I once heard somewhere that energy can neither be created or destroyed. It can only be transformed.
I just watched a documentary about Jupiter and it’s moons. Apparently one of Jupiters moons (I think it is Io) has an eliptical orbit. That means that the gravity it experiences from Jupiter varies with it’s proximity which stretches and distorts the entire plannet. The resulting friction heats the rock to melting point.
If energy cannot be created, where is this energy coming from? What is generating it?
This also got me thinking about how electricity is generated on Earth. In almost every case (except for one that I can think of) the source of energy can be traced back to the sun. Fossil fuels came from vegetation which used sunlight for photosynthesis. Even Uranium was created in the suns furnace billions of yeas ago (or am I wrong about that).
What about if you took a huge body of water, like a harbour, with a small inlet connceting it to the ocean and you placed a turbine that used currents resulting from the changing tide to generate electricity? As far as I can tell, the ultimate source of energy here is the moons gravity.
Eventually the sun will stop shining but the moon will never stop exerting a gravitational force. Where does this energy come from and how can it be unlimited?
Maybe I’m missing something really obvious here, help me out.
A moon orbiting a planet has a considerable amount of kinetic energy, which probably can be traced back (ultimately) to the big bang. As it orbits, various inefficiencies in the system sap out its kinetic energy, (energy in the form of motion,) and turn it into friction. Eventually, when that kinetic energy runs out, Io will crash into Jupiter and be swallowed. I think that’s right.
The energy in a closed system is conserved. In that sense, you cannot create or destroy energy. But as you know, I’m sure, you can “create” energy from mass according to the formula e=mc^2. So we really need to think of energy and mass as the same thing, but in different states.
All orbits are elliptical, so this is nothing unusual. In the rare event that you have a truly circular orbit, well, that’s just a special case ellipse with both focci at one point. 
This reminds me of what was once a conundrum among scientists. It may not have really been a paradox, but it seemed like one to some scientists.
Someone in astrophysics came up with the idea of using Jupiter’s gravitatation to boost the momentum/speed of a spacecraft. Just think of Jupiter as having an attached trampoline of -ahem- Jovian proportions. Said craft would bounce off the figurative trampoline if it crossed the orbit close to Jupiter at a trailing point.
But some scientists objected that such would involve energy-from-nothing! Yet it was hard to see why it would not occurr as “expected.”
The answer was that Jupiter would lose energy in the process, hence it’s orbit would be affected.
(Me!)
I’m no expert but as far as I know, when you convert mass into energy, as with a nuclear bomb, you lose the mass. The moon is not losing any mass to my knowlege.
I find it dubious that any actual scientist would come to this conclusion. It’s basic high-school level physics.
I was addressing the more general question about conservation of energy.
The energy comes mostly from Jupiter itself. As the moon is distorted and stretched, Jupiter loses a little energy, either rotational or orbital (I think, I’m no expert on this), depending on what kind of changes in motion the moon experiences. Since Jupiter is so big compared to the moon, the loses are negligible in the short term. Long term, Jupiter’s rotational velocity and orbit will be slightly affected. Just as the tides on our planet have a very small but continuing affect on the Earth.
Already being done, although not on a large scale. See this Wikipedia article:
See above, the Moon’s (and the Sun’s] tidal effect on the Earth does take energy from the Earth, Sun, and Moon, which over very long periods of time will add up to a noticeable level. So there really is no such thing as a free lunch.
But can we exploit them and use some of that energy? Sure. It’s just an engineering, social, political, and economic problem.
>A moon orbiting a planet has a considerable amount of kinetic energy, which probably can be traced back (ultimately) to the big bang. As it orbits, various inefficiencies in the system sap out its kinetic energy, (energy in the form of motion,) and turn it into friction. Eventually, when that kinetic energy runs out, Io will crash into Jupiter and be swallowed. I think that’s right.
I think this is all correct, except that I’m not confident the energy traces back to the big bang, other than in the sense that everything does. I’d try to trace the energy back to gravitational interactions that assembled the bodies from an accretion disk around a forming star, with these things having been set up by whatever supernovas scattered the materials in this neighborhood to begin with.
You missed the gravitational potential energy in the field between Io and Jupiter, and the kinetic energy in Jupiter from its motion around the center of mass of the Io Jupiter subsystem. I think that there are equal amounts of gravitational potential energy and kinetic energy in this 2 body system. I also think that the fraction of kinetic energy in Jupiter’s motion as a fraction of the total kinetic energy would be equal to Io’s share of the total mass of the system.
Note that after the collision all these energies are zero. Some of the energy goes into heating the resulting mess, but much of it goes into tides on the two bodies and the radiating gravity waves that go out into the universe and make little vibrations everyplace.
>But some scientists objected that such would involve energy-from-nothing! Yet it was hard to see why it would not occurr as “expected.”
This isn’t believable to me. This would be like a professional magician falling for that “I’ve got your nose” hand trick.
FWIW my degree is in physics and astronomy. That being said, I’ve not been doing the astronomy part for a quarter century. Io hasn’t aged much, but I have…
Energy/Mass is indeed conserved. However, people sometimes get confused because of the way we commonly use the word “energy” as opposed to the way physicists officially define the term. What many people mean when they say energy is the potential to perform work. To perform work- that is, to create a local decrease in disorder- you have to take a low-entropy source of energy and use it to perform work. In the process, you convert the energy from a low-entropy form to a high-entropy form, thus increasing the overall entropy of the system as a whole. Energy in low-entropy forms is basically a gift of the Big Bang, and high-entropy energy is what we call “waste heat”.
Obviously they were creationist “scientists.” :smack: