# Is Energy Finite? thoughts ideas

1. Aug 11, 2011

### CDCraig123

As I understand Energy is finite. This is a problem for me tho as I can see one way to create energy. In fact in my eyes energy is created everyday in are very own solar system. Anyway I want to get some points of view on the subject from others before I make my self a fool because maybe you have the answer after all if i was right that would be a problem.

2. Aug 11, 2011

### Pythagorean

it's actually quite important to most of us that energy is conserved and not created. Are you sure you're seeing energy be created? or are you just seeing it change form?

3. Aug 11, 2011

### CDCraig123

its hard to say i understand the idea that energy is transferred i started to study the idea of energy being infinite and i ran across some stuff online about Io a moon of Jupiter and the more i thought about it harder it became its burning a hole in my head now gravity has to be energy and not a force to make everything work in my head or nothing makes sense and if gravity was finite then every orbit would fall apart and everything would expand in the universe like it is doing right. If centrifugal force (kinetic energy) is created by an orbiting body and gravity counters the kinetic energy it has to be a? force? a force is not energy so where does that kinetic energy go?

4. Aug 11, 2011

### Pythagorean

It sounds like you're doing more thinking than studying :)

kinetic energy can be stored as gravitational potential energy (see potential energy in general) but there's also dissipation.

You seem to cinfuse force and energy. Energy is the distance an object is moved times the force used to move it.

So applying 10 pounds of force to a ice block over ten feet takes half the energy it does to move the block ten 20 feet.

If I go the twenty feet at only .1 pounds of force, I save a lot of energy for the same distance (but it takes longer).

5. Aug 11, 2011

### Gravitas

Energy is neither created or destroyed. It can be free-floating (ie in the form of heat) or it can be contained as a part of something else (ie atoms). There are two main forms of it, Kinetic Energy (abbreviated to K.E.) and Potential Energy (abbreviated to P.E.). The first is the energy of motion and the second is stored energy. Potential Energy becomes K.E. as soon as it is used. (To remember the difference, think 'it has the "potential" to be used'.) A battery would be an example of energy stored in chemical form. There is a third form called Binding Energy, that holds atoms together and is released when an atom is destroyed. The sun (as are all stars) is a giant fusion reactor. In this process two lighter elements are fused, to create a heavier element. When this happens, there is always some energy left over and this is what is actually being released and radiated. In fact, this is what you are seeing, when you see light emanating from the sun.

6. Aug 11, 2011

First of all, I think you're looking at gravity like (I don't know how better to put it!) some sort of health potion in a video game. The fact that any form of energy is finite does not mean it empties over time, rather that it is converted into another form of energy. And other forms of energy are regularly converted into the first. So everything wouldn't fall apart and the universe wouldn't expand over time just because gravity is 'finite.'

Secondly, centrifugal force doesn't exist so long as you're measuring everything from outside the whole system. And, speaking of kinetic energy, whatever is apparently lost is gained as instantaneous gravitational potential energy as the object (Io, in this case) falls towards the centre of the rotating system (Jupiter) every instant, hence retaining its circular motion.

I remember a thread on the same question of gravity being either a force or a form of energy: https://www.physicsforums.com/showthread.php?t=138215

I think it's better this way on the long run!

7. Aug 11, 2011

### ZapperZ

Staff Emeritus
Before wrapping your head around something like this, maybe you need to step back and learn a bit of basic mechanics.

For example, would it surprise you to realize that "force" can be defined in terms of "potential energy"? It is true! The gradient of potential energy is directly related to force, i.e.

$$F = -\nabla V$$

So while force is not energy, force CAN be defined in terms of it.

Zz.

8. Aug 12, 2011

### CDCraig123

Here is my problem when I do the math maybe someone could look over this also keep in mind that there are some parts of what i am trying to figure out witch is where earths energy goes some of the numbers are missing.

I converted the earths kinetic energy into joules witch ranges 2.0184e+36J - 1.815e+J
This would be relative to something witch would have zero velocity.

How I did this is takes earth mas 6e+24 kg
earths velocity would be 300 km/s for the Milky ways speed around the local cluster. 250km/s for are suns speed around the galactic core. Finally 30km/s for the earths speed around the sun when added its 580 km/s.

When i did this i realized that the earths speed relative to an object that had zero velocity would by at least have to drop by 30 km/s when it was orbiting the sun the opposite way as the sun was orbiting the galactic core because of this the earth would lose energy i figured it to be 10.08% this is where i get lost. To use this information i had to find where that lost energy was stored and how it could be used to later accelerate the earth back to its max velocity so i turned to the earths orbit to see if it was stored as potential gravitational energy i found at its closest earth is 147,093,602 km and farthest 152,097,053 km a difference of 3.3% I do not know how to make these number relative to me earlier numbers i tired to earlier and i found that there was not enough potential gravitational energy to accelerate earth back to max velocity witch lead me to the idea that gravity could some how pull energy out of someplace.
maybe if i could find the length of earths orbit i don't know the longer i look at this the more it seems that energy is doing something its not suppose to that's why i am seeking an answer. If you read all this thank you

9. Aug 12, 2011

### Staff: Mentor

The Earth doesn't need to be accelerated back to 30 km/s in the opposite direction using energy, as its angular velocity always stays at a fairly flat amount. Gravity provides the force necessary to change the direction the Earth is moving. It accelerates the Earth inwards towards the sun at just the right amount to keep the Earth in orbit with its current velocity. This does NOT require energy to function. The fundamental forces of nature do not use or expend energy to exert their force on objects.

Energy is simply a measure of the ability to do work. For example, if an asteroid hits the earth at 50 km/s, then it has X amount of energy. (Kinetic energy)
However, depending on the orbit of Earth, it could have hit when the planet was heading away from it or towards it and hit with 20 km/s or 80 km/s instead. No energy was lost or gained here, as you could not say the asteroid had 50 km/s of kinetic energy without having a frame to compare it against in the first place.

10. Aug 12, 2011

### CDCraig123

You said that "gravity provides the force necessary to change the direction the Earth is moving". If the amount of force necessary to change the Earth's direction varies depending on the position in the orbit, are you implying that gravity is not a constant?

You also said the earth does not need to be accelerated back to its max velocity.

Any object with mass with any velocity has energy (kinetic energy)
To change any objects velocity with mass it takes X amount of energy

The earths velocity changes when it orbits the sun relative to an object with no energy or something that is not moving. Therefore it has to gain and lose energy. Where does this energy go and how is it used to accelerate the earth? The earths velocity relative to the sun never changes; however, relative to the rest of the universe, it does. Remember the sun also has its own velocity and orbit.

I understand the basic laws of energy and forces. I know that the earth is gaining and losing energy. For that matter any orbiting body with a point of orbit which is also orbiting something must lose and gain energy as it would lose and gain velocity relative to something that is not moving or something which has no energy.

It is easy for us to say that this energy is stored in gravitational potential energy. Has someone ever really sat down and crunched the numbers. I am trying to put this all into paper to prove that this is right. I am having problems doing this. There is ether not enough information to complete this. My skill level is not high enough to complete this and someone on this forum can show me. Or we do not have a complete and totally understand of the relationship between gravity and energy.

11. Aug 12, 2011

### ZapperZ

Staff Emeritus
Er.. you need to not only learn conservation of linear momentum, but also conservation of ANGULAR MOMENTUM. Something that is spinning tends to want to stay in that spinning state. It is when you want to stop it that it requires energy. So something orbiting something else due to a central force is similar to something moving at a constant velocity per Newton's First Law!

Again, remember what I said earlier that you really do need to go back to basic kinematics.

Zz.

12. Aug 12, 2011

### Staff: Mentor

It is not. The strength of gravity falls off with distance. The gravitational constant, which is different than what I think you are talking about, is the same though.

To change the direction of an object it takes a FORCE. Not energy. Consider a rotating sphere. The atoms on the outside of the sphere are constantly being accelerated inwards from the atomic bonds (and a little bit due to gravity) of the material. No energy is being expended to cause this to happen, as the sphere will rotate forever without an outside force to stop it.

The amount of energy it has or gains/loses is directly related to what you compare it to. Compared to the moon, the Earth doesn't really have that much change in kinetic energy, as the moon is orbiting the sun with us.

First of all, explain to me, in your understanding, what Energy is. Be specific.

13. Aug 12, 2011

### CDCraig123

Does everyone here think that Earths Kinetic energy is constant never changing and you can show me with numbers earths velocity and mass and convert it into joules. I don't think i am posing my question very well.

If I am looking at the Earth, relative to the Sun:
It would take X amount of energy to stop the Earth from orbiting the sun (say it ran into a superficial brick wall). Since the sun is also orbiting around the Galactic core, shouldn't the position of the sun's system have an effect on the amount of "X" energy needed to stop the Earth from orbiting the sun? Energy is a force times a distance. So when the Earths orbit is the same direction as the sun's orbit around the Galactic core, it would have a greater velocity then it would when it was orbiting the opposite way.

If you are swinging a bucket of water around yourself the bucket would have the same velocity always. But if you were to swing the same bucket around your self on a merri-go-round you would have increasing and decreasing velocity on your bucket.

14. Aug 12, 2011

### Staff: Mentor

No matter where you look at it from, the amount of energy needed to stop the earth from orbiting the sun is the same. Again, imagine the rotating sphere with the Sun as the center and the Earth as one of the atoms on the equatorial edge. No matter where the force is applied, the same amount of energy is needed to stop it from rotating. This is because you are comparing the Earth with the Sun specifically. You cannot choose another frame otherwise you would not be talking about the angular momentum of the earth anymore, but about something else.

Not from your point of view. You would always see the bucket as swinging at the same speed relative to you.

15. Aug 12, 2011

### CDCraig123

I agree with you. However when looking at the earth as its orbiting that galactic core (bearing in mind that its still orbiting the sun), there would would still be an increase and decease in velocity on the earth. Just like if you were a different child in the play ground not standing on the merri-go-ground the bucket would increase and decrease its velocity.

16. Aug 12, 2011

### Staff: Mentor

Of course. That has never been in question.

17. Aug 12, 2011

### CDCraig123

If the earths velocity is increasing and decreasing and because the earth also has mass this means that the earth has to be gaining and losing energy somewhere. What I want to know is where and how this energy is lost and gained. Furthermore I want the process shown to me in number so I can understand so I can see the energy move from one body to another. A side note I have read a few people compare earths orbit to a sphere by definition the earth is not even a sphere and earths orbit is not a sphere.

18. Aug 15, 2011

### IsometricPion

Earth's orbit is nearly circular wrt the Sun. I think what you are missing is that (assuming the Earth and Sun are all that exist and we choose a coordinate system so that the center-of-momentum of the system is moving at 600 km/s) the Sun is travelling in the opposite direction wrt the Earth. The momentum vectors of the Earth and Sun add such that the total is always constant (in both magnitude and direction). Furthermore, the total energy of the system should be the sum of the kinetic energy of the system treated as a point particle at its center-of-mass and the energy of the system treated as if the center-of-mass were at rest. Since the former is not a function of time (by definition in the setup of this situation) and the latter is constant (since we all agree that the energy of the Earth-Sun system in its center-of-mass frame is conserved), the total energy is conserved in this situation as well.

19. Aug 15, 2011

### Pythagorean

This is incorrect.

My first post, the second in this thread, mentioned 'potential energy'. Objects can gain and lose velocity if there energy is converted between potential and kinetic energy, not gained or lost.

What you possibly mean to say is that the objects kinetic energy is being lost and gained.