# Pendulum, part two.

sonhouse
Posers and Puzzles 14 Sep '06 02:34
1. sonhouse
Fast and Curious
14 Sep '06 02:34
A regular pendulum, on say a one hundred meter long could assume no more than a one hundred meter change in gravity (altitude).
1/R^2 and all that. So how would a pendulum's frequency change if it were mounted above an airless body (no air friction to worry about) and a VERY long string, say its above the moon and its hanging from a thousand KM altitude. Does that change its fundamental frequency of swing? You can see a new variable here, greatly changing gravity at the extremes of its swing vs closest approach to surface level.
2. IM4Y2NV
Dadohalic
14 Sep '06 02:46
Originally posted by sonhouse
A regular pendulum, on say a one hundred meter long could assume no more than a one hundred meter change in gravity (altitude).
1/R^2 and all that. So how would a pendulum's frequency change if it were mounted above an airless body (no air friction to worry about) and a VERY long string, say its above the moon and its hanging from a thousand KM altitude. ...[text shortened]... re, greatly changing gravity at the extremes of its swing vs closest approach to surface level.
I do not believe this is possible outside of the earth’s atmosphere. Wouldn’t the pendulum motion simply turn into an orbit? If a pendulum were not unique to earth. wouldn't there be an existing example in the universe?
3. XanthosNZ
Cancerous Bus Crash
14 Sep '06 04:27
Originally posted by IM14u2NV
I do not believe this is possible outside of the earth’s atmosphere. Wouldn’t the pendulum motion simply turn into an orbit? If a pendulum were not unique to earth. wouldn't there be an existing example in the universe?
What? There exists no pendulum in the universe outside of Earth therefore they cannot exist? Also let's say you are in the influence of a bodies gravity (say on the surface of Mars), why would the pendulum act any differently than on Earth?
4. 14 Sep '06 05:41
how would you start such a pendulum? If you just let go at the top of the arc it would never reach a vertical position if you push it then it will escape the gravitational field at the end of the first swing.
5. IM4Y2NV
Dadohalic
14 Sep '06 10:58
Originally posted by XanthosNZ
What? There exists no pendulum in the universe outside of Earth therefore they cannot exist? Also let's say you are in the influence of a bodies gravity (say on the surface of Mars), why would the pendulum act any differently than on Earth?
Also let's say you are in the influence of a bodies gravity (say on the surface of Mars), why would the pendulum act any differently than on Earth?
Sonhouse was not asking about a pendulum on another planet. Nor did I comment about one.

What? There exists no pendulum in the universe outside of Earth therefore they cannot exist? That is what I stated I believe.
6. XanthosNZ
Cancerous Bus Crash
14 Sep '06 11:18
Originally posted by IM14u2NV
Also let's say you are in the influence of a bodies gravity (say on the surface of Mars), why would the pendulum act any differently than on Earth?
[b]Sonhouse was not asking about a pendulum on another planet. Nor did I comment about one.

What? There exists no pendulum in the universe outside of Earth therefore they cannot exist? That is what I stated I believe.[/b]
He's asking about a pendulum above an airless body which would indeed exert gravity on the bob. So the pendulum will no longer be damped (no air resistance and gravity is restorative) but it would still swing back and forth like a pendulum does on Earth. Difference being that at the bottom of the swing gravity is much stronger and therefore the bob undergoes more force than at the top end of the swing.

Agnis, I can't see why the bob would work as you said, say I start it at horizontal then it will fall/swing through a 180 degree arc before starting to fall back again. That's how a pendulum works even if there is no air resistance. It won't go past the halfway point as that would require more energy than the pendulum contains.

One of the major things about pendulum's on Earth is that their period is not related to angle swung through. I suspect (but cannot prove) that this would not be the case for a pendulum such as sonhouse suggests.

PS. IM, your bit about pendulum's not existing outside of Earth is hilarious. I hope it's a joke.
7. sonhouse
Fast and Curious
14 Sep '06 13:07
Originally posted by XanthosNZ
He's asking about a pendulum above an airless body which would indeed exert gravity on the bob. So the pendulum will no longer be damped (no air resistance and gravity is restorative) but it would still swing back and forth like a pendulum does on Earth. Difference being that at the bottom of the swing gravity is much stronger and therefore the bob undergo ...[text shortened]... your bit about pendulum's not existing outside of Earth is hilarious. I hope it's a joke.
Actually, there are plans afoot to use a giant pendulum just as I said as a way to sling spacecraft out of earth's orbit. It would literally be a thousand or more miles long and one end high in orbit and the other end just dippinig into the upper atmosphere say about 160 Km high. So a rocket takes off and does not have to achieve even orbital velocity and the swinging line is timed in such a way as to snatch a payload and run with it into orbit or beyond. I was thinking about a satellite in orbit with a swinging line on it, orbiting the moon. You could also think of it as on a thousand mile high tower and dangling below just like ones on earth, of course thats just hypothetical.
8. IM4Y2NV
Dadohalic
14 Sep '06 13:30
Originally posted by XanthosNZ
He's asking about a pendulum above an airless body which would indeed exert gravity on the bob. So the pendulum will no longer be damped (no air resistance and gravity is restorative) but it would still swing back and forth like a pendulum does on Earth. Difference being that at the bottom of the swing gravity is much stronger and therefore the bob undergo ...[text shortened]... your bit about pendulum's not existing outside of Earth is hilarious. I hope it's a joke.
No I am not joking but I ask you to help me understand as sonhouse just posted above after the pendulum reaches escape velocity. Why would it reverse its swing? There is not an atmosphere to slow it down and gravity is no longer a factor once you pass escape velocity.
9. sonhouse
Fast and Curious
14 Sep '06 16:56
Originally posted by IM14u2NV
No I am not joking but I ask you to help me understand as sonhouse just posted above after the pendulum reaches escape velocity. Why would it reverse its swing? There is not an atmosphere to slow it down and gravity is no longer a factor once you pass escape velocity.
If it reached escape velocity it would lose the same energy going out. Don't see how it would pass the 180 degree mark very much. I don't think it would go in a complete circle. Picture a tripod a thousand miles high on the moon and a pendulum swinging from the peak. If you start at 180 degrees, parallel to the moon's surface, how could it go greater than the opposite parallel? However the gravitaional attraction would get much less as it approaches horizontal so the force varies with the altitude. Just wondered how that effects the frequency of the swing. Or the amplitude.
10. IM4Y2NV
Dadohalic
14 Sep '06 18:09
Originally posted by sonhouse
If it reached escape velocity it would lose the same energy going out. Don't see how it would pass the 180 degree mark very much. I don't think it would go in a complete circle. Picture a tripod a thousand miles high on the moon and a pendulum swinging from the peak. If you start at 180 degrees, parallel to the moon's surface, how could it go greater than t ...[text shortened]... th the altitude. Just wondered how that effects the frequency of the swing. Or the amplitude.
Thanks sonhouse I was not thinking the string was connected to the moon prior. Now that it is, I understand.
11. sonhouse
Fast and Curious
15 Sep '06 02:36
Originally posted by IM14u2NV
Thanks sonhouse I was not thinking the string was connected to the moon prior. Now that it is, I understand.
I don't think it makes much differance to the dynamics of the pendulum whether or not its tethered to a pole or tripod vs dangling from a mass in orbit. If the system is in orbit around a spherical mass the swinging motion would be the same from the pendulums POV, I think. Imagine a large mass in orbit 1600 Km above the surface of the moon and it has a swinging bob on a line 1500 Km long. It would have to be set in motion, if it started out horizontal so the center of the line was at a tangent point then the two objects would be at the same orbital velocity and you would have to add energy to get it to go down! Wow, this is complicated! I know this kind of system has already been worked out for capture of sub-orbital objects to bring them up to orbital velocity but think about the space shuttle when it goes back to earth, it has to fire retro-rockets to decrease its velocity, it has to expend energy in other words, to go down!
12. 15 Sep '06 07:36
Originally posted by sonhouse
Actually, there are plans afoot to use a giant pendulum just as I said as a way to sling spacecraft out of earth's orbit.
I don't think so, no serious plans, anyway.
But I could think this could be a hyphotetical exercise for constructors, but not more, I'm afraid. The fonding simply isn't there.
13. sonhouse
Fast and Curious
15 Sep '06 12:191 edit
Originally posted by FabianFnas
I don't think so, no serious plans, anyway.
But I could think this could be a hyphotetical exercise for constructors, but not more, I'm afraid. The fonding simply isn't there.
Yes, of course. There are dozens of plans around like that, one being the space elevator, work is actually proceeding on that front, carbon nanotube research is lengthening the cable made and figuring out ways of gluing them together. You need a cable about 50 times stronger than steel to do the job. Another project is the idea of a fusion reactor in space something like a mile long, basically a long linear accelerator which theoretically generates hundreds of megawatts to run an ion rocket which is by its nature very efficient at producing thrust. These things probably won't see the light of day but the theoretical work can still proceed in some engineer's office or at home.