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Nothing will ever appear to be moving at the speed of light from our point of view, because if it's moving at C away from us information will never reach us.

The hubble horizon is the distance where something would be moving faster than C due to expansion right now. But that's 14+ billion lightyears away, so even if we could receive information from those galaxies to see them, it wouldn't arrive for many billions of years.

But it will never arrive if it was emitted after the object was already past the hubble horizon.

Saw an article a while back about Earth's core possibly reversing directions. What affect would that have on day to day lives of people?

I hope a true subject matter expert will chime in but as I understand it there is a strong suspicion that sexuality has a lot to do with epigenetics.

As another post pointed out, there is good data that if one male identical twin is gay there's a higher than typical chance (20-50%) that the other twin will also be gay.

If there we a "gay gene" we would expect all identical twins to have the same sexuality, so this is strong evidence against a "simple" genetic cause for sexuality.

But the fact that if one identical male twin is gay there's an increased chance that the second twin will be gay suggests genetics are involved somehow.

So the suspicion is that sexuality is linked to epigenetics, basically you can think of epigenetics as the dials and switches which regulate how strongly a gene is "turned on". These switches and dials can be affected by environmental factors, which in the case of sexuality means environmental factors inside the womb during fetal developmemt.

Given identical twins share the same womb at the same time they are exposed to similar environmental factors so it's thought they are more likely to have those dials and switches set in a similar way than non identical twins.

As I understand it (again I hope a real expert will chime in) epigenetics is currently the leading hypothesis for the cause of sexuality, but it's still a bit of a mystery.

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A lot of long-range shooting enthusiasts use projectiles known as "boat tails" for the same reason.

Instead of having a flat back, they taper down a bit.

By making the surface area of the rear smaller, the size of the negative pressure zone is also made smaller, reducing the effect and raising the ballistic coefficient.

How deep can you find usefull minerals?

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Is a dying star the only known source for iron to be made?

The axial tilt of the earth is a significant contributor to the production of seasons. The tilt is decreasing, so we should be seeing less extreme seasonal changes. Due to climate change we are seeing more extreme seasonal changes. How do models that predict future impacts of climate change account for the changing axial tilt? It seems like we must have some indications of the severity of seasonal fluctuations from 40,000 years ago to show what would be happening absent our impact. Are those measurements precise enough to be used in models of a few decades or less?

Faster every time. Large rivers only look like they are moving slowly because of their size. Streams only look fast because there are stationary objects visible. Water moves faster when it’s sliding against more water as opposed to objects.

How does a wireless charger work? And why don't we see it being used for other appliances?

I see. So the characteristics of the bed material will factor in the role of sediments in river flows? Due to flooding, there should be an increase in loose sediments that the water would carry/get hindered by.

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Depends how you're measuring and where in the cross section. Assuming no tailwater downstream, you'll see an increase in velocity in the main channel/floodway. As water levels increase upstream, your hydraulic gradient increases, and per Mannings equation, increases your velocity.

In the wider floodplain, water will typically be mostly stagnant as more trees, vegetation, and buildings increase the Mannings roughness.

Therefore, average velocity of the entire cross section could be significantly slower if the floodplain is significantly wide.

Tailwater effects also play a significant role. If your downstream water is high, say due to flooding downstream or a reservoir, the main channel may not actually increase much at all until the downstream levels recede. You see this often near deltas where storm surge or tides provide resistance to flow coming off fairly flat topography.

>For a spinning projectile, this total shear stress value is larger than if it was not spinning, so by this reasoning a spinning bullet should experience more drag than a stable, non rotating bullet.

But the direction of this increase in drag should be orthogonal to the translational motion, no? If I'm right, this drag would contribute to slowing the rotation, but not to the arc of the projectile.

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I didn’t say they are propelled by it, I said they are pushed by it. It exerts a force, which reduces its acceleration due to drag. This is called pushing.

Yes, if you look at the shallow water equations for river flow, the right hand side will basically be predominated by a) slope driving flow and b) friction resisting flow. The friction resisting flow will be determined by the water velocity, the characteristics of the bed material, and the water depth. As water depth increases (e.g. during a flood) the frictional forces decrease as h^(4/3) where h is water depth.

The main channel of the river speeds up. Basically, as the river floods, the cross sectional area of the river increases, eventually flooding the areas adjacent to the main channel. While these move slower than the main channel, they a still moving faster than they did before.

While gravity and mass of water produce the flow, friction with the riverbed slow it down. Thus, deeper channels move faster. As they deepen, they speed up.

Okay but that doesn't really answer my question. How far away would they have to be to be moving at, from our point of view, the speed of light?

I see your line about the Hubble constant etc but it seems to be just a commentary about that distance, not how far away a galaxy needs to be to be moving at SoL.

And I use the word appears, because it would appear to be moving at the speed of light from our point of view on earth.

That depends on what you call "appearance".

If you take the distance today and consider how fast it increases then it is the speed of light divided by the Hubble constant, around 14 billion light years. We can see light these galaxies emitted in the past, it's only ~1/3 the distance to the edge of the observable universe, the current location of the matter that emitted the oldest light we see today.

We can see things where the distance between us and them always increased faster than the speed of light because of the expansion history of the universe: Initially the distance between us and the emitted light increased but as the universe got older and the expansion rate decreased the light started catching up.

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Cost.

Longer answer to an almost identical question a few weeks ago.