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Friction and work

  1. Oct 31, 2005 #1
    A person pulls her 70 N bag 200 meters at a constant speed.
    The force exerted is 40 N at 50 degrees.

    I can figure out the work the person does (5142 J).

    But I'm having trouble figuring out the 'work done by the force of friction'

    I know that Wfriction = delta KE + delta PE
    ...Since the speed is constant KE should be 0, but I'm not sure what to do about PE.. I know PE = mgy and delta PE = PEfinal - PEinitial but since it's a horizontal surface wouldn't that be 0? I must be doing something wrong because it can't be 0..
     
  2. jcsd
  3. Oct 31, 2005 #2
    The net force is ____ because the bag's velocity and speed are _____ which means the force by the ______ equals the force of _______.
     
  4. Oct 31, 2005 #3
    Just to make sure I understand

    The net force is _0___ because the bag's velocity and speed are __constant___ which means the force by the _parallel component of force(40N*cos(50))_____ equals the force of _friction______?
     
  5. Oct 31, 2005 #4
    You've got it.
     
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