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Work done negative and positive

  1. Dec 25, 2015 #1


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    1. The problem statement, all variables and given/known data
    I was just wondering whether the following notes I took on work in physics are accurate? Or have I got my understanding wrong?
    Note on friction

    Following from the example of the dragging object above…

    The object stops because it loses kinetic energy due to friction.

    How does the friction force do work?

    Ok, so I push an object, then let go; I have transferred kinetic energy to it.

    Now it moves a certain distance in a direction opposite to friction force acting on it.

    So, in W=Fd, F will have a negative sign as it is in the opposite direction; the magnitude of F is the magnitude of the friction force.

    Hence, energy transferred will come out with a negative sign.

    What does that mean?

    The W=Fd formula tells you that if you move something with a force, you are transferring some energy INTO THAT OBJECTàthe fact that W is negative here is because the point of application of force is losing energy; the kinetic energy of the object is being transferred out as heat energy (primarily), BECAUSE THE FORCE IS ACTING OPPOSITE TO THE DIRECTION OF MOVEMENT.

    If W has a positive sign, that means we are transferring energy INTO the object the force is acting on to make it move; if W is negative, energy is being transferred out.

    Thanks; your help would be much appreciated! :)
    2. Relevant equations

    3. The attempt at a solution

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  3. Dec 25, 2015 #2


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    What you have described is how I look at it: positive (negative) work occurs where kinetic energy is transferred to (from) the object.
  4. Dec 25, 2015 #3


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    Thank you! :)
  5. Dec 25, 2015 #4

    Mister T

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    If you want to treat the friction force as a force that does negative work and thus decreases the kinetic energy of an object, that is fine as far as that goes. But saying that the energy is transferred out of the object as heat energy is a problem. The fact is, some of the kinetic energy lost by the object does appear as internal energy outside the object, but some of it appears as internal energy inside the object. That is, an increase in temperature of both the object and its surroundings is observed.

    These increases in internal energy are associated with increases in temperature, but they are not associated with a transfer of heat energy.

    This is the reason that some introductory physics textbooks do not speak of work done by friction. They are trying to develop a dynamical definition of work that's consistent with the thermodynamic definition. If you describe situations where kinetic energy is dissipated as work done by friction you cannot have a definition of work that generalizes to work done in thermodynamic processes.
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