wasnt working too well):

  2. jcsd
  3. Dec 4, 2005 #2


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    Homework Helper

    Forget about the angle of friction.

    Draw a free body diagram.

    You know that the friction force will be acting up the plane. That's one force.

    Resolve the weight of the block into components parallel to and perpindicular to the slope of the plane. That's two more forces.
    Ditto for the force P. The last two forces.

    In the case of limiting friction, the friction force will be just balanced by the total force acting down the plane.
  4. Dec 4, 2005 #3
    Yep, resolving perpendicular and parralel gives 2 simulataneous equations which can be solved and plus one gets R which is needed to calculate Fr.
  5. Dec 4, 2005 #4
    Thanks for the replies.

    Im still not having any luck, I need a new major:(
  6. Dec 4, 2005 #5
    Thanks for the replies.

    Im still not having any luck, I need a new major:(
  7. Dec 4, 2005 #6
    whoa, I didnt mean to do that, sorry
  8. Dec 4, 2005 #7
    Come on mate - cheer up its not that hard. Go to mathnet.net or mathnet.com (cannot remember which site it is) and look at M1 or M2 that is A-Level UK text books and has this exact statics stuff in - and its well explained on the site with examples. Any more help needed after that post again!!

    But remember Hasta La Victoria, Siempre
  9. Dec 4, 2005 #8
    I went to both of those sites and I couldnt find the information.

    If someone could give me a visual process of what I have to do (equations etc), I can probably pick it up. Im a visual learner. I have to see the process done before I can repeat it.
  10. Dec 4, 2005 #9
    Ok, I found the site and so far it has been helpfull..but, on one of there example problems, I dont see how they got a certain number.


    For step (a), could someone please explain to me how they are getting R = 117.7
  11. Dec 5, 2005 #10
    IT shows steps if you click the applet button "steps"
  12. Dec 5, 2005 #11
    I guess one of the reasons I havent been getting much feedback is because I havent been showing any work. Heres where im at now, sorry about the poor handwriting:

  13. Dec 5, 2005 #12


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    Staff: Mentor

    First, calculate the other force - the force normal to the plane, and with that, the friction force.

    Next, build some equations for what force "p" does - they'll look similar to what you did for the weight's components. You'll end up with two relevant forces due to the block's weight (normal and parallel to the plane) and two equations for the same two forces due to force "p".

    After that, you'll need to combine the equations you generate and solve them together with algebra. Hopefully when you see the equations, it'll be obvious how to combine them to make the variables drop out...
    Last edited: Dec 5, 2005
  14. Dec 5, 2005 #13
    I took your advice and this is where I got. The book gives an answere of 17.2 and im getting 16.3. What im a doing wrong?

    Last edited: Dec 5, 2005
  15. Dec 5, 2005 #14
    Please help, I think im missing something simple.
  16. Dec 5, 2005 #15


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    Homework Helper

    The force P is pulling the block to the right.
    If you resolve this force into components down the plane and normal to the plane, then you should see that the normal component is tending to lift the block up from the plane. In other words, the normal component of P is reducing the Normal Reaction, N, of the plane against the block and this should be taken into acount when caculating the friction force.(F = mu.N)
    Similarly. The force P has a component down the plane. So there are two forces acting down the plane, not just one. This fact should also be taken into account when calculating the friction force.

    In the attachment, Fig1 is the FBD showing all the forces acting on the block.
    Fig2 shows the forces resolved into components normal to and parallel to the slope of the plane.

    Attached Files:

    Last edited: Dec 6, 2005
  17. Dec 6, 2005 #16
    I got it, wow. that took forever. Thanks for all the help.

    On another note, I was studying the problem because the teacher hinted off that he was going to quiz us on it during the next class. Well, today was quiz day and he gave us a centroid problem, yeah a freakin centroid. Haha, easy A
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