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Consider the situation when a body is placed on the ground. Let this be the state of reference. Now I lift the body to certain height h above the ground. Here, I am applying force on the body. This provides enough energy for the body to reach the height h above the ground. Here, the force is providing energy to the body to set it in motion.

Now, consider the case when the body is dropped freely from the height h to the ground. Here, the gravitational force acting on the object directs the conversion of its potential energy into kinetic energy. Here, if the gravitational force was to impart extra energy to the object, then the total mechanical energy of the object would not be constant. So, the source of kinetic energy for the object in this isolated system is only the potential energy that was supplied to it to reach that height h above the ground.

Thus, here comes a dilemma - whether force provides energy to an object to move or directs the conversion of energy that was already present in the object. How do I solve this? Is there any concept that I have not understood well? Thanks in advance for any help...

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    $\begingroup$ Remember it is the energy of a closed system that is conserved, there is no energy really just present in a single body , a single body can carry kinetic energy or be in a potential energy state within a system. In your object at height h coming to the ground potentila is concerted to kinetic, but the potential is in a system "earth object". $\endgroup$
    – anna v
    Commented Jun 24, 2017 at 12:28

2 Answers 2

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All forces affect the kinetic energy of the object, increasing or decreasing it. The trouble comes when you want to consider the total energy, i.e., kinetic + potential. I would say there are two possible answers depending on the level of depth you're looking for:

  • The macroscopic answer: the difference is that gravity is a conservative force, while the force exerted by your hand is not. Conservative forces exchange kinetic and potential energy, while non conservative forces simply affect the total energy directly.

  • The microscopic or fundamental answer: the force exerted by your hand is really an electrical force between atoms. Your body has potential energy distributed in a very complicated way among all your atoms and their electrical interactions, and when you push you're giving some of that energy to the body.

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  • $\begingroup$ This is exactly what I was looking for... Conservative forces do not provide extra energy to the object, but just direct the conversion of the energy which is already present in it. On the other hand, non-conservative forces provide extra energy to the object to set it in motion. So, that solves my doubt. Are there any other types of forces that behave differently than the two mentioned above. Please tell me for further study. Thanks!!! $\endgroup$
    – Hariharan
    Commented Jun 24, 2017 at 15:51
  • $\begingroup$ @Hariharan all friction or viscosity as well as any force that do not derive from a potential, by definition, will remove KE but do have a potential to convert to. But these are simplified models. In the real world all those forces do convert energy to some other form, like heat, Electromagnetic radiations, chemical structure etc. $\endgroup$
    – MrBrushy
    Commented Jun 26, 2017 at 14:59
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External force changes the total energy of a system. It is a characteristic of external force. Total energy is conserved unless no external force is applied on the system. For example, if you hit a cricket ball it gains energy.

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