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Questions about forces on surfaces

  1. Sep 17, 2005 #1
    I've been trying to go through the problems in our assigned textbook, College Physics, but like any other Serway book the material is not in-depth enough for me to be able to solve the problems easily.
    This one particular problem has got me thinking about how blocks would move across a surface when there are two surfaces:
    A small block of mass m = 2kg rests on the left edge of a block of length L = 3m and mass M = 8kg. The coefficient of kinetic friction between the two blocks is mu_k = 0.3, and the surface on which the 8kg block rests is frictionless. A constant horizontal force of magnitude F = 10N is applied to the 2kg block, setting it in motion. a) How long will it take before this block makes it to the right side of the 8kg block? (Note: Both blocks are set in motion when F is applied.) (b) How far does the 8kg block move in the process?
    My main concern is what kind of a force would be applied on this block with mass M; would it be that exact 10N force or something else?
    I'd really appreaciate if someone could shed some light on how one can comprehend a problem such as this, as I can't make sense of the method my teacher used.
    Thanks alot for any replies.
  2. jcsd
  3. Sep 17, 2005 #2

    Doc Al

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    Staff: Mentor

    The applied force is given as 10N, so we must assume that's what it is! :smile:

    Now, to understand how each mass will accelerate and move, we have to look at all the forces acting on each. The top block has two forces acting on it, while the bottom block has only one. (Considering only the horizontal forces, since there is no vertical acceleration.)

    To figure out when the top block reaches the other side, compare the accelerations of each block.
  4. Sep 17, 2005 #3


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    The upper block exerts on the lower one a force of f = mg Newtons, so there is a friction of f*mu_k between the two blocks. It is this force that moves the lower block. Since the upper one is acted upon by a force of 10 N, you can calculate their respective accelerations and the time the upper block will take to cover the distance of 3 m.
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