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Block Slides on Rough Horiz Surface with Pic

  1. Dec 14, 2009 #1
    1. The problem statement, all variables and given/known data
    A block slides on a rough horizontal surface from point A to point B with a force of magnitude 2 newtons acts on the block between points A and B. Points A and B are 1.5m apart. If the KE of the block at A is 5 joules, and KE at B is 4J, how much work is done on the block by the force of friction as the block moves from A to B

    my crude rendering of said block:

    http://img69.imageshack.us/img69/2411/30326150.th.png [Broken]

    2. Relevant equations

    change in E mechanical=Work done by other forces - Force of friction * distance

    3. The attempt at a solution

    change in KE = Force * distance + Work done by friction
    -1 = 2 cos 40 * 1.5 + Work done by friction
    -1 = 2.29813 + Work done by friction
    -3.298J = Work done by friction
    is this correct?
    i got -3.298J
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 15, 2009 #2


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    Science Advisor
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    Gold Member

    This equation is incorrect. The correct equation (Work-Energy Theorem) says

    Change in KE = Sum of the work done by all the forces.

    How many forces are doing work and what is the work done by each force? Call the unknown work done by friction Wf, write the Work-Energy Theorem correctly and solve for the unknown Wf.
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