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Homework Help: Easy Acceleration / Friction question

  1. Apr 22, 2009 #1
    1. The problem statement, all variables and given/known data

    A roof has an incline of 10 degrees. If the µ of friction between a 50kg box and roof is .15, how fast will the box move?

    2. Relevant equations

    Ff=µ * Fn

    3. The attempt at a solution

    I have no idea how to get this. I know it very easy.
    I figured out Fg = 490N (Fg=9.8 * 50kg)
    I figured out Fn = 482.56 (Fn=490 cos 10)
    I figured out F//=85.09 (F//=490 sin 10)

    I figured out the Acceleration with no friction is 1.70 m/s2 85.09/50

    Any help is appreciated, thanks
    Last edited: Apr 23, 2009
  2. jcsd
  3. Apr 23, 2009 #2


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    Welcome to PF!

    Hi dglenn9000! Welcome to PF! :wink:
    Hint: just use good ol' Newton's second law …

    total force (in the direction of the slope) = mass times acceleration :smile:
  4. Apr 23, 2009 #3
    I have a questions : there is an air track and on that track there is a body at rest upon which acts a constant force for a given time it gains a velocity. My question is if that force acts again upon that body for that amount of time will the velocity be 2 times the first velocity? My opinion is that starting from Newtons law v=v0+at, but if we use two times this quation we obtain that v1=at+at=2v0. Am I right?
  5. Apr 23, 2009 #4


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    Welcome to PF!

    Hi DIrtyPio! Welcome to PF! :smile:

    (but please start a new thread in future)
    yes, but it's better to look at it from the words of Newton's second law …

    force = rate of change of momentum = mass times rate of change of velocity,

    so (from the meaning of "rate of change") ∫force times time = mass times change of velocity. :wink:
  6. Apr 23, 2009 #5
    Re: Welcome to PF!

    That is how I got how fast it will move without friction.

    a= F/m
    1.70 m/s2= 85.09/50kg

    But my question is how do I figure out the acceleration with friction of .15? Where do I apply the .15 in my equation?
  7. Apr 24, 2009 #6


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    Hi dglenn9000! :smile:

    just woke up :zzz: …
    You multiply it by the normal force to get the (magnitude of the) friction force: Ffriction = µN.

    You then chuck the friction force into the "total force" in Ftotal = ma :wink:
  8. Apr 24, 2009 #7
    Hi everyone do you do any problems considering phyisc?
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