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Inclined Pulley Situation

  1. Mar 22, 2009 #1
    1. The 1.0 kg physics book in figure is connected by a string to a 500 g coffee cup. The book is given a push up the slope and released with a speed of 3.0 m/s. The coefficients of friction are Frictionstatic=0.5 and FrictionKinetic 0.2.

    question 1, a) find how far it goes up the slope
    question 2, b) after stopping does it stay at rest or go back down the slope



    2. The kinematic equations..
    picture of the situation http://www.webassign.net/knight/p8-38.gif.




    3.
    Fc=force of cup Fb=force of book

    Fcy = .5*9.8=4.9N (force of the cup due to gravity)

    Fbx = N*sin(angle), (using vectors)
    N=1kg*9.8ms^-1=9.8N
    9.8*sin20=-8.94N (negative due to the direction downwards)

    Friction force opposing motion, N*Fkinetic
    9.8N * 0.2 = 1.96N(I'm assuming i have to ignore the static friction because the book is moving, this is correct?)

    I add the friction force, The cup force and the force of the book together to get the total force of the book down the slope, which equals 15.8N

    I know i then use the formula Vf^2=vi^2+2a*x to solve for x, but i don't know how to get the acceleration of the book up the slope!

    Do i take the total force down the slope, 15.8N
    use the formula f=ma to get the acceleration down the slope?
    what mass do i use? the mass of the book or the mass of the book + mass of the cup to solve for this acceleration?
    15.8/(massofBook+cup) = deceleration of the book as it is being pushed up

    -

    help please XD
    This is for the mastering physics knight 2e,
    I got the awnser to a) but only by capturing the SWF and hacking it to see the awnser
    it was 0.669M (the book is pushed up the slope)
    the second question b was a no brainer that dosen't require hacking lol.
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Mar 22, 2009 #2

    LowlyPion

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    Homework Helper

    As long as it is kinetic, ignore the static friction coefficient.

    Of course you can calculate right away whether it will remain at rest after coming to rest using the static coefficient. But other than that it's not needed.

    As to hacking the answer, ... why? It is straight forward enough. But unfortunately the way you are approaching it is likely making it difficult for you.

    The forces are easy peasy:
    1/2*g ... the cup
    1*g*sin20 ... the book from gravity
    μ*1*g*cos20 ... from friction

    That yields:

    F = m*a = .5*g +sin20*g + (.2)*cos20*g

    With total mass at 1.5 kg then

    a = (.5 + sin20 + cos20)*g/1.5
     
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