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Friction -- please help -- HW

  1. Dec 28, 2016 #1
    1. The problem statement, all variables and given/known data
    A 2.00-kg aluminum block and a 6.00-kg copper block are connected by a light string over a frictionless pulley. They sit on a steel surface as shown in Figure P5.64, where theta= 30.0°. When they are released from rest, will they start to move? If so, determine (a) their acceleration and (b) the tension in the string. If not, determine the sum of the magnitudes of the forces of friction acting on the blocks.

    https://drive.google.com/file/d/0B4ksjSfetGUOeUdXX0tKODRWVjg/view?usp=sharing [Broken] ---- IMAGE
    fyzika.png

    2. Relevant equations
    1) I don't know how to calculate the last part: "If not, determine the sum of the magnitudes of the forces of friction acting on the blocks."

    2) Is it so far correctly calculated?

    3. The attempt at a solution
    For the system to start to move when released, the force tending to move m2 down the incline, m g 2 sinθ, must exceed the maximum friction force which can retard the motion:

    F = f(steel) + f(copper)

    = µ1m1g + µ2m2g

    = 0.61 x 2.0 x 9.8 + 0.53 x 6.0 cos(30) x 9.8 = 38.9 N

    Force tending to cause the system to move = 6.0 x 9.8 x sin(30) = 29.4 N
    Hence it will not start to move when released.
     
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Dec 28, 2016 #2
    It just hit me.... Did I already solved the: "If not, determine the sum of the magnitudes of the forces of friction acting on the blocks." Part? :D
     
    Last edited by a moderator: Dec 28, 2016
  4. Dec 28, 2016 #3

    haruspex

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    That is not correct for the block on the slope, but I see you used the right version in the next line.
    Possibly. You calculated two forces. Which one are you suggesting as answer to part b?
    Before answering, what exactly is the standard equation relating to static friction?
     
  5. Dec 28, 2016 #4
    The formula for friction is F = coeffiicent of friction * Normal contact force. For M2, the normal contact force is not the weight. but you have corrected it lol. Where did you get the coefficients from? Online? I see its 0.61 between aluminium and steel.

    Also, I know I posted in your earlier thread that got deleted, so please try to check out these two websites:

    scroll down for mechanics:
    http://www.cimt.org.uk/projects/mepres/alevel/alevel.htm
     
  6. Dec 28, 2016 #5

    haruspex

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    Not quite, and that is important here.
     
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