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Springs and Friction

  1. Dec 6, 2006 #1
    1. The problem statement, all variables and given/known data
    A 30.0 kg block is resting on a flat horizontal table. On top of this block is resting a 15.0 kg block, to which a horizontal spring is attached. The spring constant of the spring is 325 N/m. The coefficient of kinetic friction between the lower block and the table is 0.600, and the coefficient of static friction between the two blocks is 0.900. A horizontal force T is applied to the lower block toward the spring. This force is increasing in such a way to keep the blocks moving at a constant speed. At the point where the upper block begins to slip on the lower block, determine (a) the amount by which the spring is compressed and (b) the magnitude of the Force T

    2. Relevant equations

    F=kx
    Ff= uma
    Fw=mg

    3. The attempt at a solution

    we did a problem like this with out the spring and i did it like this
    Fk1=u1mg
    Fk2=u2u1mg
    Fk1+Fk2=FT
    u1mg+u1u2mg=FT
    .9(15)(9.8)+.6(.9)(15+30)(9.8)=FT
    370.44=Ft
    taht migth be right i dont know if that is or how you ahd a spring putting a force on the top block
    please help me get started
     
  2. jcsd
  3. Dec 6, 2006 #2

    Doc Al

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    Staff: Mentor

    Start by identifying all the forces acting on the smaller block.
     
  4. Dec 6, 2006 #3
    goign to set right positive, oppsite of force T
    Force on the spring=kx
    Force of weight= mg
    Force of kinetic energy= uma

    i think thats all i know
     
  5. Dec 6, 2006 #4

    Doc Al

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    Staff: Mentor

    There are four forces acting on the smaller block; these are two of them. The weight acting down and the spring force acting to the right. What other forces act? What force balances the spring force? (Since the blocks move with constant speed, what can you conclude about the acceleration and thus the net force?)

    No such thing--this is not one of the four forces.
     
  6. Dec 6, 2006 #5
    there is no acceleration so the net forces are equal to 0
    the other forces would be the force is is being pushed toward the spring and maybe something to do with the friction force
     
  7. Dec 6, 2006 #6

    Doc Al

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    Staff: Mentor

    Right!
    Assuming you mean the applied force T, then no that is not a force acting on the upper block--it only acts on the lower block.

    Yes, that's the most important force for solving this problem. Hint: What's the greatest friction force that the lower block can exert on the upper block before they start slipping?
     
  8. Dec 6, 2006 #7
    Force of weight on the lower block


    FT=Force of the Spring
     
  9. Dec 7, 2006 #8

    Doc Al

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    Staff: Mentor

    What does the weight of the lower block have to do with the maximum friction between the blocks? Or did you mean the weight of the upper block pressing down onto the lower block? If so, that's called the normal force between the two surfaces. How does friction depend on the normal force?
     
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