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Homework Help: Smooth slope - A level question.

  1. Apr 16, 2008 #1
    I have a question that i am struggling to come to terms with. I know how to get the answer but i am not 100% sure why what i am doing works.

    The question :

    A smooth plane is inclined at X degrees to the horizontal.
    A block of mass 5KG is held at rest on the plane by a horizontal force of magnitude 168N.

    Find :
    Angle X
    The force on Normal on the block.

    Ok, so far i have this.

    i seperated the horizontal force of 168N into a force horizontal and a force parralel to the plane.

    Horizontal : 168CosX = 5gsinX
    parallel : N = mgcosX + 168sin X

    Now somehow, and this is the part i do not understand 100%

    you get to the equation


    so then X is the tan^-1 of (168/5g).

    and then you put it back in the parallel equation to find N, the normal.

    Can someone explain to me how you come about reaching these equations :


    thank you.
  2. jcsd
  3. Apr 16, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Compare this:
    To these:
    The first of these equations has a typo. But these are all restatements and rearrangements of your original equation for horizontal (parallel to the incline) equilibrium.

    Or are you asking how one does the algebraic steps?
  4. Apr 16, 2008 #3
    oops 5gcosX=168cosX meant to be 5gsinX=168cosX,

    typo confused myself, thats why i had written down.

    no wonder i didnt know how i got that, its not possible :P NVM =) thx for having the sense to point that out though. lol.

    btw : OT

    im currently sitting Maths, mechanics and physics at AS level in 6th form.
    This was a question from mechanics, but there is similar things in physics 1.

    on about the 16th of may i have lots of exams, i have all re-sits from january because i did terrible.

    Maths : E
    Physics : U

    so now i have maths 1, maths 2, mechanics 1, physics 1 and physics 2.

    i also have ict 1 and 2, and economics 1 but they are less of a worry to me.

    I'm determined to get an A in everything i sit, so i'm going to do nothing but revise for this month and ensure that i get A's. The simple fact is, if you do this physics a lot, you will get an A, no real intelligence involved at this level.

    so thanks for the help i will be asking a lot more questions, glad theres people like you to point out my stupid mistakes 8D. I also have a scanner set up so i'm likely to be adding pictures to questions i CBA to explain.

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