Myuncle

Ain airplane is not accelerating downward. It flies level. If it dove, then you would feel weightlessness. That what they do on the "vomit comet"

 

Moving sideways in the elevator will be completely unnoticeable to you.

ah ok, I will read more about the vomit comet, maybe it's a good way to understand the subject.

The centrifugal force never enters the analysis if you look at the mtion from an inertial frame. It doesn't exist.

 

You elevator example works...for a few seconds. But what if you took that same exact example and gave it a sideways velocity. Is that still freefall?

 

Because that's all the ISS is doing.

You mean that if you are falling inside the elevator moving sideways, the orange will stay in front of you instead of touching the floor? An aircraft can move as fast as a free falling elevator, wouldn't be the same situation?

Who is "we"?

We is me...or common mortals like me. Not everyone has your expertise swansont, relax.

And that is precisely the direction of their acceleration.

 

 

Centrifugal force is a made up force to allow us to use Newton's laws in a non-inertial frame. So lets's not use a non-inertial frame. It will only confuse matters.

 

That's the direction of the acceleration. The bodies change direction — they're moving in a circle. They have to accelerate to do this. The direction of that acceleration is toward the center of the earth. They're just moving to the side fast enough that they continually miss it, as Delta1212 has said.

 

I am talking about the centrifugal force, because it is rarely mentioned as the cause of weightlessness in the ISS astronauts case. The case of free falling is when there is only gravity acting, but in the case of a satellite, shuttle or ISS, you need rockets to move that fast, that's why I would say that they are spinning, they are not free falling. Example, if you are dropped inside an elevator, you are free falling vertically (not spinning around the earth), at about 300 km/h, and the weightless sensation, in this case, is given by the free fall, your feet don't stay on the floor, and if you try to drop an orange, it won't go to the floor, but will float in front of you. Second example, you are inside an aircraft travelling at a constant 300 km/h, just like in the free falling elevator, but I guess you won't feel any weightlessness sensation, you stay seated, and if you try to drop an orange it will fall down on the floor. Third example, you are on a roller coaster, going up fast, at the top of the track, for a moment you will feel weightless, lifted out of the seat, the orange will stay afloat. Which of these example can be associated to the astronauts weightlessness? To me only the third example, for the ISS men it will feel like and endless "top of the roller coaster hill".

 

No, because an aircraft is not falling. The wings lift it.

 

Did you look at the Newton Cannonball?

Yes, the cannonball example is useful to understand why things can spin around the earth, but it doesn't make you understand the weightlessness sensation inside a ship.

The ISS is traveling close to 20,000 m/h (32,000 km/h). If the Earth's gravity didn't pull down, it would fly in a straight line. The effect of the two, gravity and forward speed, means it falls towards the Earth, but flies forward fast enough that it cannot fall into the Earth. Thus, it is in free fall (always falling) and moving forward fast.

and that's what I don't understand, how can it be considered a freefall if it never go towards the centre of the earth?

No, I think they are all in freefall. There is nothing stopping or slowing their fall.

Our fastest aircrafts travel at about 800 km/h, which is nothing compared to the ISS (27.600 km/h). But if you could travel at that speed with an aircraft would you experience the same weightlessness?

Free fall means there is no force preventing your fall under the influence of gravity. A sideways motion doesn't do that; for example, a ball thrown horizontally will fall to the ground at the same time as one that is just dropped.

 

Newton came up with the cannonball analogy to explain orbits as an example of free fall: https://en.wikipedia.org/wiki/Newton's_cannonball

Sorry, double post, how can I delete it?

Yes, in fact to me the ISS is not freefalling, as I read often. Don't you think the word freefall has nothing to do with the orbits cases of most satellites and ISS?

When we see the astronauts clips in the ISS or the old Shuttle, we think that they are floating because there is no gravity. In factthe gravity in those orbits is almost the same as here on earth. So what keeps them floating is a never ending freefall. That's what I was reading, but I am having problem to understand it well. As I see it the ISS or the Shuttle, is not freefalling, but is just moving damn fast, so fast that it can escape the gravity pull. A freefall example to me would imply that you are falling towards the centre of the earth, and not east or west. When we are dropped from a tower, sitting on a chair, or inside an elevator, we would experience a weghtless sensation, the same as the astronauts, but this is indeed a freefall, because there is a downwards direction. In the case of the ISS, to me it's not a freefall, but the weightlessness is given by a constant balance between centrifugal force, speed of the ship, and gravity. What do you think?

Let's say that I power a solar panel indoor (underground or under roof) with artificial light, and I place a concave mirror on top of both the solar panel and the light, how efficient would this be?

I have a small room. 6m by 5m.

I want to get it carpeted so I compute its area.

Area= 6m × 5m = 30m2 ( metre square).

 

But....

 

Muliplication is also repeated addition..

so, I add 6m + 6m + 6m + 6m + 6m..... but I get the answer 30m and not 30m2.

 

My point is...

 

Irrespective of how we do multiplication the answer (including the units) should be same....

 

but by repeated addition I get the same numerical value but not the same units.

 

1. Either mathematics does not depict nature/reality properly.

Or

2. I am insane.

That's interesting. When we say 6m, we never specify 6m of what, is it 6m of tape? 6m of chalk? 6m of atoms? 6m of air? 6m of nothingness? If it's 6m of atoms, then it is not just 6m, because atoms have a thickness....

 

You would take additional conversion and friction losses, lowering total energy output.

 

Instead of simply:

 

Mechanical to Electrical

 

you end up with:

 

Mechanical to Electrical to Mechanical to Electrical

If you end up with lowering the energy output, I can't argue with that. I was hoping it would be the opposite. I imagined turbines sealed in a vacuum, in a horizontal position, to lower the friction you could even put rollerblade wheels at the end of the blades. And also they could be placed underground. How much energy is required to spin them, if you can push them by hand, why not using a bit of electricity?

Why they don't seal the turbines inside a vacuum? By watching this clip, https://www.youtube.com/watch?v=WTt_hoWtMIc, is clear that a wheel would spin much longer inside a vacuum. So, let's say that you use a normal wind turbine (or solar panels) to activate 10 other turbines sealed in a vacuum. It would be like multiplying the energy. What do you think?

Yes that's a big chamber. It would be interesting to see how humans would move inside. Unfortunately there are no footages. The only clip I saw it is a spacesuit test in 1965, the spacesuit starts leaking, the man survives, but look at the way he falls at 1:43, he's almost floating, not too much, but I've never seen someone falling like that. Clip:

Within Einsteinian one needs to use time as it is inherently written into the formalism.

 

For dynamical problems how can we get away with not using time?

As a measuring tool? We can't get away from using time and clocks. But take my answers always with a pinch of salt, I am not the expert here, I just try my luck when I write...

 

If you want to collect the one million pounds I offered, you need to be at the right place and at the right time - which I am not telling you because you don't need it.

 

Anyway, this is a strawman argument as the word "need" does not form part of the theory of relativity. If you don't want to use the word, you don't need to. (See what I did there)

Well, I don't need it, but for 1 million pounds I can use it

Strange has 'hit the nail on the head' here. You have to remember that in theoretical physics one is looking for mathematical models of nature. You look at nature and realise that space (length and position) and time (or duration) are useful ideas. So you develop a mathematical understanding of these concepts and this may well feed back into your more intuitive notions based on your observations. And this 'feedback loop' goes on. All notions in physics are inherently mathematical and 'man-made'. The point is they should be useful in describing our Universe. The question of 'reality', 'existence' etc. is not really at the forefront. Pragmatically, one usually accepts that the only 'real' things are what you can measure and thus physics is full of 'non-real' concepts.

Yes, but I agree with you on this. I just don't agree with confusing the verb need with the verb use.

How is this fundamentally different to the concept of length as a measuring tool (we assign some units to length) and a human idea?

This is a great question, and I hope to answer it well: length as a measuring tool is exatcly like time, it will never be perfect, it will always be an approximation of reality, so there is no difference between measuring space or measuring movements. Space is the only dimension, and we measure it with our human concepts of height, length and width. Many ideas exist only in our imagination and fantasy, but they don't exist in reality. Space, matter and movement, not only exist in our mind, but they exist in reality as well. However, the idea of perfect measurements, or the idea of two identical unities, don't exist in reality, but only in our minds, and thanks to this idea, we can have math and measurements, so we can write 1=1, and we can measure everything, and we can agree more or less on these measurements. Measurements will never be perfect, they will never represent perfectly reality. Whatever you use to measure length in space, you need a material tool, like a tape, or a laser. A tape can be made of steel, plastic or whatever you want, but one meter long tape, will never be exactly one meter long, it will be only an approximation of one meter. A perfect meter long object is only in our fantasy.

Whatever we calculate and measure (length, movement, speed), it's only an approximation, because matter it's not stable at all, it's constantly changing and moving, because its atoms are always changing and moving, even if we don't see it. Measuring doesn't give life to anything.

 

Does that mean you don't need a where, either? You just use the place as a tool. You don't need a where, but it would be better to say that you use a where, to keep track and calculate the movements.

 

To prove that you are wrong: I will meet you at Hyde Park Corner in London and give you a million pounds. (I am pretty sure my money is safe.)

 

Nonsense. Otherwise you would be knocked down by traffic even when crossing a now empty road.

Ok, that doesn't change the fact that for two objects to collide with each other, you need space, but you don't need time. You need space, but you use the concepts of height, length and width to measure them.

Two massive objects cannot occupy the same space at the same time, but they can at different times, therefore, time is not just a tool.

Two objetcs cannot occupy the same space because they collide with each other, no matter when. Again, space is needed, time is not needed. I never said that time is only a tool, I say that time is 1) a sequence of movements, 2) a meausuring tool, 3) a human idea.

I am glad you have managed to come to some sort of resolution of your doubts.

 

 

On this specific point, I know there are people (including very serious philosophers of science) who think that time might not be "real" in some deeper sense (whatever that means). But you cannot get away from the fact that, even in everyday life, time is a dimension. If you want to organize an event (e.g. to meet someone) you need to specify 4 independent coordinates: 3 spatial ("where") and 1 temporal ("when"). This is nothing to do with relativity. Even if you think time is just movement (*) or a psychological artefact, you can't get away from that fact about the world we live in.

 

(*) This is trivially wrong, but that has been thrashed out in thousands of posts in dozens of other threads so lets not discuss it here.

Thanks Strange. If you want to organize an event you don't need time, you just use time as tool. You don't need a when, but it would be better to say that you use a when, just to keep track and calculate the movements. There are many sequences that we can calculate and measure, time is just one of this sequence. But I agree that we can discuss this in other threads .

Questionist, it looks like you are frustrated like me when I started this thread 5 years ago. After 5 years, I am less frustrated, I mean, I still think that the Relativity is not well written and not well explained, and that there are things that don't make any sense (time is not a dimension, it's just a sequence of movements). But at least I am happy to have understood (or at least I am convinced to have understood...) that there is substance in the theory. This substance can be explained in a few words: gravity bends the light (see eclipse 1919, in the photos the stars behind the sun changed position), and I understood that a combination of gravity and speed, alter the state of clocks, bodies, or any subatomic particle. Lack of gravity accelerates the clocks, and speed (of a travelling satellite etc) decelerates the clocks. This is why clocks on a satellite give a different reading than clocks on the ground. And this is why an atomic clock upstairs is slightly ahead of an atomic clock downstairs. After 5 years I learnt that our GPS still relies on this theory. So, big deal?...you might think? Yes, it's a big deal, and if Einstein was the first one to discover it and calculate it, he deserves all the credit. Hope this helps you Questionist, you and all the common mortals like me, who thought Relativity was just rubbish.

I was watching this clip for the first time (Brian Cox visits the world's biggest vacuum chamber), https://www.youtube.com/watch?v=E43-CfukEgs. I was wondering what would happen if you go inside the chamber with a space suit and try to jump: would you jump higher because of less air resistance?

 

Then perhaps you could explain how our most successful theories, which treat time as a dimension not as a "sequence", are able to work.

The theories work even if you use the word dimension, but don't you think it's a misleading word? A sequence is just a sequence, love is just a sentiment, but you would never associate the word love with dimension, unless you use it in poetry and say "love is our most important dimension".

motion and time are not the same. you need time to describe motion, for example the speed of an object is given in distance/time.

 

time =/= distance/time

 

the rest of your post uses words in an unconventional sense. i don't know what you mean when you say time describes universal motion.

Yes, we need time as a sequence to describe motion, but not as a dimension. The sequence of all the motions in the universe is what we call time, and we measure it with some constant motion (like our clocks). I think it's important to use the right word. If we replace the word dimension with the word sequence, we resolve every debate regarding time. Can we reverse all these motions? No, if we could rewind or forward every single motion in the universe, we could time travel.

Again, just to to clarify: does time exist? Of course yes, but as a sequence of all motions, not as a dimension.

• And distance between stuff changing is a consequence of time - for without time there is no concept of distance between stuff changing.

And how can you prove that distance between stuff changing is a consequence of time? You can't prove that. In fact we are moving, running everyday, and the distances are changing without any need of time.

Robin, I said that change and time are different and that you can have one without the other (both ways).

 

Time and change/motion are the same thing. The only difference is that when we use the word "change/motion" we refer to the change/motion of a single thing, and when we use the word "time", we refer to the change/motion of everything in the universe.

You CAN have one without the other? Well, you CAN have Santa Claus as well, the problem is to prove it.

I am a supporter of evolution, and I often see creationists claiming that evolution is not scientific because it fails to meet the requirements of the scientific method.

 

They claim that any scientific idea must be testable, observable, repeatable and falsifiable, and this is true.

I know that direct observation is not always possible in science, but I want to know, how do scientists test evolution (changes that create new species)? How is it observable?

 

I am not referring to natural selection, like bacteria resistance or changes within the same species, I am referring to those changes above a species level, which create new ones. How do scientists test and confirm speciation? How is speciation a repeatable idea?

 

I am not doubting evolution, it's just because these claims have made me feel consufed.

 

Creationists often claim that there are no transitional fossils in the fossil record. Is it true? I would like tests that confirm speciation (creation of new species) by geographic isolation.

 

I thank for replies!

It's difficult to convince a creationist, the first obstacle for them, is to accept the existence of genetic mutations. Try to show them these google images, and see their objections, these are animals, not bacteria:

https://www.google.co.uk/search?q=new+animal+species+discovered&biw=800&bih=471&source=lnms&tbm=isch&sa=X&ei=G6BOVM_HBbGv7AaO4YHYAg&ved=0CAkQ_AUoAg

Try to ask them, how long will these new species survive? Centuries? Millions of years? Are they going to lead to new species, are they just going to be transitional? You need millions of years to answer these questions. And if they are going to be transitional, which fossil trace are they going to leave? You have to be very lucky to find bones of transitional species, just like you have to be extremely lucky to find a dinosaur, mammoth, or hominid skeleton. But you just need a single dinosaur skeleton to prove the existence of many dinosaurs. Bones decay, otherwise, we would walk on a huge pile of bones left from the animals that lived in the past.

For a creationist is difficult to understand this, but it's not impossible, you can try. Same thing is for continental drift, 50 years ago it was an hypothesis rejected even by most geologists. Today is universally accepted as true. Our planet is cooling down, any proof? Yes, volcanoes. Try to ask a creationist how long is going to take for a volcano to become inactive, and see how it goes...

I haven't read the whole thread, but it's fascinating to see people so fascinated by time. I give my opinion. Time is the various sequences of movement, is not a poetic dimension, but even Einstein I suppose was aware of this, even when he was talking about time or spacetime.