Not Mercury (at least not yet). From http://pds.jpl.nasa.gov/planets/welcome/mercury.htm:
“In this house we obey the laws of thermodynamics!”
Oops, you’re right, I misspoke. I meant to say something about simple rations of orbit to rotation but couldn’t come up with a good way to say it so slipped into something a little different and suddenly Mercury no longer makes sense. Mercury has a 2:3 orbit to rotation ratio.
Interestingly enough 2:3 exists in a stability pocket meaning it will take a lot of energy to get it into a 1:1 ratio. So it wont be going there unless something drastic happens.
HEY! You sayin’ I’m not doing anything useful?
Here’s the truth, the whole truth and… well, anyway - it’s at least in layman’s terms!! 
found here http://www.jimloy.com/physics/perpet.htm
I love layman’s terms…!!!
By the way - layman’s terms being much clearer than my prof’s (and this is relevant to the point that there is nothing with ‘no’ gravitational pull / friction):
And if my prof is reading this (I suspect he is online here somewhere!) it still makes me smile!!
Here’s an easy way to visualize tidal forces. For an extended body in orbit only its center of mass has the correct angular velocity for the given orbit. Any part of the object that is further out from the central force is moving too fast, and is therefore trying to pull away from the CoM, and vice versa for parts that are closer in. So you get two oppositely directed force vectors trying to pull the body apart.
Also, these oppositely directed vector fields aren’t equal across the extended object so you get tangential components pointing to the center of the body and it’s these components, for instance, that cause the tides on Earth.
Actually, G is the universal gravitational constant, equal to about 6.673x10^-11 m^3/s^2kg. What you are quoting is the acceleration due to gravity on the earth’s surface. Acceleration due to gravity is a local phenomenon, while G is a universal constant.
Regarding the OP, here’s my two cents:
Planetary orbits are not perpetual motion. Here’s a way of visualizing it that may make the reason a little more obvious. If we follow general relativity we should think of gravity as a curvature of spacetime, and not as an attractive force. From Newton’s laws of motion we know that an object in motion tends to stay in motion, unless acted on by an outside force. This is not perpetual motion, just a statement about inertia.
With this in mind, we can think of a planet orbiting a star as following the straightest possible path through curved spacetime. That’s not perpetual motion. It’s just Newton’s law in relativistic terms.
This is not very obvious on the face of it because we only see it in the three spatial dimensions. To us it looks like a closed loop. If we plot the orbital motion in the two relevant space dimensions and one time dimension, however, the graph looks like a helix, not a closed loop.
Forget tidal friction (ok…don’t forget it cuz it counts). I thought the earth (or any orbiting body) lost energy due to gravitational waves. Much as a cork bobbing in the water loses energy (the ripples moving away from it) so too does the earth (or any planet). IIRC Stephen Hawking calculated that the earth is losing enough energy in this fashion to power an average toaster (how you would convert the gravitational energy to a useful form is anybody’s guess). Eventually, if you wait long enough, the earth will spiral into the sun. Not much use in waiting though as this time period will take FAR longer than the life of our sun. One way or the other we will all be long gone before we see this happen.
See now if Brad Pitt had formulated the laws of thermodynamics…
“The first law of thermodynamics is that energy can be changed from one form to another, but it cannot be created or destroyed…”
(dramatic pause)
“…The second law of thermodynamics is that ENERGY CAN BE CHANGED FROM ONE FORM TO ANOTHER BUT IT CANNOT BE CREATED OR DESTROYED”
This is not correct about Mercury. If it were, then the length of one rotation of the planet would equal the length of one revolution of the planet about the sun. A solar year on Mercury is almost 88 earth days. A synodic day on Mercury (midday to midday) is almost 176 earth days.
In comparison, our moon DOES hold the same face to its more massive partner, and so by definition, the period of one rotation equals the period of one revolution.
Hope this helps clarify…
- Jinx
I thought that friction between the Earth and its oceans was turned into heat and radiated. This cannot be the reason that the moon’s orbit is getting larger can it?
Mercury has its orbit because it was struck by a huge meteor many many moons ago
Ah, but Rarrwlbazzle, may I explain that your post is in error.
quote:
Rotation period (length of day in Earth days)…58.65
Revolution period (length of year in Earth days)…87.97
The error is that the rotation period you cite is the sidereal period. This means it is the time for a star to return to the same spot (as recorded by an observer) from one consecutive planetary rotation to the next. For earth, this is 23h 56m, not 24h 00m. You see, the sidereal period accounts for the movement of a planet in its orbit AND the rotation about its axis.
Although the sun is a star, sidereal time does NOT include the sun. The measure of the sun to return to the same spot from one consecutive planetary rotation of Mercury to the next is its synodic period of almost 176 earth days. The synodic period of Mercury is so great because it is moving so fast along its orbit.
I hope this helps clarify…? Sidereal time and sidereal period can be a tough concept to understand. :eek:
- Jinx