A discussion on TFES forum

[teo123]

Well, the rockets do accelerate, right? And according to the Newton's first law, that's only possible if the force acting on them is external. The reaction mass, as your diagrams show, only applies the force on the rocket while it's inside the rocket, so the force acting on the rocket isn't external. So, what is going on?

[brimstoneSalad]

Well, the rockets do accelerate, right?

The rocket body accelerates in one direction, and the reaction mass accelerates in the opposite direction.

^
rocket body, +1

reaction mass, -1
v

The sum of the two (momentum) is zero, if you take into account the vectors. In space, there is no net change in momentum of the WHOLE.

Like if you were in space, and you threw a cannon ball. The ball would go one way, and you'd go the opposite way.

That's why a space ship can't go anywhere without reaction mass, and why our range is so limited (delta V is limited, which functionally limits range).
You can put a nuclear power plant on a spaceship and have all of the energy you could practically want, but if you don't have any reaction mass to push on, you'll just be dead in space.

The reaction mass, as your diagrams show, only applies the force on the rocket while it's inside the rocket, so the force acting on the rocket isn't external. So, what is going on?

"External" doesn't mean inside vs. outside. It means external to the system. The system is the WHOLE, the rocket and the reaction mass together, which doesn't change its momentum when you add it up. If you want a real change in momentum to the system (where the numbers don't cancel out anymore), then you need a force from outside of that confined system to act upon it.

[teo123]

Well, thanks. I really thought that I understand the Newton's laws of motion, but my misunderstanding of them (what is an external force) stopped me from understanding the reaction mass.
You know what bothers me the most now? There are people who claim to be computer scientists, and therefore probably understand science, but believe that the Earth is flat.
https://www.theflatearthsociety.org/forum/index.php?topic=66603.0
John Davis is a Christian, and Conker is an atheist, both of them are Flat Earthers (and meat-eaters). You probably can't reach them, right? Isn't it very sad?
So, can you show me where is the error in my math showing that the vertical sunrays should appear parallel?
http://philosophicalvegan.com/viewtopic.php?f=7&t=1829&start=140#p21250

[brimstoneSalad]

Well, thanks. I really thought that I understand the Newton's laws of motion, but my misunderstanding of them (what is an external force) stopped me from understanding the reaction mass.

That's how it goes. Fundamental misunderstandings of the basic laws of science and logic are pretty much behind all of these beliefs, including religion. The trouble is these people can't accept that they do have misunderstandings of these laws.

You know what bothers me the most now? There are people who claim to be computer scientists, and therefore probably understand science, but believe that the Earth is flat.

Don't worry, they don't understand science. Not even a little bit, if they are flat Earthers. "Computer science" isn't usually a science in the strict sense, it's more of a math or logic field: the two are the the most conservative fields in stem (loads of theists). Computers operate on their own rules. Compare to physics, biology, chemistry (almost no theists in those fields, the fewest probably in physics and biology).

John Davis is a Christian, and Conker is an atheist, both of them are Flat Earthers (and meat-eaters). You probably can't reach them, right? Isn't it very sad?

If I took the time I probably could, but it likely would not be worth the effort.

So, can you show me where is the error in my math showing that the vertical sunrays should appear parallel?
http://philosophicalvegan.com/viewtopic ... 140#p21250

Without perspective, they should appear parallel. But with perspective, parallel lines appear to converge. They appear exactly how they would be expected to appear.

If you were in the sky looking down on the Earth, the sun rays (if you could see them) would converge toward the ground.
On the ground looking up at the sky, they converge toward the clouds.

Convergence of parallel lines doesn't care which direction you're looking: that wouldn't make any sense. Up, down, over. All parallel lines converge with growing distance.
Go to a city some time and look up at a tall building (or find photos of this). Or look down from the top of one.

[teo123]

Hey, I tried to deal with the perspective in my math.

[brimstoneSalad]

Hey, I tried to deal with the perspective in my math.

You can't do that unless you know the exact position, distance, and angle of the camera, as well as the kind of lens... and then it would be the kind of math that would melt anybody's brain.

You have to use computer modeling for this kind of stuff. Download a 3d program if you're interested in understanding how these things work.

[teo123]

Why would you need to know all of that? You only need to know the position of the eye (camera), projection plane (apparently called "image plane" in English), and have a description of an object such that you can calculate the coordinates of all of its points (like a linear function for a straight line).
https://en.wikipedia.org/wiki/Perspective_(graphical)#Overview
That's the same explanation the computer science books give for the 3D graphics. I have never bothered to make some program or a game that uses 3D graphics, but I think I would know that. But, yeah, everyone has some preconceived notions about how optics works, so I might have misunderstood that explanation because of them. I wanted to raise a question: when you told me not to believe the ad-hoc explanations, how did you think I could know whether something is an ad-hoc explanation? Seriously, what difference can I percieve between the explanation that the sunrays appear to converge because of the perspective and the explanation that ships disappear bottom first because of, well, that nonsense with visual angles the Flat Earth Theory is telling? Both of them are made to explain away some apparent contradiction, they sound scientific, they are easy to understand, and make sense all until I try to analize them (draw diagrams and do some math).

[brimstoneSalad]

Why would you need to know all of that? You only need to know the position of the eye (camera), projection plane (apparently called "image plane" in English), and have a description of an object such that you can calculate the coordinates of all of its points (like a linear function for a straight line).
https://en.wikipedia.org/wiki/Perspecti ... )#Overview

That's the same information.
Projection plane includes the kind of lens, and the angle. Position of the eye is given by the distance and angle.

You do not have that information for any of the photos, but convergence of parallel lines is what you'd expect for any photo taken from the ground of the sky, because the sky will almost always be farther away than the ground.

If you were on a very high mountain, they might appear to bulge a little in the middle, converging both in the sky and on the ground.

Both of them are made to explain away some apparent contradiction,

There's no apparent contradiction if you already know parallel lines converge with distance. You would never expect for parallel lines to look parallel on such a scale, so there should really be no confusion (just as you expect a road to visually shrink in the distance).

It's just an explanation of your misunderstanding. If roads (with parallel lines) didn't also appear to converge at a distance, then we would have a contradiction since perspective wouldn't explain this, and some ad hoc explanation like, "well perspective only works for light, not for physical objects" would be, in fact, ad hoc.
Ad hoc means made for -- it has to happen after the error is revealed to explain it away, not be a pre-existing fact in the theory.
If the theory already predicted this observation before the "contradiction" was pointed out, and it was only a "contradiction" because you didn't understand the theory properly, explaining what the theory always was before you misunderstood it is not an act of creating an ad hoc explanation.

they sound scientific, they are easy to understand, and make sense all until I try to analize them (draw diagrams and do some math).

If you have a misunderstanding of the science, that doesn't make the science ad hoc. It existed long before you misunderstood it. You don't have to do the math, you just have to understand the science. If you reject the science once you understand it, then you can do the math to prove it's true.

[teo123]

If you were on a very high mountain, they might appear to bulge a little in the middle, converging both in the sky and on the ground.

I am so confused. Are you saying that the perspective itself can make the straight lines appear curved?

[brimstoneSalad]

I am so confused. Are you saying that the perspective itself can make the straight lines appear curved?

Of course. You're making some false assumptions about perspective here, but I don't know what they are.

You need to spend some time in reality examining perspective. Bring your eye really close to things to get exaggerated perspective.
You'll notice it works in any direction (look up along the length of a pencil by lying on the floor and putting the eraser between your eyes, pencil standing up). Close one eye and examine the pencil. You can see the pencil apparently getting smaller with distance. Same thing when you sit up and hold it on your nose, the pencil horizontal and pointing away from you.
Perspective doesn't 'care' what direction you're looking, it 'cares' about distance.

To see the bowing effect, you need something longer than a pencil. Try to find a long pole or something. Hold it horizontally close to your face. Look left, and look right: it's smaller in both directions. The middle is apparently bulging.