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Homework Help: Work, Friction and Direction

  1. Sep 3, 2009 #1
    Since friction is in the opposite direction as the direction of movement, wouldn't you expect the work to be negative? When I put it in the work formula, W = F * cos(180O) * d
    it comes out positive because F is negative and cos(180) is negative too.

    This means that if I have another force pulling something across a surface, then it would do more work if there is friction.

    I read somewhere that the work is equal to the change in kinetic energy between two points but I was working on some homework and I realized that the only way I could get the right answer is if I made is so that W = F * cos(0O) * d (instead of theta = 180) in the friction work formula; that way friction gives negative work and when I add the two it equals to the change of kinetic friction.

    Is there anything wrong in my original reasoning that W = F * cos(180O) * d?
    I'm generally confused about work, could someone explain to me how it works?
  2. jcsd
  3. Sep 3, 2009 #2


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    in a case of no friction

    total work done= change in kinetic energy

    with friction

    total work done= change in kinetic energy + work done against friction

    or if you want

    total work done= change in kinetic energy - work done by frictional force
  4. Sep 3, 2009 #3


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    If the kinetic energy of the body increases, the work is done on the body. If the kinetic energy decreases, the work is done by the body.
    So you have to decide the direction of the net force and the displacement before using the formula.
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