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Homework Help: Force on a block

  1. Oct 27, 2009 #1
    1. The problem statement, all variables and given/known data

    A block is pulled along a rough surface by the force as shown in attachment

    1)draw freebody diagram

    2)write an expression for the normal force

    3)write an expression for coefficient of kinetic friction

    4)sketch graph of v vs. t & x vs. t

    5)if the force is large enough, the block will lift of ground. find expression for the greatest acceleration that the block can have and still maintain contact with surface

    2. Relevant equations

    3. The attempt at a solution

    1)FBD ill just explain

    Left = uN
    Right = FcosQ
    Down = mg
    Up = N

    2) N = mg - FsinQ

    3)F(friction) = uN = u(mg - FsinQ)

    u = F/(mg-FsinQ)

    4) ill explain

    v vs t ... im not sure, is the acceleration constant

    x vs t ... the is a slanted line coming out of the origin to the right & up

    5) FsinQ - mg = ma

    a = (FsinQ-mg)/m

    Attached Files:

    • 1.pdf
      File size:
      1.3 KB
  2. jcsd
  3. Oct 27, 2009 #2


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    Homework Helper

    Part 1) is incorrect; you are neglecting something. I suggest identifying each force that acts on the block, and then, for each force, identify its x and y components. Then, determine what are the x and y components of the acceleration.

    Parts 2) and 3) seem to be correct, somehow, inspite of part 1).

    Part 4): is the mass constant? is the force constant? is x vs. t a straight line?

    You need to think about part 5) some more. You might get a better idea after you fix part 1).
  4. Oct 27, 2009 #3
    1) should the up be N - FsinQ

    4) mass is constant. force is constant. so the acceleration is constant.

    then the velocity is the increasing line up to the right, and displacement is the right side of a parabola
  5. Oct 27, 2009 #4


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    Close, but no cigar. Which way does N point: up or down? Which way does F point: up or down?

    Brilliant induction, Watson!

  6. Oct 28, 2009 #5
    both go up so... N + FsinQ
  7. Oct 28, 2009 #6


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    Now you're talkin'.
  8. Oct 28, 2009 #7
    should it be cos

    you use sin for the x, and cos for y correct
  9. Oct 28, 2009 #8


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    You are using sin for y (up/down) and cos for x (left/right). There is no rule that dictates this (although it is typical); it is the choice that you have made. Is your diagram exactly as given, or should there be an angle defined on the diagram. That might make a difference.

    What about part 5)?
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