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Friction Problem

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  1. Jan 27, 2016 #1
    1. The problem statement, all variables and given/known data
    The mass of A is 5kg and the mass of B is 10kg. If the acceleration of A is 2m/s^2 which is smaller than acceleration of B, where A is placed on top of B, then the value of frictional force applied by B on A is:
    A) 50N. B) 20N. C) 10N. D) None of these

    2. Relevant equations
    a(max)=meu*g
    frictional force=meu*normal force


    3. The attempt at a solution

    normal force= 5*10=50N
    meu= 2/10 = 1/5
    frictional force= 10N
    But I had to assume that 2m/s^2 was the maximum acceleration. Can I do that? Because otherwise there are too many unknowns.
    Thanks in advance!
     
    Last edited: Jan 27, 2016
  2. jcsd
  3. Jan 27, 2016 #2

    Suraj M

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    Where did you get the normal force from?
     
  4. Jan 27, 2016 #3
    sorry, I forgot to mention the masses. The mass of A is 5kg and B is 10kg
     
  5. Jan 27, 2016 #4

    Suraj M

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    You don't need to assume that 2 is the max acceleration, they've asked for the frictional force in this situation, not for the maximum frictional force that can exist, so you don't need to worry about that.
     
  6. Jan 27, 2016 #5
    But how would I do it if I don't assume that it is maximum? Because the a=meu*g formula is only for maximum a. So if I don't assume it to be the maximum how would I go about solving it?
     
  7. Jan 27, 2016 #6

    haruspex

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    You are assuming you need to find mu, but you don't.
    Forget B, forget friction. A mass m is undergoing a (horizontal) acceleration a in consequence of a net horizontal force F. What equation connects the three?
     
  8. Jan 27, 2016 #7
    F=ma
     
  9. Jan 27, 2016 #8

    haruspex

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    Right. So apply that.
     
  10. Jan 29, 2016 #9
    In this case it doesn't matter if 2m/s^2 is the maximum acceleration or not
    When two blocks are moving relative to each other,the friction between them is always the maximum (when they start their motion or are sliding w.r.t. respect to each other)
    If they have a certain relative acceleration,the friction acting is kinetic and is always a constant force

     
  11. Jan 29, 2016 #10
    are the two blocks on the ground?Does the ground possess friction?
    is it frictionless??
     
  12. Jan 29, 2016 #11
    assuming the ground /surface offers friction is going to make the problem too cumbersome/hard to solve(2 equations,3 unknowns!)

    I'll try to do it assuming the ground is frictionless!:)
     
  13. Jan 29, 2016 #12
    and yeah no implicit assumptions
     
  14. Jan 29, 2016 #13

    haruspex

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    As I posted (#6 and #8), you do not need to think about B at all. You know the mass of A, you know its acceleration, and you want to know the force acting on it.
     
  15. Jan 30, 2016 #14
    truly said,haruspex
    it's friction which acts forward and provides the acceleration for A
    and thus F=m*a=5*2=10 N is the magnitude of the frictional force!:)
    I was just complicating things,wasn't I,for the OP?
    My bad
    :)
     
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