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Homework Help: 2 boxes stacked, determining forces on each other.

  1. Oct 1, 2013 #1
    1. The problem statement, all variables and given/known data
    The mass of the upper block in the stack is 0.85 kg. The mass of the lower block in the stack is 1.67 kg. The coefficients of friction between the two blocks are: static 0.75, and kinetic 0.61. The child's mother, who likes to encourage his experiments, has oiled a small strip of the horizontal floor so that it is very slick; the coefficient of kinetic friction between the oiled section of floor and the lower block is only 0.12 and the coefficient of static friction is insignificantly different.

    Before the pushing starts, here are some questions about the vertical forces acting on the two blocks.

    b) What is the vertical component of the contact force on the upper block by the lower block? HINT: You must apply Newton's First Law.

    2. Relevant equations

    Since at this point in time the boxes are stationary

    3. The attempt at a solution

    I know the Fg of upper box is 8.33N, I also know that along with the normal force on the upper block there is also a FB2B1 block 2 being the upper block.
    I believe Fg,total= 24.696N, after many other attempts I tried to say Fg,total = NB2, therefore
    ƩF = n + FB2B1 - Fg,B2
    = 24.696N + FB2B1 - 8.33N = 0
    this gives me a negative result for FB2B1 which I know is incorrect.
    Here are my free body diagrams, I believe I may be missing something in my FBD which is why I cannot get the answer I am looking for.

    Attached Files:

  2. jcsd
  3. Oct 1, 2013 #2

    Simon Bridge

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    Looking at the upper block, where do each of the forces come from?
  4. Oct 1, 2013 #3
    I ended up getting the answer, I was over-thinking the problem as I do sometimes, especially when it's late. Thank You for your reply
  5. Oct 1, 2013 #4

    Simon Bridge

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    Well done. Would you like to provide the actual method for the benefit of others?
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