askscience 内の AutoModerator によるリンク Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

The stars existed for billions of years before the first human (or even creature with complex thought) existed, so the night sky was plenty full of stars before anyone could have their mind blown in such a way (I think you get this, but just clear up confusion if anyone else read your reply)

askscience 内の AutoModerator によるリンク Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

I think I was not clear. The distance between the Sun and Earth changes way more than the distance between the Earth and Moon.

What is important isn't how much the distance changes between them, what matter is the difference of gravitational pull on the side of the Earth close to the Moon/Sun vs the gravitational pull on the side of the Earth far from the Moon/Sun. The Sun us pulling the same hard on both sides (since the force profile is more "flat") while the Moon is pulling a little harder on the side closest to it that the side of the Earth a little further away.

askscience 内の AutoModerator によるリンク Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

It is a well known (in the physics world) that an orbit of 3 or more objects cannot be solved perfectly. We call it the "1, 2 and you're done" rule. So, as soon as you have something like the Sun, Earth and Moon, there is no equation that tells you where the objects will be as a function of time. Throw in all the planets of the Solar System and it gets even worse.

But, since we have powerful computers, this is ok, we can still simulate it to really high accuracy. And in fact, when you start to do simulations, the math actually gets easier instead of harder- you just have to do something easy a whole bunch of times (so much of studying physics and engineering is doing complex math so that you can approximate things without having to do them in a simulation which can take a lot of processing power).

So, to determine orbits we do something like this. First, take all of the positions, velocities of the planets you're looking at, and calculate all of the gravitational forces acting on them right now. This is easy, you're not saying "how is the gravitational force changing?" you're just saying "what is it now?" so you can use Newton's law of -G*M*m/r2 , and do that for all the combinations. That allows you to calculate an instantaneous acceleration for every object, and then by using standard motion formulas, you predict where the object will be in some "small" time step in the future (and what "small" is is determined by what you're working with and how accurate you want it to be. For something like planets, you might say "small" is an hour, and for something like a satellite orbiting the Earth you might say "small" is a second). So, you simply say p_dt = p_now + v_now*dt + 1/2*a_now*dt2 (position after some time "dt" is equal to the position now plus the velocity now times that "small" change plus one half of the acceleration now time that "small" change of time squared).

Then, you simply repeat the process, over and over again, until you're as far into the future as you want to me. The further you predict, the less accurate you will be, and the smaller of step size you make the more accurate you will be.

I must say, this is a simplification of how real orbit propagators work- but it does capture the basics (and it is how simple things like Kerbal Space Program works). Real orbit propagators track too many objects to do a full simulation like this- it would take too much processing power. So instead they propagate the orbit assuming that only the biggest mass nearby matters (like say, the Sun) and then all of the other masses (the other planets) act as small perturbations to that predicted motion. The nice thing about this is that motion of one planet around one sun can be predicted easily, and then just small changes to that motion are added up to see how it changes thing. But this is a much more complicated process than described above, and the basics of it are the same.

askscience 内の AutoModerator によるリンク Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

The Moon causes a tide and not the Sun because the gravitational pull of the Moon changes an appreciable amount over the radius of the Earth, while the Sun's doesn't.

To understand this, look at a graph of 1/x2. Note that the force of gravity is related to this graph, since the force of gravity is equal to -G*M*m/r2. If you call -G*M*m a constant (k), you can see that F_g = k/r2. Thus, the force of gravity as a function of distance away will look something like the graph linked above.

Well, we're a lot closer to the Moon than we are the Sun. Thus, even though the force of gravity is much greater from the Sun than it is the Moon, the force of gravity from the Sun is coming in a much "flatter" region of the graph, while the pull from the Moon is in a more "curved" region. Thus, the Sun pulls on all of the Earth basically uniform, while the Moon pulls harder on the part of the Earth (and water) that is close to the Moon than it does on the part that is far from the Moon. It is that difference in pulls which leads to a tide, not the strength of the pull.

All of that being said, there are slight tides from the Sun, they are just so miniscule that they are overwhelmed by the Moon's tide.

askscience 内の AutoModerator によるリンク Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

[–]Weed_O_Whirler 1ポイント2ポイント  (0子コメント)

It is, but less true than what most people claim.

Light from stars travel at (well, writing this now seems silly) the speed of light, so anytime you see anything, you're not seeing it as it is "now" but as it was at a time d/c (its distance divided by the speed of light) ago. While this applies to everything, it is only on stellar/galactic distance where it starts to matter.

However, most of the stars you can see standing outside are stars in our own Milky Way- which means that they are no more than 100,000 light years away (most are much closer). So, at most you are seeing them how they were 100,000 years ago. That might sound like a long time, but it is blip to the lifetime of a star (which lasts billions of years). Thus, while it is possible that some of them have died before you have seen them, it is but a very small fraction.

askscience 内の AutoModerator によるリンク Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

[–]Weed_O_Whirler 2ポイント3ポイント  (0子コメント)

Total energy is lost- which sounds like it violates conservation of energy... and it does! Turns out, conservation of energy doesn't apply in an expanding universe.

Conservation of energy is derived from the assumption of time invariance- that is that if I do an experiment now, and I do an experiment tomorrow, the experiment will behave the same way both days. But in the expanding universe, that doesn't hold true, so energy is not conserved.

But that doesn't mean conservation of energy is thrown out the window. Locally, the universe is not expanding. Throughout our entire galaxy, and even in our galactic super cluster, things are not flying apart. We're bound together by gravity. So any experiment that takes place entirely in our galactic supercluster will obey conservation of energy. But this red-shift effect happens from galaxies outside of our local super cluster, and thus energy is no longer conserved.

nfl 内の msaltveit によるリンク Albert Breer on Instagram: “Looks like the Eagles have adopted Rugby-style tackling, like Seattle. Watch head placement ...”

[–]Weed_O_Whirler 3ポイント4ポイント  (0子コメント)

I hear this complaint a lot, but I think there is a reason for it: in Football every yard matters, since there are a set number of plays allowed to gain that magic 10 yards. So there is a big difference between a 3 yard run and a 5 yard run. Not the case in Rugby. It's a much more fluid game and until you're right at the end zone, the difference between 3 and 5 yards is negligible.

That's why Football values big hits that stop a player in their tracks.

AdviceAnimals 内の wtffffffamidoinghere によるリンク Girlfriend cheated on me and lied about it. Now she and her friends expect me to forgive her because "forgiveness is a part of love".

[–]Weed_O_Whirler 8ポイント9ポイント  (0子コメント)

I've seen relationships where one person cheated on the other, and forgiveness happened and the relationship (seemingly) fully recovered. But every time I've seen that happen, there has been one common trait: the person who did the cheating did not expect to be forgiven. The apologized and realized they did not deserve to be forgiven. That's the level of guilt the cheater needs to feel for their to be a chance of it working out.

movies 内の profound_whatever によるリンク Great Moments in Nude Scenes [NSFW]

[–]Weed_O_Whirler 353ポイント354ポイント  (0子コメント)

I think you're confused by what he means by "crop." I thought the same as you, until he mentioned the pink sock.

When film is sent to the theaters, the film is larger than what is supposed to be projected, and the theater is supposed to line up their projector in such a way that only the part the director intended is shown. When projectionists don't line it up correctly, you can see things you're not supposed to. Roger Ebert talked about this a lot, how sometimes movies are ruined by bad projectionists who don't crop the film correctly.

nfl 内の bceltics933 によるリンク Who is the worst coach to win a Super Bowl?

[–]Weed_O_Whirler 1ポイント2ポイント  (0子コメント)

In Caldwell's defense (I can't believe I'm defending the guy who ensured that Manning's last game as a Colt was a loss to Sanchez, but...) the call to bench starters came from Polian, not Caldwell.

cordcutters 内の Luke1BK によるリンク Netflix Starts Testing Pre-Roll Ads

[–]Weed_O_Whirler 1ポイント2ポイント  (0子コメント)

This actually would be why they would need to raise prices/add ads. Netflix has to pay per thing they show you- both in licensing and in bandwidth. If they had as many subscribers as they do now, but were using substantially less of the internet traffic, that would be a sign they are making lots of money.

Colts 内の Wham_Bam_Smash によるリンク Colts Fans, come tell us why you're going to win the 2016 Super Bowl, as well as why you won't.

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

This is said a lot, but on a lot of our blowouts last year, the offense was anemic. We put up 7 against the Patriots.

nfl 内の ImOnTheRadioo によるリンク Who was the player, that whenever they touched the ball got you out of your seat in excitement?

[–]Weed_O_Whirler 2ポイント3ポイント  (0子コメント)

Tony Dungy was interviewed about that last year. Interestingly, they planned all week to not ever kick off to him, but the night before the Superbowl Dungy decided that he was going to kick it right at him to show they weren't scared.

Oops

askscience 内の justcallmetom13 によるリンク Are the four laws of Thermodynamics absolute? And if not, what are some exceptions?

[–]Weed_O_Whirler 2ポイント3ポイント  (0子コメント)

The laws of thermodynamics are statistical laws only, and they only apply when you have large collections of particles.

Think if I said it was a "law" that if you flip a fair coin, 50% of the times you'd get a head and 50% of the times you get a tail. Well obviously if you only flip a coin 1 time, you won't get such results (since it will either be 100% heads or 100% tails). If you flipped a coin 10 times, you are likely to be around 5 heads and 5 tails, but really it still isn't surprising if you have more heads than tails (and it isn't even that weird if you got all 10 heads or tails). But imagine instead if you flipped the coin a thousand times (or even a million times). If it is fair, you are likely to get almost exactly 50% heads and 50% tails- and the more times you flip the coin the closer to 50% you'll be.

The laws of thermodynamics are the same. They explain what happens "on average" so they really only apply when you have enough particles so that the average thing happens. For instance, thermodynamics says that if we had a box with a divider in it, and if one of the box is at a higher pressure than the other and we open up the divider, then the pressure will equalize. But, if we have a box and there are 10 air particles on 1 side and 5 on the other, and then open the divider, it wouldn't be outside of the realm of possibility that air would actually travel from the 5 particle side to the 10- because there just isn't enough particles to guarantee that the most likely outcome happens.

askscience 内の hawkman561 によるリンク Is quantum superposition a state in which the values are indeterminate or just a state in which we are unable to make precise measurements?

[–]Weed_O_Whirler 2ポイント3ポイント  (0子コメント)

This is a common question, and it is one that goes pretty deep. Here is what we know, when it comes to superposition. First, we know that it is more than just "we do not yet know." Bell's Theorem has solidly proved that there is something going on beyond just us not knowing. And this is more than just theoretical. Bell's Theorem is the basis of Quantum Encryption being secure against eavesdroppers, so it is an important proof.

But, Bell's Theorem doesn't (as is commonly said) disprove that the states are not "known/predetermined" by "the universe." What Bell's Theorem states is that there are no local-variables which determine the state of a particle. That is, there isn't something unknown about the electron which determines its spin, but it doesn't disprove that there may be something connecting all of the electrons involved in the experiment with some "global variable." So, we don't yet know if there is something "bigger than the electron" which determines its state, but we do know that the electron itself is in an indeterminate state until a measurement takes place.

askscience 内の MapsAreCool によるリンク Why aren't there more 3rd and higher order differential equations used in physics?

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

The third derivative of position is called jerk, and it really does show up a lot in physics. The reason it doesn't appear to show up as much is because in beginner physics you don't get into the cases where there are non-constant accelerations.

Newton's second law is often times written F = ma, and then we often times will re-write it to solve for a by saying a = F/m. This is all fine and good, if the force is constant or the mass is constant. But if neither of those things is constant, you'll find that acceleration is not a function of time- and if acceleration is changing, that means it has a non-zero derivative, and thus there is jerk.

Probably the easiest system to see jerk on is a rocket ship taking off. The first booster provides a (pretty much) constant force, but acceleration is not constant at all. This is because 'm' is not constant, thus you have to use the "real" version of Newton's second law which is F = dp/dt, where d means "small change in" and p is momentum- which depends on both mass and velocity (and you can see if mass is constant, than dp/dt = ma, which is the simplified version of Newton's second law).

If you follow the link above, you will find lots of other examples where jerk happens. For instance, if two rigid bodies interact, there is no jerk. But, since bodies aren't actually rigid, when two gears mesh together, for instance, their rotation has a non-constant acceleration. Also, in manufacturing, not only do most systems have a maximum allowable acceleration, they also have a maximum allowable jerk.

AskReddit 内の mikemclovin によるリンク Reddit, what is something that is purchased often but seldom used?

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

But it's not a dollar per channel, it's $5 for the channels you want, and $0.1 for HGTV

Android 内の johnmountain によるリンク Stalking Your Friends with Facebook Messenger

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

The accuracy of a GPS measurement is not the same as the accuracy of your GPS state. There are several things you can do in order to improve your accuracy.

First, most of the error in GPS positions are in the up direction. If you assume that the person is on the ground, you have removed a lot of error. Also, since you get a report every second, and the noise is white you can quickly filter your location to much higher accuracy than a single report.

Android 内の johnmountain によるリンク Stalking Your Friends with Facebook Messenger

[–]Weed_O_Whirler 1ポイント2ポイント  (0子コメント)

This isn't quite true. The accuracy of both systems is the same (true) but the difference is that the military system broadcasts on two frequencies. Since atmospheric scattering is a function of frequency, by measuring the difference in receipt time of the two signals allows for a atmospheric correction which isn't possible with only getting a single signal.

Android 内の johnmountain によるリンク Stalking Your Friends with Facebook Messenger

[–]Weed_O_Whirler 0ポイント1ポイント  (0子コメント)

No single GPS report is super accurate, but considering that you can update your position once a second on GPS, it doesn't take very many reports until whatever filtering algorithm Android is using (probably a Kalman Filter) can narrow down your accuracy to a very accurate and precise location- especially if you aren't moving.

movies 内の AndrewTheCyborg によるリンク [Interstellar Spoilers] How the ending of Interstellar was filmed. The lack of CGI is surprising.

[–]Weed_O_Whirler 6ポイント7ポイント  (0子コメント)

That bothered me at first too, but upon a second viewing at home it appeared to me that the space station was rotating. Not very quickly, but even a small rotation would add a little centrifugal force which could cause the behavior.

movies 内の AndrewTheCyborg によるリンク [Interstellar Spoilers] How the ending of Interstellar was filmed. The lack of CGI is surprising.

[–]Weed_O_Whirler 12ポイント13ポイント  (0子コメント)

Of all the YouTube channels which are people making snarky comments about movies, CinimaSins has to be the worst one. It's a "mistake" that people were impressed with the movie. It's a "mistake" that someone cheated on George Clooney. It's a "mistake" that NASA would send up a non-astronaut.

Yes, Gravity allowed jet packs to have way too much delta-v, and yes Gravity put a lot of space objects on the same orbital plane, and yes it had the run-away space debris scenario happen orders of magnitude faster than would actually happen, but honestly, those are pretty minor compared to the way space travel is handled in most movies.

nfl 内の mrmagoo512 によるリンク Who's stats are the biggest lie?

[–]Weed_O_Whirler 5ポイント6ポイント  (0子コメント)

What I don't get is how everyone shits on QBR and loves DVOA- both are proprietary formulas made by for profit websites which refuse to release their formulas.