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Work done question

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data
    Is it possible to have a scenario like this. An object is pushed up a slope and it has no initial speed. When it reaches the top the object's speed is also zero. So it's like work done by push+work done against friction=mgh. It doesnt seem plausible to me.

    2. Relevant equations
    Work done=force X distance moved in the direction of force


    3. The attempt at a solution
    I dont think it's possible as the object has 0 kinetic energy at the end. That means that the applied force can only counter the friction so it shouldn't be able to start moving at all unless it initially has speed already then it would be work done by push+work done against friction+KE=KE+mgh in this case even though numerically is the same it is different as now when there is no net force the object still has velocity so by newton's first law it will continue to move up to the top with a constant speed.

    The only possible way when the object has no velocity at first is if there is kinetic energy at the end because this means that the object accelerated which means there is net force acting on the object which makes it plausible for it to move.

    Are these correct? Thanks for the help!
     
  2. jcsd
  3. Oct 31, 2012 #2

    haruspex

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    The acceleration doesn't have to be constant. The object may start at rest, be pushed up the slope, and allowed to come to rest again.
    No. Try that again.
     
  4. Oct 31, 2012 #3
    but if we look at a constant force then that's not possible right? Since if we just look at the forces acting on the object, the object experiences no net force.
     
  5. Oct 31, 2012 #4

    haruspex

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    If the force were constant it would not be possible since the object would keep moving faster. But your problem statement doesn't say it's constant.
     
  6. Oct 31, 2012 #5
    Oh sorry I forgot to say I was taking O level so we deal with constant forces only. Thanks
     
  7. Oct 31, 2012 #6

    haruspex

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    I accept that you are only expected to deal with forces that are constant, but that's not quite the same as only dealing with problems in which all forces are constant. If you can solve a problem without having to consider forces, that might be within your syllabus.
     
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