We know that when a mass is pulled by gravitation it's kinetic energy increases. This is also the case when a photon is being radiated towards a massive body,its wave length decreases and it's energy increases. Since I know that energy is conserved,my question is,where does this extra energy come from?


It comes from the potential energy from gravity arising from the configuration of the objects.

When an object gains kinetic energy due to the gravitational force, the overall gravitational potential energy of the system decreases by an equal amount.

The graviational field can be thought of as "storing" work energy. You can increase the potential energy of the system by investing energy into re-arranging the system, and you can decrease the potential energy of the system by extracting it as kinetic energy.

You can, for example, imagine taking a book from the floor and placing it on a shelf directly above the original spot. This takes a certain amount of work, $W$. Later, if you decide to remove the shelf and let the book fall back to the floor, the book should accumulate kinetic energy $\frac{1}{2} mv^2$ exactly equal to the original work $W$ you put in.

The "original source" of the energy that the book gains when falling, then, is the work energy that you originally invested in putting it on the shelf. Numerically, they are equal. The gravitational potential took that original energy, stored it, and releases back the same amount when the book falls.

  • $\begingroup$ If a photon reaches from one side of the seen universe to a massive star on the other side of the seen universe ,and because of gravitational blue shift the photon increases its energy . this energy came from somewhere since this photon and this star are meeting first time since the creation. what is the meaning of potential energy . where was this energy kept all these 13.7 billion years until the star and the photon met for the first time? even when there is a potential energy it should be kept somewhere like a battery or oil tank for a more visual example. $\endgroup$ – Eran Sinbar Jun 28 '18 at 12:48
  • $\begingroup$ Funny that you mention a battery or oil tank. Those are good analogies; how do you think chemical batteries store their energy? It actually comes from the physical configuration of the molecules inside the battery. The energy is not stored inside the molecules or chemicals themselves, but purely from their arrangement in the battery. We put energy into configuring the atoms as they are, and then later extract it by letting it move to a configuration with less and less energy until there is no lower configuration easily possible. $\endgroup$ – Justin L. Jun 28 '18 at 21:31
  • $\begingroup$ Justin, a you wrote we insert energy into the battery to charge and arrange it with electrical /chemical energy. but in the case of gravity , stars and even black holes that apply enormous gravitational force and potential energy are formed without any added new energy to the universe . it is as if charged batteries were formed naturally by themselves and charged naturally bu themselves and no one would care where did they get their potential energy from . $\endgroup$ – Eran Sinbar Jul 1 '18 at 6:50
  • $\begingroup$ Their potential energy came from kinetic energy or other sources of energy. If we don't consider dark energy, energy is not created or destroyed; just transformed. It continuously transformed; from the initial energy we had at the time of the big bang, it transforms between potential energy from configuration, and kinetic energy of motion. kinetic energy creates potential energy which creates kinetic energy, etc; the potential energy we observe has its origins from other energy sources. The potential energy didn't come from nowhere; it's just a manifestation of an always-changing energy. $\endgroup$ – Justin L. Jul 2 '18 at 7:01
  • $\begingroup$ The universe didn't magically appear in its current configuration as it is now; the current configuration comes from the unverse-old process of swapping kinetic energy and other forms of potential energy back and forth. Remember, kinetic energy can be expended to increase potential energy (by affecting spatial configuration), just as often as potential energy can be expended to increase kinetic energy. $\endgroup$ – Justin L. Jul 2 '18 at 7:01

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