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Incline problem

  1. Oct 12, 2007 #1
    1. The problem statement, all variables and given/known data
    Two packages at UPS start sliding down the 20 degree ramp shown in the figure. Package A has a mass of 7.00 kg and a coefficient of kinetic friction of 0.180. Package B has a mass of 12.0 kg and a coefficient of kinetic friction of 0.150.

    How long does it take package A to reach the bottom?

    [​IMG]

    2. Relevant equations

    F=ma (sorry but thats all I know for this question)

    3. The attempt at a solution
    I hate incline problems with a passion, and I have no idea how to start one. All I have done is draw free body diagrams for this question. I know you are supposed to apply Newton's second law, but it get confused when i do this on an incline question
     
  2. jcsd
  3. Oct 12, 2007 #2
    On your free body diagram, if you take the incline as your x-axis, this should help simplify things.
     
  4. Oct 12, 2007 #3
    can you show us what you got for Newton's second law when applied to each block individually?

    For this problem you should first find the acceleration of package A. From there you can work out the time it takes to fall down the 2m @ 20* incline.
     
  5. Oct 13, 2007 #4
    k well for box A i got

    Fnet = F B on A - Ff

    and for Box B i got

    F net = -F a on B - Ff

    but what would F b on a be equal to if i broke it down into f=ma format? Would it be f= mass of b times the acceleration of b?
     
    Last edited: Oct 13, 2007
  6. Oct 13, 2007 #5
    your missing the acceleration due to gravity in your equations.

    and yes the force B exerts on A will be the net force that B is experiencing.
     
  7. Oct 13, 2007 #6
    oh crud i forgot, it should be Fgx in both questions.
     
  8. Oct 13, 2007 #7
    so i should work with diagram b right? to find the acceleration of block a.
     
  9. Oct 13, 2007 #8
    it doesn't matter which one you work from, they both experience the same acceleration.
    And since you need to substitute in force of block a on b, or force of block b on a. Eventually you will get down to the same equation with the same answer.
     
  10. Oct 14, 2007 #9
    k but like what do you mean substitute?
     
  11. Oct 14, 2007 #10
    i got 4.3586 as my acceleration..
     
  12. Oct 14, 2007 #11
    could you show the work please? I got something like lower than your value for acceleration.

    When you have the 2 formulas for the net force of A and B you need to consider the effect that B has on A because A has a higher coefficient of friction it will probably be slowing down B so B will be pushing down on A and thus giving it a bit higher accleration.
     
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