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Trying to understand Work and Energy

  1. Nov 6, 2014 #1
    I understand that:

    1) For work to be done there must be a force (or a component of it) exerted upon an object, causing it to move. Work done on a system puts energy into it.

    2) If the force is applied in the opposite direction to that of the object’s displacement, then the work done by that force removes energy from it.

    3) If work is done by a system, energy is removed from it as well.

    But here is where I get confused:

    If I put a box on a table, it gains PE. If I let it fall from that height, it will lose its PE. My question is, when falling what force in the upward direction is being applied to the box, removing energy from it? On what is my system (the box) doing work?

    My textbook asks for giving an example where the work done by the system removes energy from it. I've been thinking in a car toy after having pushed it. It’s easy to think about it as friction doing work on the car with a direction opposite to its motion, and hence removing KE from it; but if my system is the car toy, on what is this doing work? On the ground?

    And ultimately (and my today’s silly question), Can a system do work on itself? I guess it can’t, because Newton’s Third Law would be violated. If this conclusion isn’t wrong, then what about the act of eating? A person eating is putting energy on themselves, but how is this interpreted in the context of work?

    Thanks !!
  2. jcsd
  3. Nov 6, 2014 #2
    When the box on your table falls, a downward force is applied to your box and your box moves downward. As a result work was done on your box and it gains kinetic energy. Associate work not with potential energy or energy in general, but rather specifically with kinetic energy. As an aside, your falling box is also doing work on the earth while it is falling.
  4. Nov 6, 2014 #3
    Aah, true.

    About the car toy, the only work done is by friction against its motion, right? Can there be an explanation where the car is the thing doing work on something (like the ground) removing energy from it?
  5. Nov 6, 2014 #4


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    Science Advisor

    Kinetic energy is not an invariant (aka absolute) quantity. It depends on your choice of reference frame. If you choose a reference frame in which the ground is moving and in which the car is [initially] at rest then friction between the two will remove energy from the ground while imparting energy to the car.

    There is an invariant quantity here -- the difference between the total starting kinetic energy and the total ending kinetic energy does not depend on the choice of reference frame. This lost kinetic energy appears as heat due to the friction.

    Note that when the car slows the ground down, the amount of work it is doing on the ground is negative.
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