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Coefficient of Friction from only time given

  1. Feb 8, 2015 #1
    [Moderator note: Moved from General Physics so no template.]

    An object slides down an inclined plane where there is friction. The time it takes the objects to slide down the plane is 5/4 times longer than the time it takes the same objects to slide down the plane without friction. Calculate the coefficient of friction.

    I am completely lost here. If anybody could help me where to start it would be great, like an equation to use it would be greatly appreciated! :D
    Last edited by a moderator: Feb 8, 2015
  2. jcsd
  3. Feb 8, 2015 #2


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    How long does it take an object to slide down an inclined plane of length l and angle alpha without friction?
    How long does it take with friction?
  4. Feb 8, 2015 #3
    I'm just completely stumped here, I'm sorry.
  5. Feb 8, 2015 #4

    Simon Bridge

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    Welcome to PF;
    Are you saying that you don't know how to do the problem without friction?
    Have you never seen problems about blocks sliding down frictionless slopes?
  6. Feb 8, 2015 #5
    It's been like 3 years since I did this last and that time it was in icelandic. I'm having difficulty with the word problems.
  7. Feb 8, 2015 #6

    Simon Bridge

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    Are you saying that you don't know how to do the problem without friction?

    Do you remember about drawing free body diagrams? Newton's laws of motion? Solving simultaneous equations?
    Are there words in the problem you do not understand due to the language change?

    Have you tried googling for the frictionless version of the problem?

    How would you normally calculate the time for a distance to be covered?
  8. Feb 8, 2015 #7
    I remember the free body diagrams. Simultaneous equations isn't that like substitution? or elimination?

    Mostly the confusion is that in Iceland we sometimes use different letters in equations so I put in the wrong value but I'm getting the hang of it more and more everyday. Mostly it´s difficult for me to translate what I want to know from Icelandic to english.
  9. Feb 8, 2015 #8


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    I can sympathise with you. I'd be lost, too, without a large diagram. :)

    Your first step should be to draw a clear diagram, and mark on it what you are given, and what you are asked to find. Often, this process alone will jog your thought processes into recognizing the procedure to be followed.

    One of the equations you'll need here is F = m.a
  10. Feb 9, 2015 #9

    Simon Bridge

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    I'm with Nascent here -
    step 1: draw free body diagrams.
    step 2: Newton's second law
    ... do everything with symbols (the letters for variables), you'll end up with a couple of equations with a couple of unknowns - one of the unknowns will be the friction coefficient s solve for that.

    as long as you remember the physics, the actual letters shouldn't matter: though it can help to note down what each letter means i.e. as labels on your diagram.
    You'll remember, and will have seen recently, that physics involves drawing lots of diagrams.
    Once you start doing that again, you'll find it's like falling off a bicycle.
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