In what sense does the moon rotate?

Factual Questions
41

Nope, it would still rotate on its axis, owing to conservation of angular momentum. It would also maintain the same orbit about the Sun that the Earth currently does (presuming that it doesn’t interact gravitationally with the Earth). There would be some slight addition of of energy from the transition of orbital momentum (about the Earth) to tangential momentum, but this would only be a small fraction of total orbital momentum about the Sun; it’ll make the orbit slightly more eccentric than the ellipse described by the Earth, but only on the order of a few percent difference.

Stranger

42

I think you’re right. Due to the long radius of the orbit about the sun compared to that about the earth, it wouldn’t take much of an increase in tangential velocity to make up the former orbital momentum about the earth moon system. But I’m not convinced that it would make the orbit more elliptical. It seems to me if the earth disappeared while at aphelion, it would make the orbit less eliptical. The reason beaing that the tangential velocity is lowest at aphelion, so incrasing it then would tend to round out the orbit.

43

True, it would tend to circularize the orbit in that regime. Regardless, the Moon would keep turning at its 27d, 7h rotation rate, neglecting any minimal tidal effects that may speed up or slow down the rotation.

Stranger

44

And in that country, he could build an airport, with a giant treadmill for a runway.

45

Oh no, you di’int!

46

Have some mercy, guys. My thought experiments operate on a fairly restricted budget: I can afford a softball and some rope, but that’s about it.

I begin to see where my comprehension jumped the track. My error was in misunderstanding the concept of the moon rotating “about its axis.” I was envisioning the Earth-moon system as sort of a three-dimensional equivalent of a phonograph record with a circular sticker on it, near the edge (this is the sort of thought experiment I can afford! --notice how at no time does anything exhibit any highfalutin’ properties such as exceeding the speed of light). As the record turns, the sticker obviously turns along with it, but the axis of rotation is at the center of the record, not in the vicinity of the sticker itself. For the sticker to keep the same orientation relative to an observer, there’d have to be some sort of additional arrangement keeping it turned counter to the motion of the record itself, and to me this seemed more like “rotation” than the static version.

In summary, then, would it be accurate to say that “rotation” in the astronomical sense is defined as motion relative to the total star field? So if the moon actually did have a dark side, always presenting one face to the sun, it would still be rotating, since it would still be in orbit around the sun. If the moon instead were stationary relative to the axis of the galaxy, it would still be rotating, since the galaxy itself is turning relative to other galaxies. If so, then I’m guessing that there are no observed non-rotational objects in nature (though it might be possible to construct one using sufficiently precise measurements)?

Q.E.D., Stranger On A Train, CurtC, and others: thanks for the suggestions! Revenge is sweet! Muhahahaha!!!

47

Aha!!! Success! Took a wooden poker chip, floated it in the center of a cup of water, then spun around in a circle. Poker chip retained orientation relative to surrounding room! Eureka!

I so proud of me.

48

If this were to happen for some reason, we’d certainly have more pressing problems than worrying about the rotations of heavenly bodies.

Won’t someone think of the children? :smiley:

49

And show Showgirls as the in-flight movie. :smiley:

50

If Monty Hall rides his bike fast enough on a treadmill, will he eventually take off and fly?
:smiley:

51

CurtC, QED, would you please explain how the electricity travels, and how the bicycle stays upright?

I am only a layperson. Doesn’t electricity transportation involve there being an increased amount of electrons in one end of the line, as compared to the initial situation? Or else, what on earth is going on?

52

Easy question first:

Bicycles maintain balance with a juggling act involving gravity, centrifugal force* and friction. In a turn, for instance, the rider leans into the turn. Friction keeps the wheels from sliding out from under him, and the centrifugal force (which is really inertia trying to keep the bike going in a straight line) caused by the turn balances out the force of gravity trying to tip him over. The gyroscopic force exerted by the wheels at any reasonable speed is so small in relation to the weight of the bike and the rider that it has very little effect. A plane on a treadmill will take off, but a bike on one will fall over.

Electricity is another matter. It’s been well explained in previous posts, but I’ll try to simplify it for you. Electricity is created by moving charges. In normal electric circuits, those charges are electrons. When electrons move, they created an electromagnetic field around the conductor which propagates along the wire at the speed of light in the the dielectric medium around the wires. In free air, a bare wire has a propagation speed of about .99 c–we say the velocity factor is .99. With various types of plastic insulation, the velocity factor can typically be anywhere between .6 and .8. This moving electromagnetic field is what we call electricity and what does the work, such as making an electric motor turn. The electrons themselves move fairly slowly and individual electrons don’t move very far. The typical drift velocity–the speed at which the electrons move–is dependent upon the current and is typically in the tenths of a meter per second. You can walk faster!

Hope that helps.

53

That should read “…those charges are carried by electrons.”

54

Well, in a sense the charges are the electrons, or are at least an intrinsic quality of the electrons.

Oh, damnit, I’m going to go and complicate your (excellent) explaination with a big more detail. It’s strictly the pedant in me, but the alternative is getting down and dirty into writing perl scripts for system log maintenance, so…

Okay, first of all, charge is an inate property of the electron; it’s the same value (e=-1.6x10[sup]-19[/sup] coulombs) regardless of what you do to the electron or where it goes. Note that a proton also has a complementary (positive) charge; however, unlike the electron (which is a fundamental particle), the proton gets its charge from the quarks that combine to make it up. There are other charged particles, but they’re not a part of everyday life, so we’ll ignore them.

Now, most people have the notion that ‘electricity’ equates to the movement of charges; however, as Q.E.D. notes, the actually electron current or drift velocity is very, very slow, and in an AC (alternating circuit) the net movement is actually zero, or very close to it. So how does electricity get from Point A to Point B?

First, you have to realize that electricity is an idealization; when we talk about electricity, we don’t mean that Little Blue Blobs go from A to B. Instead, what we mean is that you tranfer the ability to do work from A to B; in other words, you’re moving energy around. In order to move energy or do work, as Thermodynamics I students are aware, you have to have a potential difference that causes energy to move in a particular direction. For thermal processes like refrigeration cycles or steam engines, that means a temperature difference. For electricity, it means a difference in charge potential, otherwise known as voltage (or sometimes ElectroMotive Force, which is a misnomer because it isn’t a force). Voltage comes from having a difference in net charge between A and B. Note that in order to transfer energy (or information) you don’t actually need to move the charges from A to B, you simply have to effect a change in the potential between the two points.

Let’s analogize it to water pressure for an everyday visual cue. When you turn on your faucet, water comes out. Why does the water move, instead of stay in the pipes? Because there is pressure behind the water. The pressure comes from a water tower somewhere (or a pump, if you’re on a well system). The water in the tower exerts a force on all the water everywhere else in the pipes. However, when you turn on the faucet, you aren’t getting water straight from the water tower, but water that is already in the pipes. Water does flow out of the tower (force has to effect a movement in order to do work) but only very slowly. If you remove the tower or shut off its valve, water won’t flow out of the pipes. So you magically get water flow out of your faucet because of an energy source (the water in the elevated tower) miles away from home, without those water molecules in the tower ever flowing through your faucet. Freaky, eh?

This is roughly analogous to DC (direct current). By making an electrical connection (via conductive metal wires) between to distant points, the electrons bounce up and down in their orbitals, thus generating an electromagnetic field which pushes adjacent electrons to an elevated position and thus transfering energy without going very far. Think of a line of 2nd graders in which the last one pushes the one in front of him and so forth until the one in front falls flat on his face and starts crying. The one in the back didn’t touch the one in the front, but he’s still culpable for starting the reaction. With AC, the electrons jump back and forth instead of all going one direction, but the effect is similar; energy is pumped throught the metallic lattice by forcing electrons to be more energic and then allowing them to relax. Note that during all of this, the charge never changes in terms of the individual electrons; its just that the electrons become more reactive or energetic. So the electons are just a medium for the transfer of energy, but they don’t make energy themselves; we call this transfer of energy ‘electricity’. Note that it doesn’t have to be electrons that do this; it could be protons, though free protons only occur in a highly energized plasma. In a chemical bond, however, like metallic lattice, it’s always the electrons doing the work (like grad students), and the nuclei just sit around and bullshit like tenured professors.

As for bicycles and centrifugal force, I have nothing to add to Q.E.D.'s succinctly excellent explaination except to reinforce the point that it has only the most marginal effect on vehicle dynamics at reasonable speeds. On a light-weight shaft drive motorcycle at high speed you can tell a little bit of difference between turning right or left owing to the inertia of the rotating shaft, but it’s just a twitch. The real reason a bicycle stays upright is, well, complicated in vehicle dynamics terms, but it depends upon a balance of forces including active control by the rider.

Stranger

55

That was excellent. Thanks.

Inertia would also be my guess on why the bicycle stays upright, not the gyroscopic thing.

56

True enough, but you know how some the folks here can be (and I should know, I’m one of 'em). If I’d have left that stand, sure as Hell, someone would have come along to nitpick it. :wink:

57

Well…sort of. What happens is that when the bike is standing still, any slight imbalance of force will cause it to roll to one side or the other. When in motion, however, there’s a large inertial force component on the bike CM (center of mass) tire from the road, and a complementary force on the tires via the road. When the bike tends to tip to one side or the other, the force on the front tire tends to push the bike back in line; this gives it a certain measure of self-righting ability (sort of like the force of moving water on the keel of a sailboat) but its just a small component of force balance in the sideways direction; by itself, it won’t prevent the bike from toppling over when it starts leaning. However, when you have a trained rider on the bike, he can intuitively juggle all the forces using his body mass like a counterweight, and this small force on the tire becomes the key controlling factor. Unlike car tires, which are ideally flat across their outer section, a bicycle or motorcycle tire needs to be rounded and treat running radially outward in order to make use of this.

If you want to get down and dirty into the topic, check out Pacejka’s excellent Tire and Vehicle Dynamics or Gillespie’s Fundamentals of Vehicle Dynamics.

Stranger

58

The rotational periods of Earth and Moon are interconnected by a chaotic effect known as mode locking.

University of Niš

59

I don’t think the emphasis here is correct. I thought that steering trail (which is what causes the self-righting ability) is very important: without it, most people find riding a bike impossible.

60

I’m not sure what you mean by steering trail, but if you are referring to the caster angle (that is, the angle which the “steering axis” makes through the hub of the wheel–connected to, but not always identical with the rake angle on two wheeled vehicles) then yes, the angle is very critical to the dynamic response characteristic; A shallower angle makes for a steeper “pole” (i.e. the system is more difficult to control dynamically), whereas a sharper angle makes it easier to control and/or more self-righting, but at the expense of manuverability. Think of a child’s tricycle, which typically has a zero caster (and in this case, rake) angle and so is very maneuverable at low speeds but not stable when moving fast, and a “chopper” motorcycle, which has a very large caster angle and is thus hard to steer at low speed but is good for highway cruising.

However, for the purposes of explaination of why a bicycle can stay upright at all (rather than a discussion about the merits of various steering configurations) I elected not to go into that discussion. The dynamics, even for a machine as simple as a solid frame bicycle are quite complex, and without diagrams and models it is very difficult to make a comprehensible description of what is going on.

Stranger