Energy produced using Gravity

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Hi,

I would like some help with the following question that has been bothering me.

If energy is not created or destroyed then how does the following work:

Consider an asteroid traveling close enough to the earth to have its trajectory changed due to the gravitational interaction with the earth. The asteroid is far enough so it passes by, and does not fall to Earth or travel through the atmosphere. Energy was used to change its trajectory that is an outcome of the gravitational interaction (force was exerted and work was done). The gravitational field of Earth or the asteroid are not change since they are related to their respective masses which were not changed. In fact you can have a new asteroid travel like that every second with no depletion of Earth’s gravitational field. This seems like unlimited free energy being used.

Any ideas on where the energy that is used to change the trajectory is taken away from? Where do we see depletion in energy?

Thanks for your help.
 

Answers and Replies

  • #2
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You should consider picking up an introductory conceptual physics textbook.

The energy would begin as 'potential energy,' (stored in the gravitational field between the two bodies), and would then be converted to kinetic energy (although that doesn't necessarily happen in all cases, e.g. a stable orbit).
 
  • #3
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I understand that.
I am looking for the source of the potential energy.
Once the asteroid received the potential energy it will spend it as kinetic. That’s obvious.
Remember that the potential energy source is the gravitational field. As it is “giving” the energy to the asteroid it is not depleted in any way.
 
  • #4
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Hi,

I would like some help with the following question that has been bothering me.

If energy is not created or destroyed then how does the following work:

Consider an asteroid traveling close enough to the earth to have its trajectory changed due to the gravitational interaction with the earth. The asteroid is far enough so it passes by, and does not fall to Earth or travel through the atmosphere. Energy was used to change its trajectory that is an outcome of the gravitational interaction (force was exerted and work was done). The gravitational field of Earth or the asteroid are not change since they are related to their respective masses which were not changed. In fact you can have a new asteroid travel like that every second with no depletion of Earth’s gravitational field. This seems like unlimited free energy being used.

Any ideas on where the energy that is used to change the trajectory is taken away from? Where do we see depletion in energy?

Thanks for your help.
There is no violation of the conservation of energy here. Every mass in the universe has energy between every other mass in the universe. This energy is called gravitational potential energy. The change in gravitational potential energy and the change of kinetic energy between any two objects will always be zero in isolation. Just because an asteroid speeds up, doesn't mean that energy is being created and added to the asteroid. The potential energy is being converted into kinetic energy. Therefore, energy is still conserved
 
  • #5
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The thing is, these two things are contradictory:

That’s obvious.
Remember that the potential energy source is the gravitational field. As it is “giving” the energy to the asteroid it is not depleted in any way.
The gravitational field of Earth or the asteroid are not change since they are related to their respective masses which were not changed.
Grav. field can't both "give" away its energy, and have the grav field "not change"d in anyway.
As I said. You should get an introductory conceptual physics textbook.
 
  • #6
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An asteroid passing close to our earth WILL alter the earths' rotation, trajectory around the sun, or both.
However, considering the mass difference, this effect may not even be measurable with current instrumentation; thus, the effect might have a negligible influence.
 

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