Thermodynamics and gravity

[Nitack]

I was just hit with a rather interesting quandary and I figured some one here would be able to answer the question. Fair warning, I am not a professional scientist, this is a hobby, so forgive me if I am ignorant of something rudimentary to you.

 

Premise 1: Conservation of energy states that the total amount of energy in an isolated system remains constant. Another way I have heard it said is that energy can neither be created or destroyed, only changed.

 

Premise 2: Gravity is a natural phenomenon that causes items with mass to be attracted to each other. Gravity is constant and can not be lost or gained. Gravity is not energy.

 

Premise 3: The gravity that the moon exerts on the earth is responsible for the movement of the tides.

 

Now that those three premises are laid out, and accepted scientific fact. We have for centuries been able to harness the power of the tides. It is used by boats launching, more recently we have been able to use the movements of the tides to produce electricity by converting the kinetic energy. My question is, where did that energy come from?

 

The kinetic energy is a product of the gravitational pull on the water, but the gravitational pull of the moon has lost nothing in the transaction. So, how did this energy come to be?

 

Either, this is an exception to the law of conservation of energy, or the energy came from some where else. And here is where I am stuck, because I can not figure out where that energy comes from, and I do not believe that the first law of thermodynamics is wrong.

[Little Bang]

The moon moves a little over an inch farther away every year. The moons gain in orbital velocity is where the energy is created. This is a rudimentary description, you can probably find a complete description at wikipedea.

[Nitack]

OK, I might be able to wrap my head around that.

 

Question though. If the energy were created in the moons gain in orbital velocity, wouldn't the moon be losing energy in the equation? Wouldn't the drag of the oceans then slow down the moon slowly sapping it's Kinetic energy? The energy has to come from some where, and in this case slowly sapping the moons velocity seems like the most likely candidate.

 

So, if the moon is every so gradually slowing, then the laws of thermodynamics are upheld.

 

This brings another wacky idea to mind, although it requires technology beyond ours, but is it possible to sap the kinetic energy of planets orbiting a star to the point of actually slowing them down?

[Nitack]

Ok, so I have found out that the moon is in fact slowing down. The example has been satisfied, but it has led me to a further question. If you were to take two massive objects, say two mars like planets, and they were suddenly to be placed in space with no velocity yet in reasonable proximity to each other. Wouldn't the gravity of the two objects draw them to each other creating kinetic energy? In that very hypothetical case where has that energy come from?

 

The heart of my question is that I am trying to understand how gravity can exert a force on an object but not be a form of energy.

[Pyrotex]

Ok, so I have found out that the moon is in fact slowing down. The example has been satisfied, but it has led me to a further question. If you were to take two massive objects, say two mars like planets, and they were suddenly to be placed in space with no velocity yet in reasonable proximity to each other. Wouldn't the gravity of the two objects draw them to each other creating kinetic energy? In that very hypothetical case where has that energy come from?

 

The heart of my question is that I am trying to understand how gravity can exert a force on an object but not be a form of energy.

This is a problem that has frustrated engineering and physics students since the time of Newton. The nature of "energy" is not clear cut at all, and it takes a heap of pondering to understand.

 

First of all, energy is not force and force is not energy. Gravity exerts a force between two bodies and it is mutual. Each body is pulled toward the other (according to its mass). So, if you studied the Center of Mass (C/M) of the two-body system, you see that it does not move at all. Eureka! There is nothing being moved and therefore no energy required!

 

Another way of handling this (and some don't like it) is to include Potential Energy. In a constant gravity field (that is, over vertical distances of a few miles on Earth) PE = Gh, where h is the height of your second object over the surface of the Earth (or any arbitrary "zero" height location). As kinetic energy of the falling object increases, its potential energy decreases in perfect synch. So the sum of PE and KE is always a constant. (excluding friction, of course)

 

When you "Place" two Mars-sized planets in space, the PE comes from YOU!!! You had to expend energy to place them there. They acquire PE from your efforts to place them and how far apart. Not elegant, but the math works; ponder this long enough and it will make sense.

 

Remember, Kinetic Energy is not the only "energy" or source of energy. Energy is not a substance or a fluid or a thing. It is not motion or force or momentum. It IS rather arcane and esoteric, especially since it is non-linear in nature. (E = 1/2 m v^2) Energy is as energy does. A little Zen there. :shrug:

 

In the Earth-Moon example, one can assume that total energy is constant to a first approximation. But you have several energies to sum up: Energy of (EO) rotation of Earth about the C/M, EO rotation of the Moon about the C/M, EO revolution of Moon about its own axis, EO revolution of the Earth about its axis, the kinetic energy of the oceans around the Earth's axis. This all makes up a Mass Energy Kinetic System (MEKS).

 

The tides DO slow down the Earth's spin, and this not only heats up the oceans but increases the Earth-Moon distance. The non-assymetry of an Earth with tidal bulges, causes secondary forces on the Moon. :confused: This affects the Moon's orbital speed. The Moon is face-locked to the Earth, but as those other secondary forces try to twist the Moon out of face-lock, this bit of assymetry creates tertiary forces on Earth's spin. And so-on and so-on.

 

However, extraordinary predictions can be made by summing these all up and assuming the sum to be a constant. Then it can be "seen" that Energy moves within the Earth-Moon MEKS. It oscillates from Earth to Moon and back. Think of the gravitational bonds as invisible rubber bands. Think of pendulums. Rotational energy becomes spinning energy, back and forth in slow ponderous cycles. The tides slow the spinning Earth, then push against the Moon then slow the spinning Earth, back and forth, back and forth in slow ponderous cycles. It is extremely dynamic.

 

So, the Moon can "pull" all it wants for all of eternity. No energy necessary. A force is not energy and energy is not a force. Energy is in the dynamic relationship between moving (or potentially moving) parts of a System.

[Nitack]

Great response and thank you for the insights. Unfortunately my question is still not answered and was avoided by semantics. I am not concerned with how the two planets got there, but that they are there.

 

If gravity draws two objects with mass together there will be kinetic energy and eventually heat energy (friction when they collide). Where does the energy come from that drew them together? Potential energy does even out the equations, but does not satisfy the logic as far as I can see. If there is one planet then there is no potential energy because there is nothing to be drawn to the other. If there are two planets yes there is potential, but does it appear because there are two? Perhaps I am not explaining it well.

 

Maybe gravitational force is not energy, but it still is able to create effects that appear to be energy.

 

And I just had a thought that is going to drive me crazy to boot. Take the two objects of mass in space, gravity draws them together, is that now a degree of negative energy because it will cost external energy to restore them to their original spots?

 

Hmmm... perhaps a better way to phrase the idea I am trying to understand. Take your two objects of mass, now try to keep them at a close yet constant distance from each other. Is there not a constant input of energy to keep them apart because of their gravitational pull with out the equivalent amount of energy coming out in some way?

[Pyrotex]

Great response and thank you for the insights. Unfortunately my question is still not answered and was avoided by semantics. I am not concerned with how the two planets got there, but that they are there.

...If there are two planets yes there is potential, but does it appear because there are two? Perhaps I am not explaining it well....

Let's start with this one. One planet, no PE...Two planets, suddenly you got lots of PE!!! Where did it come from???

 

It came from the ellipsis above. The "...".

 

Planets do not appear out of nothing. You can't start a physics scenario with, "and then this second planet appears" -- or -- "there are these two planets at rest with one another". Such things do not occur in reality.

 

Okay, how DID the second planet get there? Obviously, it moved from somewhere else. It had kinetic energy of motion as it approached the first planet, swung by (with even MORE kinetic energy) then as it departed, there was a point in time where the velocity between the two bodies fell to zero. Bingo! There! There, you begin your scenario of two bodies at relative rest with each other.

 

Where did the PE come from? From the KE that the second body brought in, due to its motion. That KE became PE as the second body slowed and came to a stop some distance, D, from the first planet. You HAVE to be concerned HOW the planets got there.

 

[Actually, it would take a 3rd, unnamed planet in some kind of large orbit about the 1st planet so that the 2nd planet could come in and lose enough KE so that it didn't have enough to depart, and came to a stop instead, but those are details we can avoid for the moment.]

[Nitack]

Hmmm... perhaps a better way to phrase the idea I am trying to understand. Take your two objects of mass, now try to keep them at a close yet constant distance from each other. Is there not a constant input of energy to keep them apart because of their gravitational pull with out the equivalent amount of energy coming out in some way?

 

You are correct, I do have to be concerned how the objects got there, that was fair. My question is really in the relationship between gravity and energy though. The quoted example above is the best illustration of my question that I could come up with.

 

If we were to keep these two objects from coming any closer to each other it would require an energy input. Regardless of the amount of energy we input to keep them apart, the potential energy and gravitational force would remain the same. So where does all that energy we put into keeping them apart go?

[Pyrotex]

Great response and thank you for the insights. ...Take your two objects of mass, now try to keep them at a close yet constant distance from each other. Is there not a constant input of energy to keep them apart because of their gravitational pull with out the equivalent amount of energy coming out in some way?

Thanks! :shrug:

 

Excellent question, and this gets to the heart of the matter, doesn't it?

 

The answer is, just HOW do you propose to keep the planets separate? Some methods require energy, others don't. (!!!!!) :confused:

 

I could keep them apart with rockets. Rockets rely on accelerating fuel from relative zero velocity to some huge exhaust velocity. That takes (chemical) energy to supply thrust.

 

However, what if I tie an infinitely strong cable to each planet and tie those cables to stars at some huge distance away? Each planet is now just "hanging" from its cable like a wrecking ball hanging from its cable. The two planets still tug at each other, but neither one moves.

 

No energy is generated, expended or needed. I have essentially nailed the planets in place.

 

Energy is NOT required to oppose a force. If you oppose a static force with another static force, then you got equilibrium, and no energy is expended at all.

 

The apple hangs serenely from its branch in the tree, pulled to the Earth, yet motionless. What keeps the apple in place? The energy of an angel to whom it was delegated, "don't let that apple fall!"? The buzzing of a thousand grunting gnats?

 

What keeps the apple in place? The unmindful, effortless, slumbering tree.

[Nitack]

The answer is, just HOW do you propose to keep the planets separate? Some methods require energy, others don't. (!!!!!) :confused:

 

I could keep them apart with rockets. Rockets rely on accelerating fuel from relative zero velocity to some huge exhaust velocity. That takes (chemical) energy to supply thrust.

 

However, what if I tie an infinitely strong cable to each planet and tie those cables to stars at some huge distance away? Each planet is now just "hanging" from its cable like a wrecking ball hanging from its cable. The two planets still tug at each other, but neither one moves.

 

Exactly the question I am trying to get at. If you were using rockets and chemical energy there would be energy input, but if energy can neither be created or destroyed, what is that kinetic energy converted to if it was expended to offset the gravity?

 

I realize some methods require energy and some do not. What I am trying to figure out has to do with the ones that do require energy because I am trying to understand the relation between gravity and energy.

[Pyrotex]

...What I am trying to figure out has to do with the ones that do require energy because I am trying to understand the relation between gravity and energy.

Simple answer: gravity is a static force. Always treat gravity as just a force and nothing else, and you can't go wrong. Not very far, anyway. ;)

...you were using rockets and chemical energy there would be energy input, but if energy can neither be created or destroyed, what is that kinetic energy converted to if it was expended to offset the gravity?....

Woof! :confused: Gonna hafta straighten this question out, cause it has too many kinks in it to give a straight answer.

 

"...there would be energy input,..." > input to what? :shrug: We have energy in the chemical fuel. We are converting that chemical energy into the kinetic energy of the exhaust gases. In the process, we generate a force to counter the force of gravity.

 

"...if energy can neither be created or destroyed..." > We aren't creating or destroying energy. Really. No energy was harmed in the creation of this scenario. We have given ourselves a fully fueled rocket, so there is (potential or chemical) energy in the fuel. That energy becomes kinetic energy as the exhaust plume races away at very high speed.

 

"...what is that kinetic energy converted to if it was expended to offset the gravity..." > We expended fuel, not kinetic energy. The chemical energy was converted to kinetic energy for generating a huge accelerating FORCE on the combustion products, causing them to move away at high speed. For every force*, there is an equal and opposite force (Newton). So if the rocket engine applies a force to the exhaust gases, then the exhaust gases exert an equal force on the rocket engine. This force is transferred to the planet to keep it from "falling" into the other one.

 

*Newton used the words "action" and "reaction", which translate to the word "force" as defined in physics today.

 

So, we didn't "expend" kinetic energy. We expended chemical energy, which was used to generate a force, which balanced the force (or, served as a counter-force) to the pull of gravity. As a waste by-product, we also accelerated a lot of gases to very high speed, and these gases presumably race away and out of our little scenario. Yes, they have a lot of kinetic energy.

 

The tough answer: hot, spreading gases are very disorderly, while condensed fuel in a nice pretty tank is very orderly. Disorder is called entropy. By firing the rockets and expending fuel, we generated a counter-force -- and we generated entropy. We increased the disorder of the whole system. Any kinetic energy carried off by the exhaust plume essentially disappears and becomes entropy.

[Pyrotex]

...because I am trying to understand the relation between gravity and energy.

 

...I am trying to understand how gravity can exert a force on an object but not be a form of energy.

 

After due consideration, I have come up with a simpler answer. Gravity doesn't just exert a force, it IS a force. It is a static field of force around any massive body.

 

Energy is NOT a force. Energy has the same direct relationship to gravity as it does to any other force: none at all.

 

Magnets "generate" a force. No energy required. The field of magnetic force is static.

 

Pressurized gas in a steel tank exerts tremendous force on the inner wall of the tank. No energy required.

 

Now, there ARE indirect relationships between force and energy. Energy can be converted to some other form and generate a force as a by-product. Rocket engines.

 

A force can cause one form of energy to be transformed into another, because the force causes a massive body to accelerate or decelerate. Kinetic to Potential or Potential to Kinetic.

[von Faulkenstein]

A+ to both of you--for the question and the answer--shows the value of this discussion forum! You got to love it!

Doc

[freeztar]

A+ to both of you--for the question and the answer--shows the value of this discussion forum! You got to love it!

Doc

 

+1

[Pyrotex]

Thanx for da kudos.

 

Questions like the one that started this thread are very common, especially among the young who are trying to get their minds around this concept of "energy". There are so many contradictions and inconsistencies in the ways the word is used in science, science fiction, medicine, pop culture and literature. Given its nature as a non-linear and non-palpable (and non-THING!) expression of reality, it is not easy to acquire a gut-level understanding of "energy".

 

Like many technical concepts in science, there is a strong temptation to trust one's personal experience rather than the book definition. We all have experience at tossing balls around, expending energy (effort) to make things move, etc. And we have experience at forcing, shoving, lifting things to get them to move. So, shouldn't energy be "like" a force?

 

And the answer is "no". Clues to the difference are found in equations that are used to calculate force and energy of moving objects:

 

Force = Mass * Acceleration, F = M a

 

Energy = 1/2 Mass * Velocity squared, E = 1/2 M v^2

 

Look at the "units" of these concepts, remembering that "velocity" is meters per second and "acceleration" is meters per second per second, or meters per second squared:

 

Force == kg * meters / seconds squared

 

Energy == kg * meters squared / seconds squared

 

By inspection, we see that the units used to express these things are very similar; there is just one difference -- there is an extra factor of "meters" in the units for Energy.

 

So, speaking just of the Units:

 

Energy = Force * meters

 

How do we interpret this? We can say that the Energy required to get a mass moving from zero up to some final velocity is proportional to the (constant) Force we apply to that mass multiplied by the distance over which we apply that Force.

 

If Force is like a vector (a line pointed in a certain direction), then Energy is the area swept out by that line moving perpendicular to itself over a distance D, where D is the distance over which we applied the force.

 

What if the object is so massive that we can't move it? Then we apply the Force over a distance of zero, and the Energy transfered is zero.

 

That's right! A huge force that doesn't move the object, doesn't transfer ANY Energy to that object.

 

NOTE: One must be careful to speak of the Energy transfered to the object rather than some assumed energy "expended". If I get an object moving, the only Energy that I can reasonably discuss is the energy that object now has as a virtue of its velocity. Any grunting or sweating that I may do in this process is irrelevant. ;)

[freeztar]

Thanx for da kudos.

 

Questions like the one that started this thread are very common, especially among the young who are trying to get their minds around this concept of "energy". There are so many contradictions and inconsistencies in the ways the word is used in science, science fiction, medicine, pop culture and literature. Given its nature as a non-linear and non-palpable (and non-THING!) expression of reality, it is not easy to acquire a gut-level understanding of "energy".

 

First of all Pyrotex, you rock! I feel that I qualify as one of the "young who are trying to get their minds around this concept of 'energy'". I've learned this stuff in Astronomy class, but your teacher instincts are impeccable and expand my knowledge, in a strangely indirect manner. :hihi:

 

I must admit, that my classical background teaches me that energy is a result of force. I've since learned that this is not a correct model.

Like many technical concepts in science, there is a strong temptation to trust one's personal experience rather than the book definition. We all have experience at tossing balls around, expending energy (effort) to make things move, etc. And we have experience at forcing, shoving, lifting things to get them to move. So, shouldn't energy be "like" a force?

 

Yes! (emphatically)

....but not really.... :hihi:

 

Ironically, my best imagery of force is electrical impedance. The ironic part is that I do not have a good grasp on the subject of electricity. Nonetheless, electricity can be measured, it can be 'matched against' alternate forms of "energy".

 

I understand that impedance is 'obstruction' of the flow of electrons across the circuit. This is a corollary feature of the resistors in the circuit.

How does electrical "energy" factor into your theory?

Clues to the difference are found in equations that are used to calculate force and energy of moving objects:

 

Force = Mass * Acceleration, F = M a

 

Energy = 1/2 Mass * Velocity squared, E = 1/2 M v^2

 

Look at the "units" of these concepts, remembering that "velocity" is meters per second and "acceleration" is meters per second per second, or meters per second squared:

 

Force == kg * meters / seconds squared

 

Energy == kg * meters squared / seconds squared

 

By inspection, we see that the units used to express these things are very similar; there is just one difference -- there is an extra factor of "meters" in the units for Energy.

 

Which implies a third dimension.

So, speaking just of the Units:

 

Energy = Force * meters

 

How do we interpret this? We can say that the Energy required to get a mass moving from zero up to some final velocity is proportional to the (constant) Force we apply to that mass multiplied by the distance over which we apply that Force.

 

If Force is like a vector (a line pointed in a certain direction), then Energy is the area swept out by that line moving perpendicular to itself over a distance D, where D is the distance over which we applied the force.

 

I understand your logic, but I don't see 'Energy' being related to an 'Area" vector(s). Perhaps it is the result, but we are left with a necessity for explanation.

 

What if the object is so massive that we can't move it? Then we apply the Force over a distance of zero, and the Energy transfered is zero.

 

Like a Black Hole?

 

That's right! A huge force that doesn't move the object, doesn't transfer ANY Energy to that object.

 

NOTE: One must be careful to speak of the Energy transfered to the object rather than some assumed energy "expended". If I get an object moving, the only Energy that I can reasonably discuss is the energy that object now has as a virtue of its velocity. Any grunting or sweating that I may do in this process is irrelevant. :lol:

 

So we are implying PE->KE? :D

[Qfwfq]

I must admit, that my classical background teaches me that energy is a result of force. I've since learned that this is not a correct model.

Work is a result of a force and a displacement. Work is transfer of energy from something to something.

 

I understand that impedance is 'obstruction' of the flow of electrons across the circuit. This is a corollary feature of the resistors in the circuit.

How does electrical "energy" factor into your theory?

Try dragging a rubber shod sled across an asphalt pavement. You do a lot of work and it all gets dissipated into heat.

 

Which implies a third dimension.

No, it implies the force being accompanied by a displacement.

 

I understand your logic, but I don't see 'Energy' being related to an 'Area" vector(s). Perhaps it is the result, but we are left with a necessity for explanation.

Here, I must say, Pyro got it a bit botched. The scalar product isn't proportional to the area, it goes by cosine of the angle and not sine.

 

Like a Black Hole?

Anything that won't budge. Imagine a spring compressed between two brick walls.

 

So we are implying PE->KE?

No, only that most of your effort might get wasted, even in your own muscles.