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Homework Help: Help with force problem

  1. Nov 28, 2013 #1
    1. The problem statement, all variables and given/known data

    A mass "b" is placed on top of a larger mass that is 2.4 times the mass of the smaller one as shown. The coefficient of friction between the larger block and the surface is 0.35. A horizontal force acts on the larger block.

    a) What force would be necessary to maintain an acceleration of 4.0 m/s2? (in terms of "b")
    b) What would be the minimum coefficient of static friction necessary to keep the smaller block from slipping?

    2. Relevant equations

    a = Fnet / m
    Ff = µFN
    Fnetx = Fa + Ff
    Fnety = Fg + FN

    3. The attempt at a solution

    Fg = ma
    Fg = 3.4b (9.81)
    Fg= 33.354b N [down]

    Fnety = Fg + FN
    0 = 33.354 N [down] + FN
    FN = 33.354b N [up]

    Ff = µFN
    Ff = (0.35)(33.354b N)
    Ff = 11.6738b N

    Fnetx= Fa + 11.6738b N

    I don't know how to get Fnetx from here, and I don't know if this is right so far

    4.0 m/s2= Fnet / 3.4b

    Also I don't understand part b)
    Thanks for the help
  2. jcsd
  3. Nov 29, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No, Fnetx = Fa. You want to find Fapp, where Fnetx = ƩF = Fapp + Ff.
    The smaller mass is to accelerate at 4m/s2. What force is necessary to achieve that? Where is that force coming from?

    Btw, I recommend working entirely symbolically, only plugging in numbers when you get to the final equation. It makes it easier to find errors, and much easier for others to follow.
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