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If an object is raised by a mechanical force on earth (ground) up a certain height, then we say that it has gravitational potential energy to do work. because of gravity, well then where does the mechanical energy provided in lifting the object goes?

i know that, i am missing out something ...any help will be appreciated ....and if velocity is frame dependent , then kinetic energy should also be frame dependent , well in that case a body at rest from one frame of reference can have kinetic energy if observed from another frame of reference?

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The object raised to that height has accumulated energy in form of potential energy, for the only reason of being at certain height in a gravitational field.

  • The energy of lifting gets accumulated in the body, as long as there's the gravitational field.
  • That energy can be released in form of kinetic energy, when you liberate the object and you let it fall. Those are the only possibilities: kinetic energy or potential energy, if we don't consider heat or chemical reactions...
  • If the object is at minimum height, its potential energy is irrelevant. That agrees with the fact that the potential reference is arbitrary. We only care about differences of potential (here differences of height), that's the only meaningful thing. That difference in energies is what can become K.E.
  • If the gravitational field disappears, so does the P.E. and consequently the object can remain in that position until something changes.

Now part 2 (next time try to separate questions, see how deep they can be ;) )

Yes, KE is frame-dependent, but that doesn't matter because, again, only changes in velocity do matter, not "absolute" velocity, if such thing existed. For example, collisions can be seen from many points of view, and all of them get the same results. KE is different for each observer, both before and after the collision. The variation of energy and momentum IS however the same afterwards, and that's what really matters.

I hope it's clear.

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  • $\begingroup$ So ..you mean to say that both gravitational and mechanical energy are responsible or converted into potential energy ??...and yeah next time i' ll try to ask seperately .. $\endgroup$ – Abhay Oct 20 '17 at 19:03
  • $\begingroup$ @Abhay The potential energy you're talking about is gravitational potential energy. Mechanical energy can get converted to potential energy. See for example, ideal projectile motion. As the objects gains height, it loses kinetic energy; while gaining gravitational potential due to it's height and acceleration it is experiencing. As it begins to fall, that is converted back to kinetic energy (and in an ideal system with no friction losses, the kinetic energy at the end has the same magnitude as the beginning; and the same magnitude as the potential energy at the apex). $\endgroup$ – JMac Oct 20 '17 at 19:09
  • $\begingroup$ Exactly my point @JMac as you mentioned " As object gains height it loses kinetic energy " and gains potential energy i.e its kinetic energy gets converted into potential energy ??? $\endgroup$ – Abhay Oct 20 '17 at 19:13
  • $\begingroup$ @Abhay Yes, kinetic gets converted to potential and vice versa in that situation. The "gravitational energy" is actually the potential energy though, not a third type (as your comment seemed to suggest IMO). $\endgroup$ – JMac Oct 20 '17 at 19:16
  • $\begingroup$ @JMac ..ok now i am getting it ...gravitational potential energy is just the name in this case given to potential energy ...does that means gravity does not gives any energy to it and even though it is named gravitational potential ..?? $\endgroup$ – Abhay Oct 20 '17 at 19:20

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