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Newtons law, interacting objects question?

  1. Mar 23, 2008 #1
    Newtons law, interacting objects question??

    A block of mass 5.00 Kg sits on top of a second block of mass 15.0 Kg, which in
    turn sits on a horizontal table. The coefficients of friction between the two blocks are
    : μs = 0.300 and μk = 0.100. The coefficients of friction between the lower block and
    the table surface are: μs = 0.500 and μk = 0.400. You apply a constant horizontal
    force to the lower block so that this force is just large enough to make this block start
    sliding out from between the upper block and the table.

    (a) Draw the free body diagram for each block, naming the forces on each of them.
    (b) Determine the magnitude of each force on each block at the instant you have
    started pushing but motion has not started.
    (c) Determine the acceleration you measure for each block.
  2. jcsd
  3. Mar 23, 2008 #2


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    Welcome to PF!

    Hi Xinio64! Welcome to PF! :smile:

    You must tell us how far you've got, so that we know what's troubling you, and how to help.

    What have you tried? have you drawn the diagram?

    What equations have you got? :smile:
  4. Mar 23, 2008 #3
    Well I drawn two free body diagrams, one representing each block. The upper block has a weight and normal force of of 49N and a static frictional force pointing to the right. The lower block has a weight and normal force of 196N and a static frictional force of 78.4N pointing to the left and a constant force F pointing to the right. I don't know if in the free body diagrams I use kinetic friction or static friction forces. Equations I guess is F=ma. That's all I have.
  5. Mar 23, 2008 #4


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    Hi Xinio64! :smile:

    To get the starting force, you must use the static coefficient.

    To get the movement, you use the kinetic coefficient (but of course you use the force you got from the static coefficient).

    So … what value force do you get, and how do you carry on from there? :smile:
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