Thread: Rockets

Im having fun thinking about rockets.

The earth is 12000 miles in diameter. So, when the rocket is sat on earth, it goes around with the earth(rotation speed) 12000mph * PI.

Now I want to know what is the rotation speed of the rocket once it's 12000 miles above the earth.

Is it still 12000mph * PI?
Or is it 36000mph * PI?

Im fireing the rocket streight up. What feels strange is the rocket on earth is at the rotation speed of the planet, when i fly off why would the rotation speed of the rocket increase to stay directly above its start point as I get further out?

In fact the rotation speed of the rocket must decrease on return. Something to think about too.

Does anyone understand what im on about here? Not only is it accelating outwards it's accelerating around too. It's got to go around faster to keep up and stay above the start point.

Don't know how I came out with this one.

Originally Posted by griffithsuk

Im having fun thinking about rockets.

The earth is 12000 miles in diameter. So, when the rocket is sat on earth, it goes around with the earth(rotation speed) 12000mph * PI.

The diameter of the earth is not 12,000 miles, it is 12,756 kilometers. That would make the circumference 12,756 * Pi kilimeters, but it is not rotating once per hour. It is rotating once in 24 hours, so the speed of the surface of the earth at the equator is 1/24th of Pi * the diameter per hour.

Now I want to know what is the rotation speed of the rocket once it's 12000 miles above the earth.

Is it still 12000mph * PI?
Or is it 36000mph * PI?

It can be going at any speed. If it is going too slow, it will fall back to earth. If it goes at the right speed it will stay in orbit. The formula for calculating that speed can be found here.
http://en.wikipedia.org/wiki/Orbital_speed

Im fireing the rocket streight up. What feels strange is the rocket on earth is at the rotation speed of the planet, when i fly off why would the rotation speed of the rocket increase to stay directly above its start point as I get further out?

It wouldn't speed up if you fired it straight up. You have to fire the rockets to get it into orbital speed. There is a particular orbit called the geostationary orbit where the satellite stays above a point on the earth's surface. See the wikipedia article.

Thanks, ive been thinking about this one all day.

Was thinking like the universe is mathamatical and not "Natural"".

Thanks man.

Another question, if the moon is going round like 3500kph and the earth gravity is like no where near that, how does it equal out and moon go happly around?

Originally Posted by griffithsuk

Another question, if the moon is going round like 3500kph and the earth gravity is like no where near that, how does it equal out and moon go happly around?

The part of your post in bold makes no sense.

The Moon oribs because it is moving at the proper orbital speed for its distance from the Earth.

The Moon. left by itself would travel in a straight line. The Earth's gravity pulling towards the Earth, preventing this. It is this balancing act between the Moon's natural tendancy to flay away from the Earth at a tangent and the Earth's gravity that creates the orbit.

The force acting towards the center that is needed to force a moving object into a circular path is found by:



Where m is the mass of the object, v its velocity and r the radius of the circle.

The force of gravity acting between two objects is:



Where M is the mass of the second object, G the universal gravitational constant, and d the distance between the centers of the objects. If M is much larger than m (such as the case of the Earth and Moon), Then for mass m to follow a circular orbit around mass M, Fc equals Fg and:



in this case d=~r so we can write:



Solving for v gives us:



G =6.673 × 10e-11 m3 kg-1 s-2
The mass of the Earth = ~6 x 10e24 kg
and d between the Earth and Moon is ~384,000 km.

If we convert the km to meters and plug then into the equation we get an orbital velocity for the Moon of 3675979 m/s or ~3676 km/sec. (your 3500 km/sec was a bit off. This means that the Moon is moving at the speed it needs to.

However, this should not be taken to mean that the Moon must move exactly at this speed or it will fall to Earth or fly off into.

Our Moon in a cicular orbit were to lose a little speed, the following will happen:

Fg>Fc and the Moon will start to drift in towards the Earth. As it does so however, like any falling object, it will pick up speed. The radius of its orbit will also decrease.

If you look at the first equation I gave, this means that Fc will start to increase. Eventually Fc will increase to the point where it will overcome the in fall of the Moon and stop it. By this time Fg<Fc and the Moon will climb away from the Earth Again. This will happen at a point which is exactly on the other side of the Earth where the Moon started to drift inward.

As the Moon climbs outward again, r increases and v decreases and when it returns to the point where it started its infall, it will right back to where it started as far as velocity goes. It will start to fall in again and repeat the process indefinitely. It will have entered a new elliptical orbit, with an average distance a little smaller than its original circular orbit distance. (As a point of fact, our Moon is already in an elliptical orbit, getting slighty closer and further away during each orbit. )

The only way to make the Moon to "fall out of orbit" is to remove so much speed that it hits the Earth before Fc can overcome the infall. This works out to removing the vast majority of the Moon's speed.

On the converse in order to cause the Moon to "fly out of orbit", you would have to increase its velocity by some 41%

Thanks for all that.

But if things fall at the same speed. What has mass got to do with it? Iv'e always had a problem with this.

Im thinking rings of saturn, im guessing that's because of gravity.

So is it the case of small dust near the inner edge of the ring and larger dust on the outer edge?

The mass of the orbiting object doesn't have anything to do with it and the rings of Saturn aren't sorted like you suggest (at least as far as I know).

Originally Posted by griffithsuk

Thanks for all that.

But if things fall at the same speed. What has mass got to do with it? Iv'e always had a problem with this.

If you notice, the mass of the Moon drops out before you get to the last equation and all you are left with is the mass of the Earth. The mass of the Earth matters because its gravity is what causes the acceleration (read deflection of the Moon's path into a circle) that leads to the Moon's orbit.

If the Earth's mass were different, you would get a different orbital velocity for the Moon. For example, Io orbits Jupiter just a little further from Jupiter than the Moon is from Earth, but because Jupiter is some 317 times more massive than the Earth, Io has an orbital velocity of 62400 kph and a period of only 1.77 days

The caveat is that the satellite's mass needs to be much smaller than the planet's. That way you can use the center of the planet as being the center of the orbit. As the satellite becomes more massive, it becomes apparent that the center of the orbit actually is the center of mass of the two bodies, and this point gets further and further from the center of the planet as the ratio of masses decreases. (At a one to one ratio, both bodies orbit a point halfway between them)

When the mass of the moon becomes large enough or you need a higher accuracy answer, you use the formula:



to get orbital speed.

Im seeing it now, it's the mass of the larger mass object which matters.
And the orbit of the lesser mass object is because of it's distance and speed.

Say I want a larger orbit, so I set the object out more. Does it have to go slower or faster?

Im guessiing that if I increse the speed it will fly off but still be effected by the gravity producing a new larger orbit.

Is there anything else about gravity?

Hows about a infinitly large orbit? So large it NEVER returns! in an infinite ...space...

Getting good at this.

lol Infinite orbit!

The only fear of an infinite orbit is some alien needing to throw something away...

Now about gravity. A nicer way of explaining it is. The force of gravity ballances with the energy of the object, the object never ever actually falls .

can an infinite orbit exist? Can things fly off forever?

I think your end up with an infinite spiral.

Now it's getting scary because of these "Spirals in the sky"...

http://www.universetoday.com/47188/w...y-over-norway/

Originally Posted by griffithsuk

Im seeing it now, it's the mass of the larger mass object which matters.
And the orbit of the lesser mass object is because of it's distance and speed.

Say I want a larger orbit, so I set the object out more. Does it have to go slower or faster?

Slower.

Originally Posted by griffithsuk

Im guessiing that if I increse the speed it will fly off but still be effected by the gravity producing a new larger orbit.

Increasing its speed will put it into an elliptical orbit, with a higher average distance. If you wait until it is at its highest point and increase its velocity again you can put it into a higher circular orbit with a lower orbital speed. I know that it seems strange to increase an object's speed twice and end up with a lower speed, but that extra speed is used up climbing higher.

Originally Posted by Janus

Originally Posted by griffithsuk

Im seeing it now, it's the mass of the larger mass object which matters.
And the orbit of the lesser mass object is because of it's distance and speed.

Say I want a larger orbit, so I set the object out more. Does it have to go slower or faster?

Slower.

Ill take your word for it. Busy thinking.

Originally Posted by Janus

Originally Posted by griffithsuk

Im guessiing that if I increse the speed it will fly off but still be effected by the gravity producing a new larger orbit.

Increasing its speed will put it into an elliptical orbit, with a higher average distance. If you wait until it is at its highest point and increase its velocity again you can put it into a higher circular orbit with a lower orbital speed. I know that it seems strange to increase an object's speed twice and end up with a lower speed, but that extra speed is used up climbing higher.

simple but clever!

Hows about a infinitly large orbit? So large it NEVER returns! in an infinite ...space...
can an infinite orbit exist? Can things fly off forever?

I think your end up with an infinite spiral.

An "infinite orbit" would result if an object moves at or greater than "escape velocity":



Note how this differs from the orbital velocity by the "2".

An object traveling at escape velocity travels in a hyperbola and not a spiral.

Originally Posted by griffithsuk

can an infinite orbit exist? Can things fly off forever?

I think your end up with an infinite spiral.

Now it's getting scary because of these "Spirals in the sky"...

http://www.universetoday.com/47188/w...y-over-norway/

Guess I should think more before posting. Couldnt help myself though

just checked hyperbola out at http://en.wikipedia.org/wiki/Hyperbola, anyone give me a quick laymans explanation, it's all way tooo much.

Or a video showing it's travel.

Im somewhere between taking your word for it and believing in the aliens...

Im sat here looking at a pie dish...

Originally Posted by griffithsuk

just checked hyperbola out at http://en.wikipedia.org/wiki/Hyperbola, anyone give me a quick laymans explanation, it's all way tooo much.

Or a video showing it's travel.

Here's an image showing the difference between circular, eliptical and hyperbolic orbits.




Notice that in this example, the three orbits all meet at one point. The speed of an object at that point is what determines which path the object is on.

The object in the hyperbolic path will come in from some direction in space, bend around the planet and then head off in some new direction less than 180 degrees from the direction it came from.

The new direction will depend upon just how fast it is moving; the higher its speed, the less it will change direction and the "straighter" the hyperbola will be. The measure of how much the hyperbola "bends" is called its eccentricity and goes from just greater than 1 (a near 180 degree turn) to infinity( a straight line.) An eccentricty of exactly 1 leads to a 180 degree turn and results in a parabola. (which can be considered a special case of a hyperbola.)

An eccentricity of less than 1 but greater than 0 would result result in a elliptical orbit and of exactly 0 results in a circular orbit. ( which can be considered a special case of an ellipse.)

These four trajectories: circle, ellipse, parabola and hyperbola, account for all free path orbits.

Been thinking today, Ive came up with "Escape Velocity" does not exist, as what your doing is either:
1. Speeding up creating a new orbit around that object.
or
2. Speeding up creating a new orbit around that object and 1 or more other objects creating a "Multiple Object Orbit".

Also can't have my "Infiinite orbit" or you would call "Escape Velocity" though unless it's constant accelerating forever. Since it's curved it will go around and return one day.

Originally Posted by griffithsuk

Been thinking today, Ive came up with "Escape Velocity" does not exist, as what your doing is either:
1. Speeding up creating a new orbit around that object.
or
2. Speeding up creating a new orbit around that object and 1 or more other objects creating a "Multiple Object Orbit".

Also can't have my "Infiinite orbit" or you would call "Escape Velocity" though unless it's constant accelerating forever. Since it's curved it will go around and return one day.

Not all classical orbits are elliptical. With enough velovity (escape velocity or faster) orbits can be parabolic or hyperbolic. They do not "return one day".

Originally Posted by griffithsuk

Been thinking today, Ive came up with "Escape Velocity" does not exist, as what your doing is either:
1. Speeding up creating a new orbit around that object.
or
2. Speeding up creating a new orbit around that object and 1 or more other objects creating a "Multiple Object Orbit".

Also can't have my "Infiinite orbit" or you would call "Escape Velocity" though unless it's constant accelerating forever. Since it's curved it will go around and return one day.

Not all classical orbits are elliptical. With enough velovity (escape velocity or faster) orbits can be parabolic or hyperbolic. They do not "return one day".

"Return one day" should be "Go around somehow forever", or "Forever Moving".

Theory: The Big Bang could be accelerating forever outwards so never return, else it will be forever moving.

So the future looks like either forever accelerating expanding or a lot of galaxys are going to be colliding.

Since there saying that the universe is expanding now beyond the speed of light, im guessing were forever accelerating expanding and could be safe.

The universe infact could be forever accelerating expanding and the galaxys forever moving. Think that's enougth of this...

Do galaxys orbit each other? Or do we not know this one yet?

Going completely on one here, anything goes.

May be the planets have pulling gravity, they want to pull together, and the galaxys has pushing gravity, they want to push apart.

Could explain why the big bang is expanding.

Im really confused by the big bang. There saying it's an expansion not an explosion.
If it's an expanding universe then the galaxys would expand in to each other. Fact! Expanding like a baloon? baloon has only a surface. With a baloon the dots are getting bigger too. If you saying everything is expanding as in getting bigger too then so is the instrument that you used to detect this is expanding (getting bigger) too.

So it must be an explosion.
But there saying there is no center to it.

Im new to science really. Ive read that the evidance of the big bang comes from the idea of "Red Shift", could it be that the universe is not expanding and red shift comes from the colors of light traveling at different speeds, giving us a "Red Shift effect?"

Originally Posted by griffithsuk

Been thinking today, Ive came up with "Escape Velocity" does not exist, as what your doing is either:
1. Speeding up creating a new orbit around that object.
or
2. Speeding up creating a new orbit around that object and 1 or more other objects creating a "Multiple Object Orbit".

Also can't have my "Infiinite orbit" or you would call "Escape Velocity" though unless it's constant accelerating forever. Since it's curved it will go around and return one day.

Here's a better image of a hyperbolic orbit (the Hyperbola in the last image is closer to a parabola):




Notice the two straight lines that cross at the left of the planet. These are called the asymptotes. The path of the hyperbola as it extends past the image will get closer and closer to these lines with out ever quite reaching them, They will not ever curve back to form a loop.

Originally Posted by griffithsuk

Do galaxys orbit each other? Or do we not know this one yet?

Yes. They form groups and clusters which orbit around their common center of gravity. (mush like the example I mentioned earlier with two bodies of comparable size.

Originally Posted by Janus

Notice the two straight lines that cross at the left of the planet. These are called the asymptotes. The path of the hyperbola as it extends past the image will get closer and closer to these lines with out ever quite reaching them, They will not ever curve back to form a loop.

I see. so can have escape velocity and my "Infinite orbit ".

Now, what's the chances of the galaxys all having a hyperbolic orbit?

Pretty slim?

So my theory may apply.

Theory: The Big Bang could be accelerating forever outwards so never return, else it will be forever moving.

So the future looks like either forever accelerating expanding or a lot of galaxys are going to be colliding.

Since there saying that the universe is expanding now beyond the speed of light, im guessing were forever accelerating expanding and could be safe.

The universe infact could be forever accelerating expanding and the galaxys forever moving. Think that's enougth of this...

Originally Posted by griffithsuk

Now, what's the chances of the galaxys all having a hyperbolic orbit?

Pretty slim?

So my theory may apply.

Theory: The Big Bang could be accelerating forever outwards so never return, else it will be forever moving.

So the future looks like either forever accelerating expanding or a lot of galaxys are going to be colliding.

Since there saying that the universe is expanding now beyond the speed of light, im guessing were forever accelerating expanding and could be safe.

The universe infact could be forever accelerating expanding and the galaxys forever moving. Think that's enougth of this...

Your posts are uniformly nonsensical. Have you ever taken a physics class or read an actual book ?

Now would be a good time to start. Try The Feynman Lectures on Physics. Then read Prussing's book on orbital mechanics.

Originally Posted by griffithsuk

Now, what's the chances of the galaxys all having a hyperbolic orbit?

Pretty slim?

So my theory may apply.

Theory: The Big Bang could be accelerating forever outwards so never return, else it will be forever moving.

So the future looks like either forever accelerating expanding or a lot of galaxys are going to be colliding.

Since there saying that the universe is expanding now beyond the speed of light, im guessing were forever accelerating expanding and could be safe.

The universe infact could be forever accelerating expanding and the galaxys forever moving. Think that's enougth of this...

The universe does not have to be "forever accelerating" in order for it to never collapse. All that is needed is that the initial expansion was fast enough. The expansion would slow over time but never reach zero and start to collapse again.

What happens is that as the universe expands and the galaxies get further apart, the gravitational hold that they have on each other gets weaker, and thus it tendancy to slow the expansion decreases. If this decrease outpaces the slowing of the expansion, gravity can never pull the universe back together.

This is the whole idea behind "escape velocity". it is the speed that something has to be moving is order for its increasing distance from a body outpaces the weakening of the gravitational pull from that body. Graviity still constantly slows the object, but never enough to remove that last little bit of velocity.

Thanks, that is how i knew it, was just having fun with a trippy effect.