# Questioning Gravity

## Main Question or Discussion Point

I'm not sure if I'm questioning gravity here, but I was thinking about the movement of the planets around the sun and couldn't explain something.

I know they say that gravity and the speed of the planets keep them going around the sun, but...how. I mean this sounds awfully like a system of perpetual motion. Everything seems to be held fairly constant.

In the process of revolutions, it seems like either the planets should be pulled towards the sun faster and faster, like water circling down a drain or the should be fast enough to escape the gravitational field.

What made me think of this was Hailey's Comet. How can an object be pulled in by the suns gravity, get very close to it, then escape again, only to return and do this systematically over time? The orbiting path seems to be in near direct opposition to the suns gravitational force. It almost comes straight in and straight back out. This is like a paper ball being sucked close to a vacuum, escaping to a very long distance, and then inexplicably being pulled back by the very same force it escaped.

I hope that came out clear enough because I've been tangling with that one for a while. I was also wondering if gravity was considered an energy because it seems to be another questionable thing since it almost seems infinite as well as proportional to mass, meaning a certain mass will create a certain amount of gravity without end. As a force or energy, it doesn't seem to run out.

Any clarity would be awesome.

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Ryan_m_b
Staff Emeritus
You're right it is like perpetual motion, but it is not a perpetual motion machine. Any attempt to extract energy for work will disrupt the machine. For example, if Hailey's comet was a magnet and we built a wrapped wire torus around its orbit the electricity produced would slow it down thus changing it's orbit.

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Orbit occurs when gravity and centrifugal force are in equilibrium. The orbit of planets is not perpetual motion; space is a vacuum, and so it does not apply any resistance to the motion of the planets, and a body in motion stays in motion. If we were to use this motion to gain energy, as we would a perpetual motion machine, then we would just be converting the planets motion into a more usable form of energy, slowing it down.

I'm not sure if I'm questioning gravity here, but I was thinking about the movement of the planets around the sun and couldn't explain something.

...I hope that came out clear enough because I've been tangling with that one for a while. I was also wondering if gravity was considered an energy because it seems to be another questionable thing since it almost seems infinite as well as proportional to mass, meaning a certain mass will create a certain amount of gravity without end. As a force or energy, it doesn't seem to run out.

Any clarity would be awesome.

If you mean that the source of gravity-matter-doesn't decrease or weaken in its ability to exert the same gravitational force as it uses that force, then you are right-it doesn't. An object of a certain mass will continue to exert the same gravitational force at the same intensity forever as long as it retains that exact mass. Only as an object loses mass, such as when a star undergoes nucleosynthesis in it's core and radiates particles away into space thus losing mass will its gravity decrease.

If planets continued in a straight line, like the TRUE perpetual motion would take them, they would be getting farther from the sun. They are, however, continually falling toward it canceling that out. Lather, rinse, repeat.

Perpetual motion is posible. Perpetual acceleration is not.

D H
Staff Emeritus
Orbit occurs when gravity and centrifugal force are in equilibrium.
No, no, no. NO. This is a terrible explanation of orbits. What centrifugal force? There is no centrifugal force in an inertial frame of reference.

This centrifugal explanation of orbits that is sometimes foisted on young students is a consequence of bad teachers, bad texts, or both. This explanation only explains circular orbits, making it a bit worthless. To make matters worse (much worse) it presumes a knowledge of non-inertial reference frames. Students who have this nonsense foisted upon them don't know the hairy intricacies of non-inertial reference frames. So why?

You don't need calculus, or non-inertial frames, to explain circular orbits. All you need are the concepts of uniform circular motion and Newton's law of gravitation. Together these tell you what the velocity must be to have a circular orbit at a specified distance about an object of a specified mass.

Where do these relations come from? That's the chief problem with pre-calculus physics. Physics at this level appears to be a whole bunch of unrelated facts, all drawn from some magical hat. Calculus based physics fixes a lot of that. Now there's a much smaller number of magical relations; most of the physics just falls out of the math. Higher level math gets rid of even more of the magic. The math starts to get devious, but the physics gets simpler.

In the process of revolutions, it seems like either the planets should be pulled towards the sun faster and faster, like water circling down a drain or the should be fast enough to escape the gravitational field.
That kind of motion would violate conservation of energy, conservation of angular momentum, or both. If you know calculus it's pretty simple to see this is the case. If you don't, the answer is unfortunately the same one your parents give you when you keep pestering them: "Because we said so."

So why are energy and angular momentum (and also linear momentum conserved)? Those conservation laws are laws of physics. In short, "Because we said so!"

Well, no. Physicists are little kids who really do not like "because we said so!" as an explanation. It turns out that there is a deeper explanation than "because we said so" for those conservation laws. Each of these conservation laws results from some symmetry. Angular momentum is conserved because physics is symmetric with respect to direction. The laws of physics are same no matter which direction you look in the sky. Linear momentum is conserved because physics is symmetric with respect to location. Step ten feet to the left, take a spaceship flight thousands of light years to the right, and the laws of physics will be the same. Energy is conserved because physics is symmetric with respect to time. The laws of physics that applied a million years ago are the laws of physics of today.

Drakkith
Staff Emeritus
If planets continued in a straight line, like the TRUE perpetual motion would take them, they would be getting farther from the sun. They are, however, continually falling toward it canceling that out. Lather, rinse, repeat.

Perpetual motion is posible. Perpetual acceleration is not.
Orbiting bodies are constantly and perpetually accelerated by gravity. Both perpetual motion and perpetual acceleration are possible. But machines that try to accomplish either one are not possible, as a machine is something that does work, which requires energy.

I'm not sure if I'm questioning gravity here, but I was thinking about the movement of the planets around the sun and couldn't explain something.

I know they say that gravity and the speed of the planets keep them going around the sun, but...how. I mean this sounds awfully like a system of perpetual motion. Everything seems to be held fairly constant.

In the process of revolutions, it seems like either the planets should be pulled towards the sun faster and faster, like water circling down a drain or the should be fast enough to escape the gravitational field.

What made me think of this was Hailey's Comet. How can an object be pulled in by the suns gravity, get very close to it, then escape again, only to return and do this systematically over time? The orbiting path seems to be in near direct opposition to the suns gravitational force. It almost comes straight in and straight back out. This is like a paper ball being sucked close to a vacuum, escaping to a very long distance, and then inexplicably being pulled back by the very same force it escaped.

I hope that came out clear enough because I've been tangling with that one for a while. I was also wondering if gravity was considered an energy because it seems to be another questionable thing since it almost seems infinite as well as proportional to mass, meaning a certain mass will create a certain amount of gravity without end. As a force or energy, it doesn't seem to run out.

Any clarity would be awesome.
Perpetual motion is perfectly OK. That's what the planets do. It is easy to confuse with perpetual motion machines, which is an unfortunate name for machines that try to get something from nothing.

I studied orbits when I was kid. What I did was take the 1/r^2 equation for gravity along with Newton's F=ma and calculate orbits. Elliptical orbits result naturally. Try it and you will see. It's easy. Thinking about it in English just doesn't do it.