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Block on Block with Friction

  1. May 16, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    F = ma

    3. The attempt at a solution

    When drawing a FBD for block 1, I couldn't figure out if the friction force between the blocks was toward the left or toward the right. Similar situation on block 2. How would I determine this?
  2. jcsd
  3. May 16, 2015 #2


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    Staff: Mentor

    Which direction do you expect for the acceleration of the top block?

    Technically you can choose both directions for the force, but one choice will give a negative value for the force.
  4. May 16, 2015 #3
    Accelerate left. So the friction force would be left?
  5. May 16, 2015 #4


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    Which way do the dishes accelerate in this video?
  6. May 16, 2015 #5
    To the right, but when I put a book on top of a heavier book and pull the heavier book right, the book goes left and falls off?
  7. May 16, 2015 #6


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    No, the top book goes right. It falls off the left end of the bottom book because the bottom book accelerates faster than the top book.
  8. May 16, 2015 #7
    Oh. So the friction on the top book would be right, and that on the bottom book would be left.
  9. May 16, 2015 #8


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    Yes, that's correct.
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