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Friction problem -- 2 Blocks Sliding on a Surface

  1. Nov 14, 2014 #1

    rf1

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    1. The problem statement, all variables and given/known data
    A block of mass m=2kg and one of mass M=8kg have initial velocity zero.
    The friction coeffecient between m and M blocks is 0.3
    Between block M and the surfasse there is no friction.
    L=3 metres
    F is a force constant and horizontal with magnitude 10 N, applied on block m and so block m starts to move and goes to left extremity of block M.
    block M also moved as you can see in the image

    1) How long does it takes to block m to arrive to left extremity of block M?

    2? What is the value of the displacement of block M?

    the image is:
    http://s3.amazonaws.com/answer-board-image/79698f93-5a84-4194-ab39-782abe24edcd.png

    2. Relevant equations


    3. The attempt at a solution

    i put the forces applied in each block:
    block 1:
    x: ma1=F-F(of friction)
    y: mg=Normal force 1

    block 2:
    x: Ma2=F(of friction)
    y: Mg=Normal force 2

    and then i took a2 (acceleration of block 2) = 0.735 m s^-2

    i tried to use v^2=v^2 (initial) +2ax
    but i dont know v neither x....any suggestion?
     
    Last edited by a moderator: Nov 14, 2014
  2. jcsd
  3. Nov 14, 2014 #2

    PeroK

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    What about looking first at the relative motion of block m with respect to block M?
     
  4. Nov 14, 2014 #3
    Although you have an image, I suggest you to draw a free body diagram representing all the forces that are involved in the situation (in the image just appears one). Then see what happens. Is there constant velocity?

    For there to be motion of something, there must be a force acting on that something.
     
  5. Nov 14, 2014 #4

    Doc Al

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    Staff: Mentor

    Good. What's the acceleration of block 1?

    Well, you do know the distance... the relative distance. Since you don't know v, you might want to choose another kinematic formula.
     
  6. Nov 14, 2014 #5

    rf1

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    I cant calculate acceleration of block 1 because it is in an accelerated frame and so the second law of Newton is not valid

    I tried to use Energy-Work Theorem but i cant because i dont have enough information to use it
     
  7. Nov 14, 2014 #6

    PeroK

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    Suppose two racing cars started at the same time. One accelerated at ##a \ ms^{-2}## and one accelrated at ##b \ ms^{-2}## and you measured the distance between them over time.

    Suppose you repeated this and the first car failed to start and the second car accelerated at ##(b-a) \ ms^{-2}## and you measured the distance between them over time.

    Would there be any difference?
     
  8. Nov 14, 2014 #7

    Doc Al

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    You can calculate the acceleration of block 1 exactly as you calculated the acceleration of block 2: By applying Newton's 2nd law from the inertial frame of the ground. Those accelerations will be with respect to the ground. It's up to you to figure out the relative acceleration of block 1 with respect to block 2.

    You won't need the work-energy theorem; stick to Newton's 2nd law and kinematics.
     
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