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Friction question

  1. Apr 24, 2009 #1
    Hi. I have a question. A body is lying on a slope. We increase the slope until the body starts to move. So my question is, if we stop increasing the slope when the body starts to move will the body maintain constant speed going down until reaches the bottom, or will it accelerate?
     
    Last edited: Apr 24, 2009
  2. jcsd
  3. Apr 24, 2009 #2

    tiny-tim

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    Hi DIrtyPio! :smile:

    Well, there's µs and µk

    so what does g*sinθ have to be …

    i] when the body starts to move ?

    ii] for the body to maintain constant speed ? :wink:
     
  4. Apr 24, 2009 #3
    The body starts to move when MU*m*g*cos(theta) <= m*g*sin (theta). I suppose that the body will be accelerateing but I'm not sure. Oh, and by the way, can you tell me why can't I use properly the advanced editing tools, even though if I leave them hw they automatically are put there it sais I've did not tuse the correctly.
     
  5. Apr 24, 2009 #4

    tiny-tim

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    Hi DIrtyPio! :smile:

    (I'm not sure what you mean by "advanced editing tools" … but I know the LaTeX isn't working at the moment.

    But why didn't you copy my µ and θ? :wink:)


    I don't think you're taking into account the distinction between µs and µk (static and kinetic coefficients of friction ).
     
  6. Apr 25, 2009 #5
    I ran into this question on an exam and if I remember correctly it did not said that I shold care about static and kinetic friction coefficients. So I don't know the answer to this question and I think I don't either know how to solve it, so if you could tell me how to solve this problem that would be great. I know that the static friction coefficient is greater than the kinetic one but if I know only the static one can I calculate the kinetic one? But as you see my main problem is theoretic, so I don't need to calculate anything, I just want to know how the gravitational force acts upon that body, so will it accelerate or the speed will be constant. Because as I said I think that by Newtons II law the force determines the acceleration of a body I think that it will accelerate exponentially. Is this right?
     
  7. Apr 25, 2009 #6

    tiny-tim

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    mmm … just because it didn't say that you should, that doesn't mean that you shouldn't have …

    the difference does matter: use F = ma to work out what happens :wink:
     
  8. Apr 25, 2009 #7
    So it accelerates with g*cosθ until it reaches the end of the slope.
     
  9. Apr 25, 2009 #8

    tiny-tim

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    No!

    It would only do that if there were no friction.
     
  10. Apr 25, 2009 #9
    Yes... I've forgot about it, it accelerates with (G*cosθ-µk*G*sinθ)/m
     
  11. Apr 25, 2009 #10

    tiny-tim

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    But what is θ?

    Remember, θ isn't given, you have to find it.
     
  12. Apr 25, 2009 #11
    θ is 90 degrees minus the slope.
     
  13. Apr 25, 2009 #12

    tiny-tim

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    Yes, but what is the slope?

    You have to find it.
     
  14. Apr 25, 2009 #13
    It is arctg θ
     
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