# I Gravitational Potential Energy & L.C.E. Questions

1. Sep 22, 2016

### victorhugo

1. A comet that passes by Earth has GPE, which will be all lost if it begins to fall towards Earth. If it's shot back up, it will now start with Ek and finish with a higher GPE.
Now, what exactly is this GPE? if energy cannot be created or destroyed, but transformed, where does it get stored at GPE? Where does it go?
Also, the comet would add the gravity of Earth as it hits Earth, thus increasing the GPE of everything else around Earth. Does this have anything to do with it?

2. Sep 22, 2016

### Biker

Okay suppose there is no GPE.
and you placed an object in the air, It will fall down right?
There is a force acting on it over a distance. So something must be doing work on it. There must be energy stored somewhere.
So potential energy is just an idea that for some forces like Gravity and electric forces. When they do work on an object, It doesnt simply remove that energy but store it in its field.
$W_{conservative force} = -\Delta PE$
The field will use this energy to do work on objects.

3. Sep 22, 2016

### victorhugo

How exactly does it get stored in the fields, especially of electric forces? I can see GPE working as now the mass that hit the earth adds a tiny amount to the total g of earth and thus increasing the total GPE of everything else within Earth's gravitational field (infinity!)

4. Sep 23, 2016

### Biker

The how and where questions leads to confusion, we can probably say how but Where is not a good question. Fields are just forces. So if they can do work they must have energy.

If you place a +Q charge beside another, you will find that it gets repelled. Now you might ask where it got the energy? You have placed the charge there and you did work on the charge to place it there ( you felt the repulsion )so that is how it got the energy.

In order for the whole energy concept works there must be something called potential energy.

That is just how I see the whole topic, maybe someone will probably talk deeply about it here.

5. Sep 23, 2016

### Staff: Mentor

Suppose you have several shelves at different heights. Then moving an object from one shelf to another changes its gravitational PE. Maybe you can answer your own question for that object. How is the PE stored for that object?

6. Sep 23, 2016

### Aaron Crowl

I think part of your question is what happens to the GPE if the comet hits the earth. Is that right? It would make a big unfortunate explosion. Lots of energy would be released.

A similar question I've heard is "what happens to the kinetic energy of cars when they collide?" The energy goes into deforming the metal of the cars.

7. Sep 23, 2016

### Khashishi

The comet has gravity before it collides with the Earth. The gravity of the Earth + comet is roughly the same before and after the comet collides with the Earth. (The mass distribution will be slightly different, but this has very little effect on other objects far from the Earth and comet.)

8. Sep 23, 2016

### Khashishi

If you really want to know where the energy "goes", you could think of the gravitational potential energy as being stored in the gravitational field. Somehow, curved space is lower energy than flat space. Greater accumulation of masses generates a lower energy field around it. So, pulling apart two masses flattens out the field a bit, and this takes some energy.

9. Sep 23, 2016

### Staff: Mentor

That's what happens to the kinetic energy. Nothing happens to the potential energy, which is the same for a comet at the earth's surface and a bunch of comet fragments at the earth's surface.

Potential energy is being converted into kinetic energy as the comet gains speed during its approach to the earth. The kinetic energy turns into a (possibly very large) explosion, while the potential energy that hasn't yet been converted remains potential.

This is true - but note that this time you specified kinetic energy instead of potential.

10. Sep 23, 2016

### Aaron Crowl

Potential energy becomes kinetic energy when something falls. Sorry, I should have clarified that.