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Friction question

  • Thread starter DIrtyPio
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  • #1
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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?
 
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  • #2
tiny-tim
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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 de body maintain constant speed going
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:
 
  • #3
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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.
 
  • #4
tiny-tim
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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.
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 ).
 
  • #5
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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?
 
  • #6
tiny-tim
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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.
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:
 
  • #7
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So it accelerates with g*cosθ until it reaches the end of the slope.
 
  • #8
tiny-tim
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So it accelerates with g*cosθ until it reaches the end of the slope.
No!

It would only do that if there were no friction.
 
  • #9
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Yes... I've forgot about it, it accelerates with (G*cosθ-µk*G*sinθ)/m
 
  • #10
tiny-tim
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Yes... I've forgot about it, it accelerates with (G*cosθ-µk*G*sinθ)/m
But what is θ?

Remember, θ isn't given, you have to find it.
 
  • #11
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θ is 90 degrees minus the slope.
 
  • #12
tiny-tim
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θ is 90 degrees minus the slope.
Yes, but what is the slope?

You have to find it.
 
  • #13
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It is arctg θ
 

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