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Help with multiple choice question (kinetic energy)

  1. Nov 7, 2008 #1
    1. The problem statement, all variables and given/known data

    Work is done on an object far out in space where it has negligible gravitational-PE. If in the process there is no net change in KE, we can conclude a) friction may have been operative b) this situation is impossible c) the energy of the object has decreased d) the object's speed decreased e) none of these.

    2. Relevant equations

    3. The attempt at a solution
    I'm guessing it's a), since for an ordinary object on Earth it's possible to do work on it and not change its kinetic energy: if there is kinetic friction working against it, for example. but it says it may have been operative, so does that imply some other force could be acting against the object in space and yet not do any work on it?

  2. jcsd
  3. Nov 7, 2008 #2
    Think of it in terms of energy conservation, there is an equation relating a bodies potential and kinetic energy with any work done on or by that body;

    [tex]K_{1} + U_{1} + W_{other}=K_{2}+U_{2}[/tex]

    Consider the potential energy as the bodies internal energy. If the K's are the same what does that tell you about the bodies internal energy?

    As for the friction: think about what would provide this friction force in the abscence of gravity. It may help to draw a diagram.
  4. Nov 8, 2008 #3
    The work caused a change (either positive or negative) in the bodies internal energy?

    Also if some other object exerted a force on the body and slid against it, that would cause friction in space right?
  5. Nov 9, 2008 #4
    That would cause friction in space. But the question satets the mass is far away from any large gravitational effects, so any bodies in it's region must be of comparable size. Now the only way to get a friction force is to press against another body. What happens when you push something in free space, when it's mass isn't much much greater than your own?

    Yes the bodies internal energy either incresed or decreased depending on the direction of the force (internal energy being mass energy in this case). Note that the objects total energy has not changed, assuming it's rest mass stays the same.
  6. Nov 12, 2008 #5
    No friction would occur since the two objects would just move through space as if they were one object...
    Also what about random space particles. would they cause a small amount of friction if they slide past the object?
    If I'm understanding what you're saying correctly, you're saying that work changed the amount of internal energy in the object. But I was under the impression that forces that take away or increase internal energy don't really count as work since they don't contribute to any translational motion. (Btw, this question was from a chapter that only dealt with work that causes translational motion)

  7. Nov 13, 2008 #6
    In space the objects would likely move apart as soon as one exerted a contact force on the other (by Newtons third law). Also that work is being done is clearly stated in the question. It is also stated that the kinetic energy hasn't changed. To do work you must change the velocity, at least in this situation. K=1/2*mv^2. So in order for K to be the same the object must have lost/gained some mass, by some mechanism. The work hasn't changed the bodies internal energy, the mass loss/gain has.
    Last edited: Nov 13, 2008
  8. Nov 13, 2008 #7
    ok so basically:

    the work changed the velocity and also changed the object's mass so that KE will remain the same but the internal energy changed so the energy of the object changed. the velocity has to increase since there is nothing in space that can counter the object's motion. if the velocity increased then that means the mass has to decrease for the object to have the same KE. So the internal energy decreases. so..the answer is c.

    am i right?
  9. Nov 13, 2008 #8
    Almost. The work didn't change the bodies mass, we don't know what changed the mass. We also don't know if the work speeded up or slowed the body down either, so we can't be certain that there was mass increase or decrease and thus we don't know if the objects (mass) energy increased or decreased. So the answer will be e)none of these.

    Hope this helps
  10. Nov 13, 2008 #9
    oh right. the object could be moving at constant speed and a force could act against it and slow down its speed.

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