if perpetual motion is impossible, why/how do planets orbit and spin continually?
In: Physics
Right now motion diminishes very slowly relative to the intial velocity, and it took a long time to slow down this much. Graphically the line would appear symmetrical along a 45 degree angle. At infinity on the vertical y-axis at the beginning of time on the horizontal x-axis and decrease rapidly until it doesn’t seem to be slowing noticably much when you are in the billions of years. It approaches 0 as the rate of change of the loss of momentum also decreases over time. Have you ever skated on ice? You’d think if you didn’t move for a while you would stop, and you do but it takes a lot longer than you’d think if you just let yourself come to a complete stop while staying straight the whole time.
When people say “perpetual motion is impossible” they are using the term “perpetual motion” improperly: what they really mean is that overunity motion is impossible. What is overunity, you may ask. I’ll explain.
Any moving object has a reserve of energy. Let’s assume that it equals 100% at the beginning. Part of this energy is spent to contrast friction, so that the object can remain in motion. Take away the friction part, and you remain with… let’s say 60% energy you can use for yourself. But energy needs to be converted (typically to electricity), and every conversion generates heat as a side effect, which is more energy that is dispersed into the environment and you cannot use. Let’s say this is a further 30%, so you end up with only being able to use the remaining 30% of the original energy.
Pretty simple, right? Well, apparently, for some people this is too complicated, or they are in denial about how the universe works. Other people know very well how the universe works, but intentionally spread misinformation in order to make profit (by persuading people to buy machines they know won’t work). According to these people, if you start with 100% of energy, you can take out some of it for your purposes (let’s say 30% again), and at the same time you supposedly can give THE WHOLE 100% of the energy back to the object. This (if it was possible, which is NOT) would mean that you started with 100% (a “unit”), but you ended up with 130% (OVER a UNIT, overunity).
Of course, if you ask this kind of people (I call them “perpetuists”) where the additional energy comes from, they either fall in contradiction and/or recur to hazy description that don’t really explain anything, so you know what they’re up to.
The others have answered in detail about the planets.
Perpetual motion is NOT impossible, in fact physics says that in theory an object will move (or spin) forever (in space) if there’s no force acting on it. And, as everyone else has pointed out, there’s always a force, however small – space does have a very low density of atoms, dust, particles, so moving through space does encounter some friction.
But anyway, the “perpetual motion” rule you quoted refers to perpetual motion MACHINES, which would be devices that spin or push or create energy forever, without a power source. You always have a power source, because there’s always friction and resistance, so the energy to push against friction and resistance has to come from somewhere.
The underlying principle is actually [conservation of energy](https://en.wikipedia.org/wiki/Conservation_of_energy). That’s why we can’t have perpetual motion MACHINES.
Planets are very very very very slowly falling towards the sun they are orbiting. It will take billions of years for the earth to fall into the sun, a very long time but eventually it will happen.
Try googling “Heat death of the universe”
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The amount of force that planets started off with is enormous, and it’s wearing down very slowly. It’s not perpetual motion it’s just going to last longer than we will.
The length of time a planet spins is finite, not continuous. They spin due to the gravitational effects of the sun they orbit. The sun has gravitational energy due to its size and the energy inside it. When it runs out, so does it’s effect on the planets. I think.
This answer leaves out stuff like solar winds and other forces because, well, it does. Sorry.
As I understand it, planetary orbits ARE changing/decaying, just at a rate slow enough to be virtually imperceptible to us. In space, massive celestial bodies don’t have air resistance or friction to slow their momentum like on Earth – they are mainly affected by the gravitational forces pulling them back and forth between bigger things. Planets in a solar system orbit the densest thing around – a star – and that star, in turn, moves through space pulled around the galactic center, an even denser mass that holds the entire Milky Way in its pull. The gravity between these things is basically fighting against the outward explosive releas pf energy that created separate planets and whatnot to begin with.
The massive scale may give the illusion of permanence, but the universe is always expanding outward with tremendous speed, and the energy that propels us spreads out, too. Eventually the Sun’s hold on us will weaken, and the momentum that started our orbit will carry us out of the system, maybe even out of the galaxy.
Or maybe the galactic center will shift, and the whole system will spin out until eventually the energy of that initial push is expended, or spread so thin so as to prevent the further growth of space.
So what we’re really seeing is just a process of energy transfer that’s efficient enough, I guess, that it seems limitless. There IS a limit, it just doesn’t mean much in the context of a human lifetime.
According to the delta T equation the earth will stop spinning in 1.9 Trillion years. The planets will not rotate for ever. It will seem that way to us due to our limited observation but they meeting minimal friction in space and they are competing with entropy so eventually everything will collapse again in the heat death of the universe. The amount of mass and inertia they carry mean that it’s going to take either significant mass, force, friction, or a combination of three to have any observable impact. Otherwise it’ll take a lot of time.
There is no friction* in space. Since the planets don’t have to do any work overcoming friction, they don’t lose energy and can continue orbiting.
*) in reality there’s a minuscule amount of friction from the tiny bits of debris in space, and planets are losing some tiny, tiny amount of energy due to gravitational waves, but these losses are so small it’s safe to ignore them for planets not orbiting in gas clouds.
Edit: a planet’s rotation around its own axis slows down due to tidal braking. That’s what happened to our moon, which is tidally locked to the earth.
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