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Work done on block by spring and friction

  1. Oct 24, 2010 #1
    1. The problem statement, all variables and given/known data

    In the system shown in the figure, suppose the block has a mass of 2.7 kg, the spring has a force constant of 480 N/m, and the coefficient of kinetic friction between the block and the floor is 0.18.

    Find the work done on the block by the spring and by friction as the block is moved from point A to point B along path 2.

    Diagram:
    http://img706.imageshack.us/img706/5935/walker4ech08pr006.jpg

    2. Relevant equations

    Wspring = 1/2kx^2
    Wfriction = -uk*m*g*x

    3. The attempt at a solution

    Wspring = 1/2(480)(0.020)^2 = 0.096
    Wfriction = -(0.18)(2.7)(9.81)(0.020) = -0.0953532

    I inserted these values into the system, but it said that my sign for Wspring is wrong, so I switched it to a negative. It stopped saying anything about my signs, but it won't say that it is correct. I'm not sure what is wrong...
    THANKS.
     
  2. jcsd
  3. Oct 24, 2010 #2
    Although the work done by the spring only depends on the initial and final positions, the work done by friction depends on the entire path taken---i.e. whats the distance during which friction does work?
     
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