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Help cant figure this question out

  1. Jun 23, 2012 #1
    1. The problem statement, all variables and given/known data
    a 20kg block rests on a horizontal surface and is attatched to a mass of 3.5kg by a cable,the pulley is frictionless ans the coefficient of static frition is 0.2

    prove that the mass is not suffcient to start the block moving,what is the additional mass that would be requaired to start motion


    2. Relevant equations
    http://img818.imageshack.us/img818/9962/37642403.png [Broken]

    3. The attempt at a solution
    20kg block Ff=39.24 Fn=196.2
    3.5kg block Fw=34.33
    i can work out the acceleration which is 6.68 m/s

    but i just dont know hwo togo about working out what it is asking to do,as in prove the mass is not sufficient to start the block moving,what additonal forceis needed

    any help would be great Thanks
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jun 23, 2012 #2
    What makes a body move from rest? Can you relate to any laws?
     
    Last edited: Jun 23, 2012
  4. Jun 23, 2012 #3

    NascentOxygen

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    Staff: Mentor

    Hi camo2412! Welcome to Physics Forums.

    I'm puzzled about the acceleration you say you have worked out. The masses won't move, because the smaller one exerts a horizontal force of (according to your figures) 34.33N, while your calculations determined that a force of 39.2N is needed to overcome the bigger block's friction.
     
  5. Jun 23, 2012 #4
    thanks nas
    the acceleration calculation i did shouldnt have been in the that was another question is was working out sorry.
    but yes from what i undertstand it wont move until another extra 5n is applied,
    therefore i think is its an extra .5kgs that is needed
    .5x9.81=4.9 which rounded up lets say 5n

    just unsure if this is correct answer to the question?
     
  6. Jun 23, 2012 #5

    NascentOxygen

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    Staff: Mentor

    Sounds about right to me. :smile:
     
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