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Work and friction

  1. Apr 7, 2015 #1
    Which of the following statements are true?
    (A) No work is done on an object if that object remains stationary
    (B) Work can only change the velocity or position of an object and not its temperature
    (C) Energy is conserved when frictional forces are applied to an object
    (D) Energy transfer as heat always accompanies friction

    3. The attempt at a solution

    I understand how A, B, and D are incorrect but I cannot figure out how C is correct. My reasoning is that friction is a nonconservative force, therefore, how can energy be conserved when friction is involved?
     
  2. jcsd
  3. Apr 7, 2015 #2
    Energy is always conserved. What friction does is that it converts the "useful" form of energy (say kinetic energy) into "not useful" form of energy (which may be the dissipated heat, or the kinetic energy of air molecules). You cannot create or destroy energy. If a force is non-conservative, it simply means that the work done by that force will be dependent on the path taken by the object on which that force is acting, contrary to conservative forces whose work doesn't depend on the path taken by the object.
     
  4. Apr 8, 2015 #3
    Ok. I understand now. A is wrong because a stationary object we may be neglecting PV work or some other type of energy on the system. B is wrong because an increase in internal energy will raise the temperature of object, and D is completely wrong because in the example of PV work, compression of a real gas will raise internal energy which can dissipate out in to the environment without the use of friction.
     
  5. Apr 8, 2015 #4
    Yes you are right. And all your explanations are correct.
    However I am having a feeling that you are considering "heat" to be something that is of no important? Heat is just a way to transfer energy at the system boundary. It is not necessary that it always gets dissipated in the surrounding. Consider this, when you heat water, it gets converted to steam which can be used to run a turbine to produce electricity (that is what we do at a thermal power plant). So heat is always to be dissipated in the environment.
     
  6. Apr 8, 2015 #5
    Thank you for the reply. Actually, I gave the example of heat transfer only as an example even though heat has broad applications. I think my confusion was the difference between conservative and non-conservative forces. There are many details in physics that when ignored can backfire when you don't fully understand. Thanks for answering my question!
     
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