J.C.MacSwell

From the original post...I skimmed through but did not see it covered and had not seen this thread prior to today (IIRC) Does an electron in free fall radiate? (My thought would be no) Does an electron forced to stay at rest in a gravitational field radiate? (It must...correct?? or am I off base?) So I'm thinking electrons do not radiate due to gravitational forces (alone)

Hi KC No one is trying to imply cat's are smarter. It is just that they have evolved to land on 4 feet very effectively and very efficiently. A simpler method of merely rotating the tail does not achieve the same result. Their instinctive method is actually very clever and a little complicated, even if they have no real understanding of what they are doing.

Here you go again...

I'll go with the cats, that control the precession about other axes while counter rotating about the main one, enabling them to land on all four feet in best orientation. You can't do that by simply spinning your tail.

Your humble opinion would be correct. While the photons emitted are themselves massless, they contribute to the mass (invariant mass) of the box/system

Right. They can't change their angular momentum without outside help (forces)

That's why you don't even need something to counter spin...it can simply translate away, on a trajectory offset from the axis ...and then you have cat's that can land on their feet from any position, with no change in angular momentum at any point in their fall

Is the ball simply released with the system in a static position? What force would be required to maintain the lower ball at rest prior to collision?

I see a lot of new members get negative reps where they are not used to the way we tend to debate. It might help them to conform, but at the same time they end up somewhat in the hole starting out. I think it would help if anyone with negative reputation received an automatic +1 once a week.

"Be a cubical photon" for the win!

At the risk of sounding inane, "nothing", as we think we know it, may not be nothing after all. (or in this case before all, and possibly again after all) Turtles may be the best guess so far: "A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise." The scientist gave a superior smile before replying, "What is the tortoise standing on?" "You're very clever, young man, very clever," said the old lady. "But it's turtles all the way down!"" —Hawking, 1988

With apologies to Wolfang Pauli, it is NOT EVEN... ..."not even wrong"

For the Earth/Moon gravity, the force is the same, but opposite, on the Earth as it is on the Moon. The Barycenter is not a massive point effecting the GPEs For the Earth/Moon the changes in GPE are pretty close to proportional to displacement (marginal change in force over the distances discussed) If you jump in the air, the COM between you and the Earth stays in place (or on it's path), but of course (almost) all the energy of displacement goes to you. It is not split equally even though the mass displacement is equivalent (opposite) ...all because the Earth is so much more massive

The increase in GPE wrt distance to the barycenter. It is less for the Earth than the Moon.

WRT the barycenter the gain for the moon is much greater than that for the Earth.

OK, let's see how you approach this. Let us know any assumptions you make that you feel might not be obvious.

Ok. Obviously that is not steady, and much more complicated than what would start out as as a constant velocity horizontal flow. I think you have to start by making some simplifying assumptions. when you have hard corners, changing cross sectional area, and varying pressure head before you even get to the airfoil.

I have seen a number of links that claim that the planets orbit the SSB exactly and not the Sun. I think the logic is flawed. If Jupiter was set in stable orbit always further from the Sun, the SSB would move further from the Sun, and at the same time make Jupiter less of an influence on the orbits of the inner planets. If Jupiter was moved far enough, the SSB would have been moved entirely outside the orbit of Mercury. Unless one argued that the orbit of Mercury was hundreds of years rather than 88 days, it would be pretty clear that Mercury was in fact orbiting the Sun. I think they are taking the results of a two body problem and assuming it holds when more bodies are involved. Reasonable guess, but that wouldn't be correct.

In this case it is steady flow, so they are the same thing.

By stream I mean the flow bounded by two streamlines, or the airfoil surface and a streamline when no boundary later is present (as in the hypothetical of inviscid flow). Edit: trying to post an image but keep getting You are not allowed to use that image extension on this community.

It would look the same as when the streams divide at the stagnation point at the front. There is a point (same point in each stream) that comes to rest (wrt the airfoil) before reaccelerating on it's path/s. There is no boundary layer in this (inviscid) case, so "they" (the streams) meet at a point after going above or below the airfoil.

The way I understand it: I think that if the Euler Equations for inviscid flows are set (certain assumptions are made with regard to the wake, or division of flow at the trailing edge) up to allow lift, they invariably include a net drag force on the airfoil. You have circulation about the airfoil, with faster moving flow from above the wing converging with slower moving air from below. In all the set ups that have zero drag (D'alemberts paradox), there is no circulation and the flows converge at zero speed wrt the airfoil...they meet at a second stagnation point and there is no net lift force

There would be no drag, and no lift, if that was the case.

What is the definition of "orbit" that we are comfortable using here? The planets are not drawn gravitationally toward the SSB. At any one time one might be by coincidence, but for the most part the gravitational force vector does not favour that point.

For our Solar System, wouldn't the Solar System barycentric frame be the closest thing we have to an inertial frame? (closest and most permanent)