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Homework Help: Conservation of energy (true or false)

  1. Oct 27, 2007 #1
    1. The problem statement, all variables and given/known data

    1.Consider a train that pulls a wagon. By Newton's third law, the wagon pulls on the train with an equal opposite force, therefore the work done by the train must be zero.

    2.Negative net work done on an object always reduces the object's kinetic energy.

    3.Two identical masses are placed on frictionless slopes at identical heights. The first slope makes an angle of 30deg with respect to the horizontal; the second slope makes an angle of 40deg. Both balls are released simultaneously. At the bottom of the slope, both balls have the same speed.

    4.If the work done on an object by a force is zero, then either the force or the displacement must have zero magnitude.

    5.The formula W=F d cos theta can be used only if the force is constant over the distance d.

    2. Relevant equations

    Conservation of Energy

    3. The attempt at a solution

    At least one of these are wrong. I don't know which one(s)
    Please check and correct me. Thanks

    3.False, (because mgx changes due to the difference in angle. Since f=ma, accleration will not be the same.)
    4.False, (it may be the direction that makes the W=0, for example an object falling where force nor displacement is zero but W=0.)
  2. jcsd
  3. Oct 27, 2007 #2
    3 is True. Gravity is a conservative force. All that matters is the change in height, not how you get there. So slope one will make the ball travel longer, but it will still have the same speed as ball 2. Now, the two balls will NOT get there at the same time, but when they do get there, they will have the same speed.

    4 is True. Work is done by the Earth to pull the object in. W = F * D. F = gravity, D is how much it falls by.
  4. Oct 27, 2007 #3
    Arr.. still wrong.. help is still needed
    Last edited: Oct 27, 2007
  5. Oct 27, 2007 #4
    Why would 1. and 2. be false? Or are they true? HELP!!
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