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Properties of Friction H.W Problem

  1. Sep 29, 2008 #1
    1. The problem statement, all variables and given/known data
    I can't speak physics very well. So when you help me be a little patient please.
    A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force F of magnitude 6.0 N and a vertical force P are then applied to the block. Fig 6 shows a picture of a block at rest and the forces applied in there directions. The coeffictients of friction for the block and surface are static friction=.40 and kinetic friction=.25. Determine the magnitude of the frictional force acting on the block if the magnitude of P is (a) 8.0 N, (b) 10 N, and (c) 12 N.

    2. Relevant equations[/b
    Fk=Uk Fn
    Fs,max=Us Fn
    (Kinematic Equations table)

    3. The attempt at a solution
    Ok I have to find the magnitude of the friction force. Usually when I come across problems finding magnitude I usually use the equation Fn=√Fx^2 + Fy^2

    do i have to find something first before i find the magnitude. Like the acceleration or something.
  2. jcsd
  3. Sep 29, 2008 #2
    Is force P pushing the block into the horizontal surface or away from it?
  4. Sep 29, 2008 #3
    it's pushing into the horizontal surface
  5. Sep 29, 2008 #4
    What you need to do is first figure out if the block is able to move at all at each given value of P (i.e. is static friction strong enough to stop the block from moving). Ask yourself how P contributes to the normal force. If the block is able to get past the static friction, then you are left with the simple equation Force of friction = coefficient of friction times the normal force. Acceleration does not appear in this equation, so you do not need to worry about that.
  6. Sep 29, 2008 #5
    Shouldn't it move because they're all greater than the force of F.
  7. Sep 29, 2008 #6
    Are you trying to say that p doesn't help the block move anywhere.
  8. Sep 30, 2008 #7
    You are correct that P does not help the block to move anywhere. In fact, it is helping to prevent the block from moving by contributing to how difficult it is to move the block. Draw a free body diagram for the block and find out what the normal force is. Think about how static friction acts to prevent an object from moving. Does the equation for static friction force allow the block to move at all?
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