Physics Puzzlers- Class In Session!

Forever, under current conditions. The moon's rotation maintains it's facing toward the Earth as is orbits our planet. So the Earth always seems to hang in the same place in the sky from anyplace on the moon from which it is visible. You do get to watch 'phases' of the Earth, from a dark 'new Earth' to a bright 'full Earth,' and back again, over about a 29 day cycle. And you can catch a sunset every lunar month....
 
Wait a sec, back on the cloud thing. I looked back and did not see anyone mention this. It sounds like everyone assumed the sun was directly overhead. If the sun was at any other angle, the shadow, as projected onto the Earth's surface, would be larger than the cloud. Same as my shadow is huge late in the afternoon.
 
Wait a sec, back on the cloud thing. I looked back and did not see anyone mention this. It sounds like everyone assumed the sun was directly overhead. If the sun was at any other angle, the shadow, as projected onto the Earth's surface, would be larger than the cloud. Same as my shadow is huge late in the afternoon.

Good point I think we have a new winner. I think TyBrad missed this one.
 
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Yes, I did not specify closely enough about the sun angle.
~ 1:1 in mid day, not 1:1 at other times.

Any other comers to the latest question?
 
I think Don Q is correct. The moon's face stays stationary with respect to Earth, in other words it takes the same 29 days (or whatever) for it to rotate once as it does for it to revolve around Earth. So the Earth would stay in the same place in the sky from any point on the Moon. You would have to take a looong walk to see the Earth set.
 
OK, this one is answered by DonQ and tox.

Now suppose that Earth keeps the same face toward the sun- the sun is stationary in the daylight sky. On the night side, where you can see the stars, do the stars

stay stationary in the sky?
seem to go around Earth in one day?
seem to take one year to move around?
 
Since the earth is rotating on its axis over a period of one year, and it takes one year to make a revolution around the sun, from the back side of the earth you would see a slow, one year long, panoramic view of the inside of the Milky Way.
 
Semi-related thread hijack, but it's fascinating!

Did you know that every single star visible by the naked eye, no matter what time of year or what direction you look, is inside the Milky Way galaxy?

Mind. Blown.
 
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On a related note, last summer I learned you can actually see an adjacent galaxy with the naked eye. It's a fuzzy blob near the NE horizon (at least where I am). Andromeda I think.
 
On a related note, last summer I learned you can actually see an adjacent galaxy with the naked eye. It's a fuzzy blob near the NE horizon (at least where I am). Andromeda I think.

Correct! I've seen it too, and it is indeed Andromeda. You need a nice dark night, away from any lights, and 20-20 eyesight. Or your glasses. :D

Btw, on a clear dark night, you can also see some of the moons of Jupiter with the naked eye. They look like little, tiny pinpoints around Jupiter.
 
OK, this one is done (rather quickly).

Suppose that all of space were empty except for two water spheres that are close together. We all agree that they will gravitationally attract one another.

Now suppose that all of space were filled with water and two bubbles of air were close to one another. What happens now? How do the air bubbles move?
 
They too would attract each other, and eventually combine, something to do with surface tension, and/or minimising surface area.
 
Hmm, good one!

There is no 'up' since all of space (except for the bubbles) is homogeneous and of the same density.

There is no attraction between the bubbles since they are lighter than the surrounding medium rather than heavier as in the case of two water spheres in empty space.

Would they wander randomly? Would the gravitational attraction of the water cause them to break up into a bunch of smaller bubbles?

Are we neglecting the air dissolving into the water?
 
The bubbles remain homogeneous and correct, there is no up/down sense and hence, no buoyancy argument here.
 
I would think the bubbles would slowly draw together and merge. I'm thinking that there is a uniform gravity field, except for the two bubbles. So water between the bubbles (but not exactly between)will have slightly less gravity on the axis between the two bubbles, and will be pulled away from that axis.
 
OK, this one is done (rather quickly).

Suppose that all of space were empty except for two water spheres that are close together. We all agree that they will gravitationally attract one another.

Now suppose that all of space were filled with water and two bubbles of air were close to one another. What happens now? How do the air bubbles move?

They don't move in that scenario.

Hiya, teach. :smoke:
 
OK, this one is done (rather quickly).

Suppose that all of space were empty except for two water spheres that are close together. We all agree that they will gravitationally attract one another.

Now suppose that all of space were filled with water and two bubbles of air were close to one another. What happens now? How do the air bubbles move?

I know of no force that would 'draw the bubbles together' under such conditions. In the homogeneous steady-state water universe, we can assume no temperature gradients, no currents, no energy sources of any kind, so basically nothing is happening, except random collisions between molecules. I would expect these to gradually dissolve the air bubbles and disperse the air molecules evenly (in time) throughout the water universe, through Brownian motion. However, we're told that's not going to happen. Instead I guess we can assume that all molecular effects are equal on all sides of the bubbles, so everything cancels out and nothing happens.

Now the presence of the two bubbles would have a gravitational effect. The homogenous water universe creates an unchanging and equal gravitational flux in all directions, except on the axis of the two bubbles. For each bubble, there is less density, and thus less gravity, in the direction of the other. So the stronger attraction of the rest of the universe might cause the bubbles to move directly apart from each other. But we are talking two bubbles in an infinite universe. The difference would likely be too small to overcome the inertia of the bubbles in their medium. And if the bubbles were of significant enough size and close enough together for this not to be the case, they would find the gravitational difference lessening the further they moved apart, and eventually it would become too weak to overcome the physical resistance of the medium, and motion would cease.

So I see this universe attaining a totally steady state, if it doesn't start in one, in which the two bubbles remain, unchanging and unmoving, a certain distance apart.
 
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Yup, we do. Gravity is a core property of matter. Two objects that have mass will be attracted to each other. However, this gravitational force is proportional to the mass of the two objects, and the force is miniscule unless the amount of mass is large. Like planet-sized large.

So, two spheres of water will be very slightly attracted to each other, and two bubbles of air, which do have mass, are also attracted to each other. However, given the very small amount of mass in two bubbles of air, the attraction is so small as to approach zero. Since all of space is full of water, there is no "up", so the bubbles will not rise.

Since the mass of the surrounding water is so much greater than the mass of the air bubbles, the gravitation attraction of the two bubbles can safely be ignored. So, what will the water try to do?

In the real world, two air bubbles surrounded by an infinite amount of water would certainly dissolve into the water and cease to exist, but I'm guessing that's not the desired answer.

Two air bubbles surrounded by an infinite amount of water, which would have infinite mass, would instantaneously compress the two bubbles to as close to a singularity as the mass of the air would permit. No, wait, that can't be correct. Water "compresses" things at the bottom of the ocean solely due to the force of gravity pulling the water "down". However, in the problem statement, there is an equal amount of water in all directions, so there is an equal amount of gravitation force on the water from all directions, so the net force on any point in the water is zero. The water will not exert any compressive force on the air bubble.

So, if there is no net force being exerted on the air bubbles, they will not move. The gravitational attraction between the air bubbles is many orders of magnitude smaller than the inertia of the water resisting any movement of the bubbles.

I'm going with "They will not move".
 
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