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Question on work and energy

  1. Mar 24, 2013 #1
    Ignorant as it sounds, what 1kW of energy spent on having moved an object from point A to point B would be really spent on ? Is it, 1. acceleration /desceleration of all moving parts of both the engine and the object itself, and 2. overcoming by all moving parts (including electrons in wires) of all possible physical resistances. Am I missing something, and the pure fact that the body has changed position - "a work" was done - itself has required energy ?
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  3. Mar 24, 2013 #2


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    First you have made a mistake. "1kW" is not an amount of energy. The Watt is a unit of power. Power is the rate at which work is done (or energy is used).

    The answer to your question is..

    Consider an ideal electric car with regenerative braking. If it's an ideal car there are no losses (no resistance in the wires, no friction, no air drag). When it starts off the electric motor converts energy stored in the battery into kinetic energy KE = 0.5mv2. Once upto the required speed the car will cruise along consuming no energy. This is because it is an ideal car so there is nothing to slow it down. When the car needs to stop the regenerative braking system can put all that KE back into the battery. The result is the car had moved from A to B without using any energy.

    Most petrol cars do not have regenerative brake systems. When they want to stop they are unable to put the kinetic energy 0.5mv2 back into the petrol tank. Instead they waste it as heat in the brakes.

    If the car is non-ideal then energy will also be lost overcoming friction, air drag, and other losses. Lets say that at speed V the air causes a drag force F on the car. Then the power required to overcome air drag is F * V.
  4. Mar 25, 2013 #3
    Thank you for the corrrection. I should rather have said 3 600 000 J of energy were spent ...and so on.
    I just thought a satellete moving on an orbit of a planet or a star, can it be viewed an "ideal" moving object ? Does not it however utilise (draw) some gravitational energy of (from) the planet / the star ? It is the planet / the star that is doing "work" on the satellete. So, the pure fact that an object changes its position in universe (sorry for using some bombastic words) still should involve spending of some subject's energy.
    (In 1 word : entrophy).
    Last edited: Mar 25, 2013
  5. Mar 25, 2013 #4
    No. An object in orbit (or free-fall) has 2 kinds of energy (that are relevant to its orbit). It has kinetic energy and gravitational potential energy. The sum of the 2 will be constant. If it speeds up it is gaining kinetic energy, but it is getting closer to the planet so it will be loosing gravitational potential energy.
  6. Mar 26, 2013 #5
    I mean more not "having" energy but using energy. Does not a rocket that overcome gravitation of a planet /star had made so not only due to the force of ts engine, but also due to attraction force of the surrounding systems (it had partially and "unwittingly" utilised their energies)?
    Last edited: Mar 26, 2013
  7. Mar 26, 2013 #6


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    No. Why does it need an energy source? It's moving in a vacuum so there is no air resistance. If there is nothing to slow it down it can keep going at the same velocity forever. eg It doesn't need an energy source to maintain it's velocity.

    I'm not sure I understand what you are asking. Are you asking if a rocket heading to the moon is pulled towards the moon?. If so then the answer is yes. Initially the rocket feels a backwards pull from the earth so immediatly after leaving earth orbit it starts to slow down, but all the while the pull of the earth gets weaker and the moon stronger. After a certain point it starts to acccelerate again.
  8. Mar 27, 2013 #7
    Thank you to read and answer my qwestions. I stongly believe that no work can be done without an exchange of some energies somewhere somehow, be it an "ideal" car (having "no losses"), or a "free falling" satellite on an orbit. And I believe the end of the universe starts at a line which no elementary particle can leave because there is lacking the outside "pull".
  9. Mar 27, 2013 #8


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  10. Mar 28, 2013 #9
    I believe in the black matter/energy. Have just a thought: try to make rotate in vacuum a 10 mm ball ("Earth") around a 1.3 m ball ("the Sun") at a distance of about 150 m apart ! Ridiculous.
  11. Mar 28, 2013 #10


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    Yes, work and energy are essentially the same, so you can't do work without expending energy. But you need to recognize what that means mathematically (W=FD). When you internalize/accept the mathematical definition of "work", you will see that neither of those examples involve the doing of work.
  12. Mar 29, 2013 #11


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    Perhaps of interest...


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