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AP Physics B Multiple Choice Newtonian (1 prob)

  1. Apr 20, 2005 #1
    Hi guys, I need help with the following problem I couldn't figure out:

    1) A block of mass 2 kg slides along a horizontal tabletop. A horizontal applied force of 12 N and a vertical applied force of 15 N act on the block. If the coefficient of kinetic friction between the block and the table is 0.2, the frictional force exterted on the block is most nearly
    A) 1 N
    B) 3 N
    C) 4 N
    D) 5 N
    E) 7 N

    The answer is E. Why?
     
    Last edited: Apr 20, 2005
  2. jcsd
  3. Apr 20, 2005 #2

    dextercioby

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    What thoughts did u have about it?Something must have crossed your mind...

    To get their answer,take g=10ms^{-2}.

    Daniel.
     
  4. Apr 20, 2005 #3
    I thought that the frictional force was given by the equation Ff = mu*m*g*cos(theta). But that only comes out to 1/5*2*10=4N. So I have no idea how to factor in the vertical force.
     
  5. Apr 20, 2005 #4

    dextercioby

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    What "cos" are u dreaming about ?All forces have right angles between them."cos 90°=0",IIRC.

    Daniel.
     
  6. Apr 20, 2005 #5
    Sorry I still don't understand. Can someone please help me get this concept?
     
  7. Apr 20, 2005 #6

    dextercioby

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    The problem is ambiguous,because it doesn't specify the direction of the forces...Assume the vertical force pushes the body into the table (it has the same direction & sense as the gravity force).

    Since the body is moving,u have to know what is the expression of the kinetic friction force.

    Daniel.
     
  8. Apr 20, 2005 #7
    You are correct, the vertical force is pushing the block into the table. I believe the force of kinetic friction is mu * Normal Force. Normal force here would be 15 N, right, so F kinetic friction = 3? Bah, I don't know.
     
  9. Apr 20, 2005 #8

    dextercioby

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    No.In the table,the pressing force is the 15N external force and the 20N force due to gravity attraction.Therefore,the normal force is 15+20=35N

    Daniel.
     
  10. Apr 20, 2005 #9
    Yes, but why isn't there an opposite reaction from the table upwards into the block of the same force of the block's weight, 20 N?
     
  11. Apr 20, 2005 #10

    dextercioby

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    That's part of the reaction force,called NORMAL FORCE...

    Daniel.
     
  12. Apr 20, 2005 #11
    Ok thank you for your help :smile:
     
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