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Fnet=ma (kind of confusing question please help)

  1. Jun 21, 2012 #1
    1. The problem statement, all variables and given/known data
    In the diagram below are two blocks. Block A sitting on top of block B has a mass mA = 10.0 kg and is attached to the wall by massless string. Block B, mass mB = 35.0 kg, is pulled by a force F = 75.0 N. The surface under block B is frictionless. The coefficients of friction for block A and B are μs = 0.34 and μk = 0.17.
    (a) Determine the acceleration of block B.
    (b) Determine the tension in the string.

    2. Relevant equations
    Fnet=ma
    Ffriction-mg*coefficient friction


    3. The attempt at a solution

    For the first part i went
    fnet=ma
    Ft=(m1+m2)a+M*g*μk
    75=45a+35*9.8*.17
    a=41.68/45
    a=.37 m/s^2

    Not sure if i am doing this correct, the coefficient of static friction is confusing me, do i need it in this equation?

    Part B
    Fnet=ma
    Ft=Ffriction+Ffstatic
    Ft=.37*35*.17+35*.34*9.8
    ft=118.82N (seems right not sure)

    I am not fully confident in my work an answers can someone tell me if idid this correct, i have no answers unfortunately given to me. Please tell me if i went wrong somewhere or not thank you!
     
  2. jcsd
  3. Jun 21, 2012 #2
    Mass A will stay on top of mass B as long the acceleration less than the static friction + the tension of the spring.
    Nothing is said about the spring.
     
  4. Jun 21, 2012 #3

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    The top block cannot accelerate in the direction of the applied force, with respect to the ground, because it is attached to the wall. So your approach is incorrect. You should use free body diagrams for each block and identify the forces acting on each , and apply all of Newton's laws.
     
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