Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: fscott on January 29, 2001, 06:39:00 PM
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Too much reading, too many differing opinions. Allow me to simplify my question.
Here are the given's.
1- The moon is falling to the earth at 750 mph (according to Hitech), ok let us all just ASSUME that is correct.
2- Forward motion has NO effect on rate of descent. Therefore, the moon's forward motion, which is greater than it's descent, does NOT prevent the moon from falling faster.
3- An object in a vacuum continues to accelerate at the rate of gravity. For the moon's sake let's just ASSUME that the gravitational pull on the moon is 750mph.
Question:
Relating to given #3, what force keeps the moon from accelerating faster and faster beyond it's constant 750mph and crashing into the earth? It would sem to me that since it is in a vacuum, it should continue to accelerate toward the earth and eventually hit the earth. However, it is apparently falling at a constant rate. Explain it.
fscott
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Thats easy,
The moon may well be coming towards the earth at 750mph which is not an acceleration measurement anyway (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
The reason its not catching up is simple its cos the earths falling away at the same rate into the Sun !!
AAAAGH we're all gonna DIIIIEEEEEEEE !!!!!
JUMP SHIP !!!!
Ok its late and i was bored so shoot me ! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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fscott: assuming 28 day moon orbit, key is to realize in 14 days the moon is falling the completly oposit direction that it was on day 1.
If a plane could create lift with out drag pulling back on the stick would create a force conceptualy the same as gravity. This force points toward the center of a cirle it would fly. Even thow the plane is undergoing an acceleration i.e. (Pilot G Load) the plane is neather speeding up or slowing down, The exact same thing happens in the moons orbit.
HiTech
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Fscott, the answer to your question is actually pretty simple IMO. The moon is being accelerated toward the earth via centripetal forces. (Eqn: F=mass((v^2)/r) r-> radius from earth's center to moon's center. v-> velocity) It is this force, which is the gravitational, that "hold" the moon in place. I think the moon is actually moving away from the Earth. This would indicate that it is slowing down.
The moon is not unlike a bucket of water that you swing around your head. The water stays in the bucket due to centripetal forces opposing gravity.
Your question is why doesn't the moon continue to acclerate because it is in a vacuum. The reason is that all the centripetal force acts toward the center of the earth. Now consider that the moon has another component of velocity in the "horizontal" direction. In effect it keeps missing the Earth. The same thing happens to satelites placed into orbit. Once they reach a certain speed the centripetal force opposes the gravitational force and they continue to rotate. When they slow down, centripetal forces decrease and they begin to fall toward the Earth.
Perhaps the best way to think about it is that two forces are balancing each other out. (That being gravity and centripetal force.)
Did that help, or did I just confuse you more??? (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
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bloom25
THUNDERBIRDS
[This message has been edited by bloom25 (edited 01-29-2001).]
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And to answere this question
The reason I ask, and how it pertains to a plane's ability to dive: If we had a zero drag aircraft, I am wondering how weight would play a factor in it's dive? It seems that in a vaccum, weight would play no role in an object's acceleration in a dive. However, I am assuming that weight would play a role in an objects maximum attainable rate of fall, or maximum mph.
However, when thrust is added into the equation, weight then becomes a factor?
Weight dosn't become a factor do to thrust, the weight becomes a factor do to the drag, it's why a feather and bolling ball fall at different speeds when not in a vacume. Even thow the ball has more drag than the feather does (assuming small feather) it's ratio of drag to weight is much greater then the feather. Planes when in a dive work the same way, it's the ratio of drag to weight that will determain it's max dive velocity. Also adding thrust when in a vertical dive is exactly the same as adding weight to the airplane. If you add 1000 Lb of thrust from the engine, that would be identical to not having any thrust but just adding 1000 Lb's of weight to the plane.
HiTech
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It DOES continue to accelerate, however due to it's angular motion, the direction that the moon accelerates is constantly changing so it's not a linear acceleration.
Grossly simplified, it's the same thing as putting a weight on the end of a string and whirling it around your head. The force you feel on your hand (the tension in the string) is the force of gravity. If you whirl the weight around harder (increasing your simulated gravitational pull), the weight goes faster. Let go of the string (simulating setting earth's gravity to zero), the weight flings itself off on a direction almost exactly 90 degrees away from the angle the string was originally stretched out, because that's the weight's actual motion is always 90 degrees away from the direction it's being accelerated.
Think about it this way - At one point of an orbit, the moon is being accelerated towards the earth, and let's call that direction "left". Imagine the moon at the 3-oclock position on a clock. For the next 90 degrees of orbit, there will be some acceleration "left". The exact amount is the sin of the angle away from the original position. In any case, the moon has had 90 degrees of orbit with a "left" acceleration. After that 90 deg is completed (moon at the 12-oclock position), the moon can be considered to be on the other side of the earth, and is now being accelerated back to the "right". By the time the moon is exactly 180 degrees on the other side of the earth (at the 9-oclock position), it's had exactly the same amount of acceleration both left and right, and this is the inherent property of an orbit, namely that in a perfect situation (no tidal forces, no drag, no third object interference, etc), the orbit will continue indefinately at the exact same energy level. In a perfectly circular orbit, this would mean that at any point in time, the moon would always have the same velocity and height above the earth. The only thing that changes is the direction of that velocity, because the earth is always pulling the moon in a direction 90 degrees away from it's motion.
If you still don't get it, you never will without a good textbook on basic orbital mechanics and someone to explain it with diagrams and strings with weights attached to them. I haven't gone into the mathematics involved because while simple, they require a good solid understanding of geometry, trigonometry and Newton's laws. Understanding vector mathematics helps too, because it's all easy to understand if you know that a vector is a speed plus a direction.
Sigh... Professor eagl now tries to describe vectors to his class without a blackboard.
Imagine an arrow. The length of the arrow is the speed, and where the arrow is pointing is the direction. Forces on a vector pull from the pointy end, and always result in acceleration of some type. If the force pulls along the shaft of the arrow, the arrow speeds up or slows down. If the arrow is pulled at an angle though, it turns too. Imagine then this arrow being pulled at the pointy end exactly 90 degrees off of the arrow's direction. Since there's no pull to "stretch" the arrow, it doesn't speed up or slow down, it simply turns. Now imagine the moon is this arrow zinging around the earth. There's no pull back or forth around the shaft, so the length (speed) of the arrow doesn't change. The pull is always 90 degrees off, so the arrow just turns. The speed of the moon simply determines it's orbital height. Elliptical orbits are slightly more complicated, but mathematically a circular orbit is merely a special type of elliptical orbit anyhow.
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eagl <squealing Pigs> BYA
Oink Oink To War!!!
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If something is being pushed to the east
While at the same time being pushed to the south
It is likely to go southeast
[This message has been edited by mrfish (edited 01-29-2001).]
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Here we go. If a comet pass close to the earth at a velocity high enough it will obviously not get trapped in earth orbit. Slow it down just the right amount and it will get trapped in earth orbit. I have to define some terms.
G= The gravitational constant 6.672X10^-11 Nm^2kg^-2
me= mass of the earth (could be mass of any planet)
m= mass of the object in this case the moon.
r= the radius of orbit from the center of the earth.
To keep the moon in the current orbit the velocity of the moon must remain the same i.e.
Gmem/r^2=mv^2/r
So as you can see from the formula if you increase the velocity of the moon it will make a larger and larger orbit until it finally departs orbit.
LOL I got on the phone while I was writing this. I was the second person when I got on the phone. LOL
[This message has been edited by Jimdandy (edited 01-29-2001).]
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Ding! Ding! Ding!
Eagl wins the prize. One of the few technical answers on a BBS that is correct!
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Rape, pillage, then burn...
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See here for a very good explanation:
Newtonian Gravitation and the Laws of Kepler (http://csep10.phys.utk.edu/astr161/lect/history/newtonkepler.html)
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Rape, pillage, then burn...
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Ok Let's end this thread. Thanks for the responses, although I think some are in conflict with others. I already understood all factors except why the moon doesn't hit the earth. I think the answer elliptical orbit explains this very well. The pull of the earth brings the moon closer, and then the moon passes the earth and there is less pull, but enough to bring it back around. The forward motion remains constant, but I take it that the moon actually accelerates its descent during it closest orbits around the earth, yet not strong enough to cause a collision.
fscott
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How do I start the engine on the PT boat?
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Originally posted by fscott:
Ok Let's end this thread. Thanks for the responses, although I think some are in conflict with others. I already understood all factors except why the moon doesn't hit the earth. I think the answer elliptical orbit explains this very well. The pull of the earth brings the moon closer, and then the moon passes the earth and there is less pull, but enough to bring it back around. The forward motion remains constant, but I take it that the moon actually accelerates its descent during it closest orbits around the earth, yet not strong enough to cause a collision.
fscott
I know for a fact my answer is exactly right. It is the math behind it. There is no reason to go into anything but a circular orbit to explain it. The forces are equal and opposite and that's all there is to it. Any deviation from circular orbit is just a tweak on the equation. It just that simple. The only thing that matters in the orbit of the moon is it's velocity, radius of orbit, the gravitational constant and the mass of the earth. If you speed the moon up it's radius of orbit will increase. If you slow it down it decreases. It's mass has no baring on it's orbit. It is simply a twist of fate that placed the moon at the right distant for it's velocity to orbit where it does. It is no different then twirling a ball over your head on a rubber band. If you speed up the rotation it tries to move farther out. If you slow it down it moves in. The gravitational force is the rubber band. The difference is that gravity only pulls directly in proportion to the mass of the object it acts on. Thus the mass of the object doesn't matter. If you look at the equation above the mass of the object cancels out. They will collide if the moon slowed down enough. Just like a bullet falling back to earth. If the bullet is fired at a high enough velocity it will go into orbit. At a higher velocity it will escape orbit. Thus the term "escape velocity". There is only one escape velocity on earth and that is 25,038.72mph. There again it is because the earth's mass is what maters. On the moon the escape velocity is 5320.73mph. The moon isn't as massive as the earth. It doesn't matter if your launching an ocean liner or a ballpoint pen. Thus if you wanted to launch the moon into orbit from the earth it will have to reach 25,038.72mph to escape. Could you imaging the thrust that would take! It's still simply equal and opposite forces. If they go out of balance then you got a problem.
[This message has been edited by Jimdandy (edited 01-29-2001).]
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Easy.
The forward motion of the moon around the Earth (called the angular velocity) is perpendicular to the force applied to the moon by gravity. The force exerted towards the earth is called the centripetal force.
Remember that forces are vector quantities. The force that gravity exerts on the moon is exactly that which is required to overcome the angular momentum of the moon to move away from the Earth as it rotates away.
Since the moon isnt moving away from the earth, the forces must be equal so,
F(centripetal) = F(gravitational)
Think of it this way....if the gravitational forces between the Earth and the moon were to instantly cease, the moons velocity would carry it in a straight line 90 degrees perpendicular to an imaginary line drawn between the Earth and moon.
-Ding
[This message has been edited by Dingy (edited 01-29-2001).]
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Originally posted by Dingy:
Easy.
The forward motion of the moon around the Earth (called the angular velocity) is perpendicular to the force applied to the moon by gravity.
Remember that forces are vector quantities. The force that gravity exerts on the moon is exactly that which is required to overcome the angular momentum of the moon to move away from the Earth as it rotates away.
Think of it this way....if the gravitational forces between the Earth and the moon were to instantly cease, the moons velocity would carry it in a straight line 90 degrees perpendicular to an imaginary line drawn between the Earth and moon.
-Ding
When using Dingy's explanation, it must be kept in mind that the universe is orbiting around a central, infinite mass, within a confined space. This mass is known as "The Infinite Ass"* to which all mass reacts.
* -- For an explanation of "The Infinite Ass" see the Infinite bellybutton Theorum, by Jigster, an analysis of the effects on one being, Dingy, upon reaching the speed of light in a diving P-51D Mustang, located elseware on this bulletin board.
- Da Bess
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(http://bigdweeb.homestead.com/files/sig.jpg)
There is no escaping Murphy's Law!
(http://bigdweeb.homestead.com/files/tiger.gif)
33rd FW www.33rd.org (http://www.33rd.org)
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Originally posted by Dingy:
Easy.
The forward motion of the moon around the Earth (called the angular velocity) is perpendicular to the force applied to the moon by gravity.
Remember that forces are vector quantities. The force that gravity exerts on the moon is exactly that which is required to overcome the angular momentum of the moon to move away from the Earth as it rotates away.
Think of it this way....if the gravitational forces between the Earth and the moon were to instantly cease, the moons velocity would carry it in a straight line 90 degrees perpendicular to an imaginary line drawn between the Earth and moon.
-Ding
Dingy if this is in response to me please look at the equation and disprove it. We are saying the same thing. I'm just trying to say I put down the math. If my math is wrong then talk to Sears/Zemansky/Young. They wrote the text I used in college. I can't help what they put in the book. That's all I have to go on.
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Originally posted by Jimdandy:
Dingy if this is in response to me please look at the equation and disprove it. We are saying the same thing. I'm just trying to say I put down the math. If my math is wrong then talk to Sears/Zemansky/Young. They wrote the text I used in college. I can't help what they put in the book. That's all I have to go on.
LOL! No this is in answer to fscotts original question. What was it that you said, was I supposed to be disagreeing with?
Yer also right about the mass of an object not affecting its distance of orbit in a later post. You cant disagree with Newton and Keppler. Dunno tho about Sears, Zemansky and Young (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
-Ding
[This message has been edited by Dingy (edited 01-29-2001).]
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Grumpy old engineer I used to fly planes with once remarked to me.. "If yah can't prove it with math it's an opinion"
The old fart was was a crummy pilot. in my opinion. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Ok Dingy you just better watch it next time. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) <S> I'm getting tired and punchy. Time to call it a night. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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For Jimdandy only,
Your equations are correct. I tend to go with HT's explanation that the elliptical orbit is what keeps the moon from falling into the earth. What you are explaining is what I already know. You haven't explained though that an object in a vacuum, such as the moon, steadily accelerate at the gravitational rate. The moon apparently DOES accelerate when the orbit is closest to the earth. The reason is gets closer is because ALL objects accelerate at the gravitaional rate. However, the forward momentum of the moon, as you pointed out, keeps it from hitting the earth. What is does do is creates an elliptical orbit.
Here's what i was getting at orginally. If you drop a ball in a vacuum with gravity being 10 ft/sec...that ball will be dropping at that rate. Faster and faster, faster until it reaches the just near the speed of sound. This is what I have learned from asking. That was my first question. Now YOU tell me, what are the obvious factors that limit an objects maximum speed in the real world? Drag comes to mind right away as it is probably the singlest most important factor. However there is no drag in space.
I am _NOT_ concerned with the moon's forward velocity at all! I understood all that stuff from the beginning. My question was since the rate of gravity upon the moon is a constant, it will continue to fall, what keeps it from falling faster and faster and faster rate until it finally hits the earth? And now I think I have a good grasp on it. The elliptical orbit was formed BECAUSE the moon accelerated towards the earth, but JUST missed it. The moon passes by, but the earth's pull is too much to allow the moon to just pass out into space, so it brings it back for another pass, and it just repeats and repeats.
fscott
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You know, most every general physics book explains this stuff. Why come on the BBS and bleat about it and then ignore the many good (and unfortunately many not so good) posts and then bleat about it some more?
Newtonian Gravitation and the Laws of Kepler (http://csep10.phys.utk.edu/astr161/lect/history/newtonkepler.html)
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Rape, pillage, then burn...
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Here's another link that might be of interest if anyone who doesn't already understand this fundamental principle is actually interested in an answer.
http://google.yahoo.com/bin/query?p=gravitational+planetary+motion&hc=0&hs=0 (http://google.yahoo.com/bin/query?p=gravitational+planetary+motion&hc=0&hs=0)
The simple fact is whether or not anyone understands it, the moon actually does orbit the earth without crashing into it. It is the exact same as how the planets orbit the sun and how electrons orbit nuclei. Gravitational forces are not unlike electrostatic attraction between charged bodies. This stuff is fundamental to physics and is taught EVERYWHERE.
Asking about an airplane's acceleration in a vaccum has no meaning as an airplane will not fly in a vacuum. However, if an airplane somehow did find itself in a vacuum and also in a gravitational force it will simply accelerate due to the action of that force. If there isn't already motion that would cause it to then orbit the source of the gravitational force then it will simply impact. The equations are simple. The concepts are likewise simple and are taught in the basic college physics courses and in links all over the web.
If there were resistance to the motion of the airplane (i.e. air resistance) then the plane will accelerate until it reaches some terminal velocity where the force of accelleration is balanced by the resistive forces of air friction/drag/etc.
OK, this *is* rocket science but it is stuff that people many hundreds of years ago, without benefit of a modern education, were able to figure out for themselves by doing simple observations. There are simple animations, detailed explanations, and even computer programs and java applets available out on the web for free that demonstrate this stuff - if anyone would bother to go look.
Why do I get the feeling this thread has such parallels to the help manual thread? "Help me! But wait, I don't want to read anything for myself. Just tell me what I need to know."
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Rape, pillage, then burn...
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Dr. 1776 run around areana handing out FREE Prozac!!!!
Screams at top of lungs'"What goes up must come down."
(http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
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Originally posted by fscott:
Your equations are correct. I tend to go with HT's explanation that the elliptical orbit is what keeps the moon from falling into the earth.
The elliptical orbit has NOTHING to do with why the moon doesnt fall into the Earth. Any satellite if put into orbit via optimum conditions (correct angular velocity and altitude) can maintain a perfectly circular orbit.
It is through this same characteristic that we have geosynchronous satellites which are able to maintain their positions precisely over a single station without moving.
The reasons that you dont see this much in nature and most orbits are elliptical are numerous. First and formost, its much more common for satellites to begin their orbits with either angular velocity or distance a bit outside of optimum conditions. If either of those two parameters are slightly off, yet the satellite is unable to reach escape velocity, you will have an elliptical orbit. Secondly, due to various reasons, orbits have a tendency to decay...space is NOT a perfect vacuum. Once a spherical orbit begins to decay, it begins to take on a more elliptical characteristic.
-Ding
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Originally posted by fscott:
Here's what i was getting at orginally. If you drop a ball in a vacuum with gravity being 10 ft/sec...that ball will be dropping at that rate. Faster and faster, faster until it reaches the just near the speed of sound. This is what I have learned from asking. That was my first question. Now YOU tell me, what are the obvious factors that limit an objects maximum speed in the real world?
Dunno where ya learned this fscott but its just plain wrong. The ball would continue to accellerate until it hit something (would be the Earth's surface). Since there is no drag in a vacuum to negate the accelleration of a ball dropping towards the Earth it will continue to accelerate.
You have already stated the limiting factors in a real world....drag. You were just working under a false assumption that objects in a vacuum reach a terminal velocity.
-Ding
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why are we discussing the moon?
::innocent look::
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Originally posted by fscott:
For Jimdandy only,
Your equations are correct. I tend to go with HT's explanation that the elliptical orbit is what keeps the moon from falling into the earth. What you are explaining is what I already know. You haven't explained though that an object in a vacuum, such as the moon, steadily accelerate at the gravitational rate. The moon apparently DOES accelerate when the orbit is closest to the earth. The reason is gets closer is because ALL objects accelerate at the gravitational rate. However, the forward momentum of the moon, as you pointed out, keeps it from hitting the earth. What is does do is creates an elliptical orbit.
Here's what i was getting at originally. If you drop a ball in a vacuum with gravity being 10 ft/sec...that ball will be dropping at that rate. Faster and faster, faster until it reaches the just near the speed of sound. This is what I have learned from asking. That was my first question. Now YOU tell me, what are the obvious factors that limit an objects maximum speed in the real world? Drag comes to mind right away as it is probably the single most important factor. However there is no drag in space.
I am _NOT_ concerned with the moon's forward velocity at all! I understood all that stuff from the beginning. My question was since the rate of gravity upon the moon is a constant, it will continue to fall, what keeps it from falling faster and faster and faster rate until it finally hits the earth? And now I think I have a good grasp on it. The elliptical orbit was formed BECAUSE the moon accelerated towards the earth, but JUST missed it. The moon passes by, but the earth's pull is too much to allow the moon to just pass out into space, so it brings it back for another pass, and it just repeats and repeats.
fscott
fscott I want to make this clear that I'm not trying to be a know it all or a smart bellybutton or anything like that. But if you really want to get to the bottom of this question I think I better make sure I understand what your saying.
"...If you drop a ball in a vacuum with gravity being 10 ft/sec...that ball will be dropping at that rate. Faster and faster, faster until it reaches the just near the speed of sound..."
I assume your using 10ft/sec/sec as an example. I think your saying that in a vacuum a ball will reach a maximum speed near the speed of sound. If this is what your thinking it is not correct. The ball will accelerate to near the speed of light. The speed of sound is dependant on the material the sound is traveling thru. If it is in a vacuum there is no material for the sound to travel thru. Sound is a product of friction between the molecules in the material you might say. The more closely packed the molecules the easier it is for them to bump into each other. Light speed as far as we know is the limiting factor in a vacuum (fiction free) environment because the equations show that the mass of an object becomes infinite at the speed of light. E=mc^2, m is the mass of the object, c is the speed of light. (Please Please don't anyone bring in the discovery of faster than light particles. E=mc^2 is still the gospel at the moment.
"...I am _NOT_ concerned with the moon's forward velocity at all!..."
You have to be that is the key to the relationship between the orbit radius and the opposing centripetal force. Look at the equation above and you will see that the key factors are the radius of the orbit and the velocity of the orbital.
"...Now YOU tell me, what are the obvious factors that limit an objects maximum speed in the real world? Drag comes to mind right away as it is probably the single most important factor. However there is no drag in space..."
Yes the atmosphere of earth (air) is what causes the drag. Simple example is putting your had out of the window of a moving car. Yes space is EXTREMELY low drag but there is some. It is a vacuum.
"...My question was since the rate of gravity upon the moon is a constant, it will continue to fall, what keeps it from falling faster and faster and faster rate until it finally hits the earth?..."
The moons velocity as I have said has EVERYTHING to do with your question. It is what creates the counter acceleration because of the constant change in the direction of travel of the velocity vector at the radius of orbit. The moons orbit isn't a perfect circle as you know. But for your question that really doesn't matter at all. It is possible that do to some outside force that the moons orbit about the earth could take on a shape that DOES put it on a collision course with the earth. Let us assume that the moon is in a circular orbit and that your driving the moon. You have a speedometer on the dash of the moon. Think of it as always trying to fly off from the earth's orbit in a straight line tangent to the circle. A simple example is a car going through a curve. If you quit in putting the force on the steering wheel (the earth's gravity in our case) it will take a straight path off of the corner tangent to the curve. Also if you go faster (on the speedometer) and don't turn harder you will go straight off the road. It equal balance that you keep on the wheel of the car that keeps it turning the corner. If you turn tighter (add more gravity) you will turn into the center of the curve and off the road. But you have to go slower (the speedometer) to turn a tighter corner or you will slide off the road (the centripetal force of the moon trying to throw it out of orbit). The speedometer on a car reads tangential velocity. It doesn't see the changing direction of the vector (the centripetal acceleration). If your accelerating it is measuring the instantaneous change in the size of the velocity vector tangent to the curve. Add a stop watch to the speedometer and you can figure out average acceleration over the distance tangent to the curve. If the tangential velocity (the speedometer on the moon) slows down the force acting to oppose the earth's gravity decreases so the moon is pulled into the earth (turning a tighter corner). As I said just take a ball and tie it to a rubber band and swing it around over your head. The rubber band simulates the pull of gravity on the moon (the ball). The rubber will stretch if you spin the ball faster and contract is you spin it slower. It is a simple model of the relationship between the earth and the moon. If you want to escape the gravity (break the rubber band) of the earth the kinetic energy of you ball has to be greater than the gravitational pull on the earth. Kinetic energy is produced by movement: KE=(1/2) mv^2. There is the critical velocity again. I gave the escape velocity example already.
"...The elliptical orbit was formed BECAUSE the moon accelerated towards the earth, but JUST missed it. The moon passes by, but the earth's pull is too much to allow the moon to just pass out into space, so it brings it back for another pass, and it just repeats and repeats..."
The elliptical orbit of the moon is do to several things. The earth isn't a perfect sphere and the mass of the earth isn't distributed equally over the earth. In the above examples that was the assumption and it works just fine to explain most thing about orbitals. There are also other planets in the solar system all of which act on the moon. In the above example that was ignored because the earth is by far the greatest factor. The fact that other forces act on the moon, the earth and the moon are not perfectly round or have a perfectly even distribution of mass cause the other than circular orbit. On top of that you can think of adding this. If you captured the moon into the earth's gravitational field as it was moving past how would it look. Back to the ball and the rubber band. The ball comes flying up and you lasso it with the rubber band. The ball will start to slow down. Then it will stop as the KE of the ball and the force of the rubber band equalizes. Then it will reverse direction and speed up. If there were no friction in the rubber band and no friction of any kind that cycle would repeat over and over forever. That along with the other factors mentioned is what places the moon in a non circular orbit. If the path of the moons obit were made tight enough by some outside force (something hitting the ball for example) it could run into the earth. It is just chance that put it in the orbit we see. Comets and asteroids are in BIG orbits about the sun. If there orbit becomes small enough or happens to cross ours when we are there they run into the earth. Never forget that there can not be acceleration with out something moving (having a velocity). Acceleration is the rate of change in the movement. The earth and the moon want to come together do to the gravitational attraction. The velocity of the orbit prevents that. If the velocity of the moon drops to zero (and I'm sure it wont in my life time (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) ) gravity will take over and pull the two together like a satellite falling back to the earth.
I'm only trying to clarify. I wanted to make sure you knew that the velocity of the moon was the key to the orbit and not secondary. I like talking about this stuff and I like it when people are interested. So if what I just said is what you were trying to tell me than that's great and I apologize for the reply. Thank you for posting something though provoking. <S>
[This message has been edited by Jimdandy (edited 01-30-2001).]
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Originally posted by Dingy:
You have already stated the limiting factors in a real world....drag. You were just working under a false assumption that objects in a vacuum reach a terminal velocity.
-Ding
Well, according to the relativity, there is a terminal velocity, the light-speed.
At this speed the mass to accelerate become infinite, and the acceleration become non-influent.
But this is theory, the P51 can warp faster (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
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Originally posted by Naso:
Well, according to the relativity, there is a terminal velocity, the light-speed.
At this speed the mass to accelerate become infinite, and the acceleration become non-influent.
But this is theory, the P51 can warp faster (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
LOL yes you are right....you can never go faster than the speed of light in a vacuum.
But what if speed were relative...its measured by the time required to travel between two points. If we were able to use gravitational forces to warp space-time thus bringing those two points closer together....
you can in essence travel the same distance in a shorter period of time. Theoretically.
Ever watch Star Trek and their "warp" engines? That is the physics premise warp drives work under...and to get you thinking even harder, physics does NOT disprove it. The real limiting factor is the amount of gravitational forces required to warp space time enough to affect a ships travel (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Yer talking gravitational forces on the order of those generated by a black hole.
-Ding
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(_|_)
Thats moon for you :P
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Originally posted by Dingy:
LOL yes you are right....you can never go faster than the speed of light in a vacuum.
But what if speed were relative...its measured by the time required to travel between two points. If we were able to use gravitational forces to warp space-time thus bringing those two points closer together....
you can in essence travel the same distance in a shorter period of time. Theoretically.
Ever watch Star Trek and their "warp" engines? That is the physics premise warp drives work under...and to get you thinking even harder, physics does NOT disprove it. The real limiting factor is the amount of gravitational forces required to warp space time enough to affect a ships travel (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Yer talking gravitational forces on the order of those generated by a black hole.
-Ding
I wasn't going to go into that on fscott because I'm sure he thinks I'm a real bag of wind already. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) I love Star Trek. I think after all this brain teasing I need a cup of Earl Grey. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
"Damn it Jim I'm an engineer not an Astronomer!" (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
[This message has been edited by Jimdandy (edited 01-30-2001).]
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Jimdandy, thanks for your explanation. I pretty much ignore any of the smartasses who tell me to go read a book and I appreciate your explanation. So, centripital force is the force which keeps the moon from accelerating faster and faster into the earth. I looked at the moon like a bullet shot out of a gun level with the earth. In one second that bullet will drop 32 feet, regardless of how fast it goes. And I know that if it goes fast enough it will miss the ground and go around the earth, What I was confused about was that since gravity was a constant, in 2 seconds the bullet would drop 64 feet, then 96 feet..I was wondering what kept it from eventually hitting the earth. So centriputal force. Ok. thanks.
fscott
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Your getting the idea fscott. Your more than welcome for the explanation. LOL I had to go get my book. It's been 10 years since I took physics so I'm rusty. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) It's hard if you haven't taken trigonometry and drawn force diagrams to explain it on words. It does come down to the resultant centripetal force being equal to gravity. The bullet your describing will finally reach a point where the force pulling from gravity will balance with the KE of the bullet and slip it into a circular path around the earth. What you just describe is the Mercury rockets, the Space Shuttle or any satellite being launched into orbit. They are basically big bullets being fired into space. Now going belond placing an object in orbit and going to other plants can get real tricky. The only reason we were able to put a man on the moon was computers. We BARELY had the computing power to do it. The lunar lander had the approximate computing power of a Commodore 64. And the Commodore was more reliable. LOL! If you look at the formula as the radius of the orbit gets REAL big the gravitational force gets very small. Eventually it is so negligible as to be ignored. But it is always there. The whole universe is pulling on it self. That is why some people theorize that like the ball on the rubber band the universe will eventually stop expanding and start collapsing in on itself.
[This message has been edited by Jimdandy (edited 01-30-2001).]
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Originally posted by Jimdandy:
That is why some people theorize that like the ball on the rubber band the universe will eventually stop expanding and start collapsing in on itself.
LOL This is fun! (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
Should we expound on this Jim? (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif) There are two theorys here...the infinitely expanding universe and the expanding/contracting universal theory. The big controversey is over the amount of matter in the universe. If there is not enough matter in the universe to exert a gravitational pull to slow the expansion of the universe, the universe will continually expand and eventually burn out to a cold, energy-less cinder.
If, however, there is enough matter out there to eventually cease the expansion of our universe and cause it begin to contract, THEN we have an interesting situation. Is the Big Bang a repeatable event? Does the universe go through a cycle of big Bang/expansion/contraction/Big Bang? (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
Right now, estimates on the amount of matter in the universe only account for about 60% of the required matter to cause the evntual contraction. BUT....scientists are finding more and more hidden pockets of dark matter which add to the critical amount required to activate this Big Bang cycle.
Neat stuff eh? (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
-Ding
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I think the amount of roadkill on various BB's around the internet fills in conveniently the 'missing mass' needed for the universe to collapse.
- MrSiD
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Originally posted by Hangtime:
How do I start the engine on the PT boat?
Hangtime That is simple RTFM (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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!!! Heater !!!
(http://www.geocities.com/heater_nl/_private/heater1.jpg)
Shit Happens All The Time
"If you have any trouble sounding condescending, find a UNIX user to show you how it's done."
[This message has been edited by Heater (edited 01-30-2001).]
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Originally posted by Dingy:
LOL This is fun! (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
Should we expound on this Jim? (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)...
-Ding
Ok. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) I personally lean to the bang/contract because of the conservation of energy. But that assumes we are in a closed loop. We don't know what's outside the universe. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) I've always like to contemplate what we were expanding into. If we are expanding that implies the is something to expand into. What is on the other side of that shock wave? (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) Is it like the age old question 'How many licks does it take to get to the tootsie roll center of a tootsie pop?' (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
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Fscott,
If the moon falls onto the Earth, I hope it lands on you for starting this nonsense and for telling the moon it WAS falling....... (http://bbs.hitechcreations.com/smf/Smileys/default/tongue.gif)
Mav
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Originally posted by fscott:
Here's what i was getting at orginally. If you drop a ball in a vacuum with gravity being 10 ft/sec...that ball will be dropping at that rate.
Fundamental issue - 10 ft/sec is a speed - distance divided by time. Gravity has units of ft/sec^2 - it is an acceleration. The actual value is approximately 32 ft/sec^2 at the earth's surface.
Without the proper units, you ain't going to get anywhere.
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Rape, pillage, then burn...
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10 ft/s is prolly meant to be the rounded form of 9.8 m/s^2 (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
I love how well the 10 fits into equations solved via the meteric system.
Meteric vs the US is so confusing sometimes...like mass = slugs
And the US system uses foot/pounds for torque, and the meteric system uses the same unit type but screws them up ... instead of meter/newtons (I still call them that anyway (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) ) vs newton/meters
Thank goodness for conversion tables.
- Da Bess
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Originally posted by Dingy:
LOL yes you are right....you can never go faster than the speed of light in a vacuum.
But what if speed were relative...its measured by the time required to travel between two points. If we were able to use gravitational forces to warp space-time thus bringing those two points closer together....
you can in essence travel the same distance in a shorter period of time. Theoretically.
Ever watch Star Trek and their "warp" engines? That is the physics premise warp drives work under...and to get you thinking even harder, physics does NOT disprove it. The real limiting factor is the amount of gravitational forces required to warp space time enough to affect a ships travel (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Yer talking gravitational forces on the order of those generated by a black hole.
-Ding
Yeah, the theory behind the warp-speed, actually is this, but the correct term is "curve" the space-time, or simpler, exit from it and cut a "shortroad" (? dunno word) out of the continuum.
BTW we already see a real application of this theory with planes warping everywhere. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
And yes, I like Star trek
"beam me up Scotty, there is no intelligent form of life on this planet"