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Problem understanding the definition of mechanical work

  1. Mar 8, 2010 #1
    Hi guys,

    I know that the simple definition of work done is just Work=Force X Displacement.
    If this is the case, there are 2 scenarios which I dont really understand.

    1. If say the force applied on a car moving at constant velocity at 5m/s is 500N for a distance of 10m (therefore 2 sec), and the friction work done on the car which is also 500N- hence the constant velocity,the work done on the car by definition is 5000J. What happens to the 5000J of energy if the car does not gain it in KE (velocity is constant)? If say another car was acted on by the same forces and same amount of time but was not moving at all- started at 0 velocity did not accelerate due to friction, then by definition no work is done on the car as there was no displacement. I can't get how this could be possible. In both cases you must have used the same effort. Can someone explain this to me please?

    2. If the heat of reaction of a chemical reaction forming an expandable gas is measured in a system of which you forced to have constant volume, then does the mechanical work done- expansion of the gas, get converted into heat energy?(therefore increase of temp) if it does not, what happens to the mechanical energy?

    Thank you very much again for answering my newbie questions.
     
  2. jcsd
  3. Mar 9, 2010 #2

    Pythagorean

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    No displacement means no work done on the car. In a real system, of course, objects can be compressed and warped and bent. (I.e., there is displacement, just not a lot). If you're talking about effort, then it's likely whatever is applying the force is doing so through internal force of it's own that generate heat as a byproduct. Your cells, for instance, have little pumps in them that act on ions over a distance. Work is being done internally. It's just that no work is being done on the object you're trying to move.

    Yes, the particles will have more kinetic energy which means more heat.
     
  4. Mar 9, 2010 #3
     
  5. Mar 9, 2010 #4
    converted to heat at the point of friction
     
  6. Mar 9, 2010 #5

    Pythagorean

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  7. Mar 9, 2010 #6
     
  8. Mar 9, 2010 #7

    Doc Al

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    If the object doesn't move, no work is done. No mechanical energy is converted to 'heat' or anything else.
     
  9. Mar 9, 2010 #8

    Pythagorean

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    Consistent with what Doc Al is saying, if the object is compressible, it will be converted to heat (and/or kinetic energy). But the object will have "moved" (that is, its constituents will have all compressed more tightly together). So there will be a force applied over a distance for each constituent (say, molecules or atoms) of the material, but the object as a whole will not have had any displacement (since we define it's displacement by it's center, which hasn't moved) but all the atoms making up the material are squeezed into a tighter area, so they have all moved through a displacement from the applied forces.
     
  10. Mar 9, 2010 #9
    Does that mean that when something uses energy to apply a force, as long as the object the force is applied on doesnt move (due to opposing force), it can keep applying that force as it never runs out of energy due to no work done? or is it the work done in one direction is totally cancelled by the work done in the opposite direction therefore work is still done and energy is still used?

    I know this a very simple concept, but I find it exceedingly difficult to grasp.
     
  11. Mar 9, 2010 #10
    Assuming if something is incompressible, does the energy working in opposite directions cancel each other out? or will no energy ever be used if there was no movement?
     
  12. Mar 10, 2010 #11

    sophiecentaur

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    You can either do work against friction (losses), in which case the energy will end up as thermal, or against an 'energy storing' system, like a spring or by accelerating an object. If an object is "incompressible" then no work can be done in changing its shape - but of course, you can still accelerate it.

    Your idea of energies "cancelling out" would apply in the second case where the energy lost by pushing was gained by the object. In an ideal case, the same amount ot total energy (mechanical, and therefore retrievable) would be there at the end as at the start.

    I think you have to be rigorous in any arguments about work and energy, remembering that you need a force and a distance moved (in that direction) by (a part of) the object.
     
  13. Mar 10, 2010 #12

    Doc Al

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    Obviously not. By your own stipulation, you are using energy to provide the force. (That depends on how you are generating the force.) That's got nothing to do with the mechanical work done by the force.
    No, there is no mechanical work being done on the object if it doesn't move.

    You are mixing up the energy required to generate a force with the mechanical work done by that force. Two different things.

    Example: I hold my hand out and support a 5 kg mass. I am using a biological system (me and my arm) to generate the force, which requires energy. Nonetheless, I do no work on the mass, since it doesn't move. I could have just put the 5 kg mass on a shelf, then the shelf would provide the support force without requiring any energy input. Again, no work is being done on the mass.
     
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