41

Physics, Astronomy & Cosmology / Re: How to calculate the orbital speed of the ISS around the centre of the earth?

« on: 03/05/2021 18:20:12 »

If you want to work it out via orbital mechanics, the equation is v^2 = u(2/r-1/a), also known as the vis-viva equation.
u is the gravitational parameter or GM (with M being the mass of the planet).  For the Earth, it is 3.987e14 m^3/s^2
r is the present distance of the orbiting object from the  center of the Earth. 
a is the semi-major axis of the orbit, or average orbital distance from the center of the Earth (this allows you to account for elliptical, non-circular orbits.)
For a circular orbit, r=a, and the equation reduces to V^2 = u/r
With the numbers given in the OP, and assuming a circular orbit, you get 7.652 km/sec.
However, the actual radius of the Earth (at the Equator) is 6378 km.  making this slight correction gives 7.664 km/sec, which when rounded up to two significant digits, gives 7.7 km/sec
The Wiki article on the ISS lists its orbital speed as 7.66 km/sec, which either of the two above answers would round out to.

Also, using the smaller, more accurate radius of 6378 km, and using chiralSPO's method, we get 7.64 km/sec, much closer to to the given value.

42

Physics, Astronomy & Cosmology / Re: Why does acceleration become so difficult at high velocities?

« on: 29/04/2021 14:49:51 »

What is it that it that resists acceleration at higher velocities. If in relativity you are only aware within a frame of reference, why is high velocity acceleration so difficult?

In Relativity, nothing, as far as the accelerating frame is concerned, inhibits further acceleration. So, if you were accelerating at 1g.   You would measure your velocity as changing by 9.8 m/s every sec.
But, this doesn't mean that if you, for example, started at rest with respect to the Earth, that after doing this for 1 year by your clock, you would measure yourself as moving at a bit over 1c relative to the Earth (it ends up being closer to 0.79c)

This is a result of how velocities add up in Relativity.
Imagine you in this accelerating ship, and after accelerating to 0.1c relative to Earth, you drop off a space buoy ( this buoy maintains the same velocity relative to the Earth you had when you released it). You continue to accelerate until you are moving 0.1 c relative to this buoy. This will take the same amount of time by your clock as it did to get to 0.1c relative to the Earth.
If you now measure your speed relative to the Earth, you will find that is not 0.1c+0.1c = 0.2c,  but (0.1c+0.1c)(1+0.1c(0.1c)/c^2) = ~1.98c
If you now drop off another buoy, accelerate until you are moving at 0.1c relative to it, you will measure your velocity as being ~0.292c relative to the Earth.
Keep doing this and the consecutive velocities relative to earth will be:  0.381c, 0.463c, 0.538c, 0.605c, 0.665c...
Each time, the relative velocity to Earth increases by a smaller amount, even though, you noticed nothing changing about your acceleration.
It is not that "something resists your acceleration", it is that the very nature of time and space determine your measurements.

Relativity is a model for time and space and their measurement, It has nothing to do with outside influences inhibiting acceleration.

43

Physics, Astronomy & Cosmology / Re: Why do we not build a light speed ship?

« on: 27/04/2021 17:36:24 »

The KE needed to get 1 kg up to just 90% of c is ~ 1e17 joules

But, that it only part of the problem. Since this would have to be achieved via rocket engine, you need to use the Rocket Equation ( as per Kryptid's post),  and it gets even more difficult.

It isn't enough to work out just how much energy it would take to get the payload up to velocity, but since you are carrying your own fuel, during the earlier parts of the acceleration you have to accelerate the fuel you are going to need for later parts of the acceleration.
As you begin to get nearer to c, relativistic effects become more prevalent and you have to account for them as well.
The Rocket equation that takes this into account gives:
M0/M1 = e^(atanh(v/c) c/Ve)
M0 is the starting  mass of the ship (ship + fuel),
M1 is the final mass ( ship)
v is the final velocity
Ve is the exhaust velocity of your rocket.

An important factor to take away is that the amount of fuel needed to reach a given velocity is heavily dependent on the exhaust velocity of the rocket.

The best exhaust velocity for any engine presently under development is for the DS4G*, with a Ve of 193,000 m/s (compared to the ~4500 m/s for a contemporary chemical rocket)

Using that in the equation above, it works out that for your ship to reach even 4% of c, it would have to use roughly 1/2 the mass of Jupiter in fuel per kilogram of ship mass. To push that up to 5% of c would increase that to 3000 times the mass of the Sun. ( For practical purposes, since v is such a small fraction of c, you don't actually need to use the Relativistic form of the Rocket equation to get a reasonably accurate answer in these examples.)

But let's say that we could push the exhaust speed up to 10% of c. Then it would take over 90 million kg of fuel per kg of ship to reach just 95% of c and 3 times as much to get up to 96% of c. ( and if you want to slow back down afterwards, this jumps to 8.1 x 10 ^15 kg and 7.29 x 10^16 kg respectively.  )

As far as times go:
If you could maintain a 1g acceleration ( as measured in the ship), then:

in 1 yrs ship time and 1.19 yrs Earth time you would achieve 0.77c and have traveled 0.56 ly.
in 2 yrs ship time and 3.75 yrs Earth time you would achieve 0.97c and have traveled 2.90 ly (not yet even as far as the nearest star system to our own)
in 3 yrs ship time, and 11.8 yrs Earth time you would achieve 0.9969c and have traveled 9.7 light years ( There are 7 star systems within this distance; most of the stars are red dwarfs .)

If you want to stop at your destination, you would need to accelerate for half the trip and decelerate for the second half.
Doing so would get you to, for example, Barnard's star ( a red dwarf with ~14% the mass of the Sun) in ~4.25
 yrs ship time and and bit over 9 years Earth time.



* and while the VASIMR has a high exhaust velocity, it doesn't produce a high thrust, and wouldn't be able to achieve an acceleration of 1g.    This, unfortunately, is a common feature with rockets,  You tend to trade high exhaust velocity (thus efficiency) for  thrust.   ION engines for example are very efficient, but have very low thrust, thus low acceleration.  The type of energy sources needed to get around this just aren't achievable/practical at this time.

44

Physics, Astronomy & Cosmology / Re: Did Cassini travel through time?

« on: 21/04/2021 16:59:54 »

If an object's velocity effects it's perceived time, did Cassini move into the future by traveling upwards of 44/km for 13 years? Does time dilation work that way under the speed of light? Was there a clock on Cassini's computer? If so did they have to recalibrate as it traveled?

At that speed, over 13 years, time dilation would only amount to a difference of roughly 4.4 seconds.
That just means that over the trip, Cassini's clock would have measured that much less time than the Earth clock.*

And there really wasn't any reason for Cassini to keep track of its own time over the trip.  It would collect data, and send it back to Earth.  It made no difference that by  Cassini's clock it collected that data at 12:00:04, and according to Earth it was collected at 12:00:00
This is different than, for example, the GPS system, which does require a tight synchronization between satellites and ground in order to maintain accurate results.


*In realitiy it is much more complicated than that. The Earth itself orbits the Sun at some 30 km/sec. Cassini didn't have a constant speed relative to the Sun over its whole trajectory. You also have to account for gravitational time dilation due to differing distance from the Sun...  But even after all this is taken into account, the total time difference accumulation would be too small to matter.

45

Physics, Astronomy & Cosmology / Re: How do you measure the distance to a star or galaxy?

« on: 01/04/2021 15:30:35 »

There are a number of methods used, depending the distance range being measured:
https://en.wikipedia.org/wiki/Cosmic_distance_ladder

46

General Science / Re: What are your opinions of Yuri Gagarin, first cosmonaut?

« on: 24/03/2021 22:31:50 »

If the US Apollo moon-landings were faked

They weren't.

If they weren't, why aren't they happening anymore?   The manned Apollo landings took place in the 1960/70's.  Half a century has passed since then.

And in all that time, not a single manned spacecraft has gone there again. I mean, doesn't that look a bit suspicious.

The program was cut when the government decided to switch resources to developing the Shuttle (this is the same reason they cut funding of development of NERVA style nuclear rockets.)  They felt having  a "space truck" was more useful.
Also, the Apollo missions had already done what they were meant to do.  Further trips to the Moon of that type wouldn't have accomplished much more, and a more permanent presence on the Moon ( the next logical step) was considered too spendy for Congress' pocket book.
Now if the Russians hadn't given up on their Lunar aspirations, and had tried to establish a presence on the Moon, things would have been different. 

47

Physics, Astronomy & Cosmology / Re: Where did all the suns hydrogen come from?

« on: 23/03/2021 18:43:58 »

Our sun is a second or third generation star, a massive star that exploded many billions of years ago created the elements that make up our solar system.
It must have been many times the size of our sun in order to create the heavy elements such as Gold, Lead, Uranium.
According to the theories it went supernova after it had exhausted its supplies of hydrogen and helium resulting in an implosion that created the heavy elements then exploded outwards and the remants of that explosion created our solar system.
But if all the hydrogen was consumed then where did the hydrogen that makes up our sun come from?

Only a fraction of the heavy elements produced by that earlier star.  Those elements were spread out into the galaxy to mix with the hydrogen already in the dust and gas cloud from which the Solar system.  The shock wave which delivered those heavy elements, also precipitated the collapse of that cloud into the solar system.
In other words, while the Supernova seeded the solar system with heavier elements, it didn't provide all the material used to form it..

48

Physics, Astronomy & Cosmology / Re: Are photons relative?

« on: 22/03/2021 15:49:58 »

If objects travel with velocities relative to each other then what is light traveling relative to? The photon is a particle and it is traveling. Relative to ...

Relative to the frame it is being measured from.  Photons don't have a valid inertial frame, so it meaningless to talk about what they measure themselves moving relative to.

49

Physics, Astronomy & Cosmology / Re: Is there a better way to power a spacecraft than a nuclear source?

« on: 19/03/2021 15:20:57 »

Surely, nuclear fusion would be the ideal method of propulsion (especially if it could scoop up fuel and reaction mass from the solar wind).
- But we can't get controlled fusion to work on Earth
- Let alone a motor small enough and light enough to fit on a spacecraft.
https://en.wikipedia.org/wiki/Fusion_rocket

A few are working with nuclear power generation and ionized gas propulsion but they are very rare

Dawn spacecraft to the asteroid belt uses ion propulsion, but it gets the power from the Sun (there is still a fair amount of Sunlight at the asteroid belt).
- Spacecraft powered by nuclear energy tend to use thermo-electric generators (TEG), which are not very efficient (but they have no moving parts, which is a bonus in space)
- TEG can't generate the 10 kiloWatts or so required for an ion drive
- For those power levels, you would need a "critical" nuclear reactor, which is a lot more complex.

The Perseverance rover on Mars has a large TEG, which generates just 0.1 kiloWatts
https://en.wikipedia.org/wiki/Perseverance_(rover)#Design

I found an artcle about a month ago that tells about ``hybrid`` rocket.  This kind of propulsion is not so advanced and used for small space journeys 

What kind of hybrid rocket are you talking about?  There are hybrid chemical rockets which combine solid fuel with liquid oxygen( Solid fuels typically contain their own oxidizer). Nuclear-electric hybrids which use a nuclear reaction to power something like an ION engine and on the more hypothetical side, Fusion-fission hydbrids, and anitmatter-Fusion hybrids. 

As far as nuclear rockets go, this gives a pretty good overview of proposed types:
http://www.projectrho.com/public_html/rocket/enginelist2.php

50

Physics, Astronomy & Cosmology / Re: Is there a better way to power a spacecraft than a nuclear source?

« on: 19/03/2021 14:59:41 »

Surely, nuclear fusion would be the ideal method of propulsion (especially if it could scoop up fuel and reaction mass from the solar wind).
- But we can't get controlled fusion to work on Earth
- Let alone a motor small enough and light enough to fit on a spacecraft.
https://en.wikipedia.org/wiki/Fusion_rocket

A few are working with nuclear power generation and ionized gas propulsion but they are very rare

Dawn spacecraft to the asteroid belt uses ion propulsion, but it gets the power from the Sun (there is still a fair amount of Sunlight at the asteroid belt).
- Spacecraft powered by nuclear energy tend to use thermo-electric generators (TEG), which are not very efficient (but they have no moving parts, which is a bonus in space)
- TEG can't generate the 10 kiloWatts or so required for an ion drive
- For those power levels, you would need a "critical" nuclear reactor, which is a lot more complex.

The Perseverance rover on Mars has a large TEG, which generates just 0.1 kiloWatts
https://en.wikipedia.org/wiki/Perseverance_(rover)#Design

As we can see nuclear power is the most useful for long space joyrneys. As far as I know Perseverance used such propulsion system to get to the Mars.

Perseverance was launched to Mars by an Atlas V-541 rocket.  This is a two-stage rocket with boosters attached to the first stage.
The first stage main engine burns kerosene and liquid oxygen and the boosters burn solid fuel.
The second stage (Centaur) burns hydrogen and liquid oxygen. All of these are chemical rockets.

51

Physics, Astronomy & Cosmology / Re: What makes it go Faster & Slower?

« on: 08/03/2021 21:35:34 »

🙄

YippeYzzz!
👻

@Janus
Hi!
🙂

Yes indeed, it is a good approach as it makes the concept graspable & much easier to understand... especially when one is not well versed with technical jargons.
👍
(which are a must nowadays to understand the complex nature of the universe)

Besides, as the far as the concept of Gravity is concerned, hypothetical theories are well defined but Space Time Curvature is Something which still isn't carved or set in stone.
✌️
Most do Not still get how Gravity works, hence just a lil bit of freedom should be allowed for a layman to absord it in.


JANUS... I've observed you making certain remarkably extraordinary calculations at times in certain OPs.
👌
If one wishes to evolve or grow into the field of Mathematics, what would you suggest should one work on or study after the basic +-*/ functions are learned.
Like what comes next?
🤔

Simple algebra.   

& Another rather absurd question if i may...i have Rarely ever seen You create an OP or ask questions...may i know why is that so?
(Do u really know everything bout everything)
😊

Far from it.  It is just that when I have a question, I tend to try and look things up for myself.  I often find that, while looking for the answer to my question, I end up learning other things as well or at the very least find out where I can find the answers to other questions. 

Anyways as always...Appreciate your inputs, they are illuminating!
👍

P.S. - Thank You!
🙏

52

Physics, Astronomy & Cosmology / Re: What makes it go Faster & Slower?

« on: 08/03/2021 00:30:29 »

Why should an object slow down, when it gets further away from a central "attracting" body.

Shouldn't the object go increasingly faster, as it escapes from the "attraction" of the central body?

No, because it has to give up kinetic energy in exchange for the gravitational potential energy it gains climbing away from the central body.  This decrease of KE is exhibited by a loss of speed.

53

General Science / Re: Why can I see the moon during the day?

« on: 07/03/2021 15:31:39 »

P.S.  When it is at its brightest, and you know where to look,  you can even see Venus during the daytime.

54

Physiology & Medicine / Re: Why is pain different for different parts of the body?

« on: 06/03/2021 16:08:43 »

Why would you not think that where you injure yourself shouldn't impact the pain you feel?
Pain is part of the body's way of protecting itself.  It's its way of saying "hey, you are damaging yourself, stop it!"
Some parts of your body, by necessity, it would want to warn you about damage more than others.  Feet, because they are so important to your locomotion, for example.  With smacking your elbow, it is a matter of hitting the  nerve bundle that carries sensory information from the hands.

55

General Science / Re: How are rainbows made?

« on: 04/03/2021 01:12:29 »

As a kid, it was my favourite time to watch a rainbow come up and wait until it disappears.
But I never understood how they were made.
And if there is really a treasure at the bottom...

Rainbows are caused by the fact that light has it's path bent when passing through water droplets. Sunlight is a mixture of light in all the visible colors.  These different colors of light bend by slightly different amounts when passing through the droplet.  This breaks the Sunlight up into it constituent color.  This causes you to see all these colors separately as a rainbow.
Seeing a rainbow relies on the angle of where the sunlight in coming from and the angle between your eye and the droplets forming.  Because of this rainbows always appear to keep a constant distance from you.  If you walk towards it it retreats from you.
So, you can never reach either end of a rainbow to get to the promised treasure. The phrase, " The pot of Gold at the end of the rainbow" is more meant to express the idea of an unobtainable goal or reward. Something you can never get.

56

Geology, Palaeontology & Archaeology / Re: Is the earth really flat?

« on: 04/03/2021 00:55:32 »

Hey guys,
it's an old topic and a am really not a flatearther.
But it's indeed a good question how the water stays on the round earth.
It should just fload down from the planet, right?

Because "down" is defined by gravity and the Earth's gravity acts toward the center of the Earth.  Water tends to, if it can, flow "down hill", and "downhill" means closer to the Earth's center. (Of course, it can't flow all the way to the center because the bulk of the Earth gets in the way.)

57

Geology, Palaeontology & Archaeology / Re: Why does the planet's water remain on Earth?

« on: 04/03/2021 00:41:40 »

Gravity.

Which is also why the atmosphere clings to the Earth, and why we aren't flung into space by its rotation.

58

General Science / Re: How far away is the sun?

« on: 03/03/2021 21:12:29 »

Put in other terms:
You could fit about 30 Earth's between the Earth and the Moon, and the Sun is 400 times further away than that.
At this distance, the Earth only intercepts 0.000000045% of the light and heat put out by the Sun, and even that small fraction is 173,000,000,000,000,000, watts or 100,000 times the total world energy usage.

That being said, the surface of the Sun is  just ~5,500̊ C or a bit shy of 10,000̊ F

59

Physics, Astronomy & Cosmology / Re: Why is light bent whent it passes a massive body in the space?

« on: 15/02/2021 06:10:56 »

I don't think the mathematical model was "lucky" or "fudged" - it just followed the evidence
- The part where Einstein did find out he had fudged the result was in picking a value for the cosmological constant to fit in with the common view at the time (stretching all the way back to Newton, and probably to Greek philosophers) that the universe as a whole (outside the Earth & Planets) was static and unchanging.

A couple of points.  Einstein tended to approach things a bit differently that other theorists.  While they would start from the experimental evidence and work back to a theory to explain it, he tended to start from basic concepts and assumptions, follow them to their logical conclusion and see if it matched the evidence.  Pretty much the opposite of "fudging".

The other thing to consider with his cosmological constant was that, at the time, he developed GR, it still had not been established that the universe extended past the Milky way.  So not only did he not know of  the expansion, but he was considering a much smaller "universe".

60

The Environment / Re: What would happen if all of humanity vanishes in one second?

« on: 11/02/2021 16:52:32 »

What would happen if all of humanity vanishes in one second? Like just vanish and leaf all of our buildings and so on, but not our decaying bodies.  Would that stop the  climate change in an instant, or are we too far along by now? Would the rainforest slowly grow back, or is it lost forever? Can almost extinct species come back and repopulate areas where humans used to live instantly?

Look up a book called: "The World Without Us"  by Alan Weisman
It covers this exact subject.  It starts from the time at which we disappear, and goes and extrapolates forward from there over time.