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Homework Help: Friction on inclined planes

  1. Jan 22, 2013 #1
    1. The problem statement, all variables and given/known data
    http://img203.imageshack.us/img203/8536/capturezo.png [Broken]

    2. Relevant equations

    3. The attempt at a solution
    I want to first make sure I understand the problem properly. Please verify each of my statements. If there is no force F acting on the (large) triangular block, then the large triangular block will move to the right. The small triangular block will slide down the large block depending on the coefficient of static friction. If the static friction is strong, the small block will stay put. If friction is too weak, the block will slide down the larger one. So far, so good?

    Once the force F acts on the large block, the situation is chaged. Now the forces acting on the large bock are:
    - Gravity
    - F
    - Normal force due to surface between floor and large block
    - Reaction force due to normal force due to surface between large and small block

    Forces acting on the small block:
    - Gravity
    - Normal force due to surface between large and small block
    - Frictional force due to friction in the surface between large and small block

    Is this correct so far?
    Next, having defined all the forces present, I want an intuitive understanding of the acceleration of each block. Obviously this will depend on F, but assuming we know F, can we correctly say that the large block will accelerate to the left with a certain acceleration we can calculate, and that the small block will accelerate up the large block? I am a bit confused about the smaller block's motion.

    What are we solving for in this problem? Also, please read between the lines of my post and post accordingly. My main confusion regards the interdependence between F, the large block, and the small block.

    All help is appreciated. Thanks.

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 22, 2013 #2


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    Staff: Mentor


    The small block should be "on the verge of sliding up" - but still stationary (relative to the large block). This simplifies the analysis, as both blocks move in the same way.

    The required force F.
  4. Jan 22, 2013 #3
    I don't understand what "on the verge of sliding up" means. What does that mean physically, and mathematically, does it mean that the frictional force on the small block cancels out with the gravitational component on the small block parallel to the surface between the large and small block?

    What is the net acceleration of the large block intuitively? Is it 0, or is it the same as that of the large block? I am very confused on these matters. Would appreciate further clarification thanks.

  5. Jan 22, 2013 #4


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    Staff: Mentor

    If you would increase F a tiny bit, the block would slide upwards.
    Therefore, the static friction is maximal in your setup (and pointing down the incline).

    I assume that you mean "small block"?
    It is the same as the large block.
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