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Homework Help: Work Energy Power

  1. Oct 9, 2009 #1
    1. The problem statement, all variables and given/known data
    Hi Guys

    I have a problem when working with conservation of energy and work energy theorem. I have a problem when friction is present in the system.

    I dont know when to add the friction to get the total work done or when to minus the friction to find work done.

    Will someone be able to help me out?

    1. Lets say someone is going UP a ramb and friction is present.
    2. a skier is going down a slope and friction is present along the course.
    3. a person is going down a slope , friction present and they ask you to find the skiers velocity at the bottom.
    4. a car is moving up a slope and friction is present and you need to work out the velocity at the top

    My Answer.

    When someone is going up a ramp we add the work done by friction to obtain the work done.

    2. when someone is going down a ramp we minus the friction to find work done.

    Im not to sure about this. In my exam i lost the 13 marks for this question. Can someone just maybe give me some equations to use when friction is present for the different situations because when friction is present i get really confused.

    Thanks alot for reading..looking forward to your replies
  2. jcsd
  3. Oct 9, 2009 #2


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    Homework Helper

    Work done = F*d*cosθ.
    Frictional always acts in the opposite direction to the displacement.
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