how does earth stay "in place" in space?

science_newb · Dec 6, 2008

<p>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move".</p><p> </p><p>and last (keep in mind i'm not an electro-magnetic expert) but i was using my portable radio two days ago by my microwave while cooking something, and as i leaned in to the window to look at what i was cooking, the radio signal began to "white noise", it made me wonder why isn't all our radio signals always "white noise" if were exposed to all kinds of electro magnetic radiaton every day on earth </p>

Mee_n_Mac · Dec 6, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move". Posted by science_newb</DIV></p><p>The Earth is "free falling" around the Sun just as the ISS is around the Earth.  The Earth isn't in the same place tomorrow as it is right now.  I'm not sure I understand what you mean by up, down, middle.  You don't feel the Earth moving in it's orbit anymore than you feel it rotating (night/day cycle) because you're moving right along with it. Same thing as sitting in an airplane cruising along at 500 mph. You don't (usually) sense you're moving but looking out the window will sure show you something's moving.  You and everything around you is "stuck" to the Earth by it's gravity.  You and everything are moving together so you don't notice the motion unless you look at somthing truly stationary.  I can get more detailed if you wish.</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> and last (keep in mind i'm not an electro-magnetic expert) but i was using my portable radio two days ago by my microwave while cooking something, and as i leaned in to the window to look at what i was cooking, the radio signal began to "white noise", it made me wonder why isn't all our radio signals always "white noise" if were exposed to all kinds of electro magnetic radiaton every day on earth <br />Posted by science_newb</DIV></p><p>I don't know what you heard with your radio but it was more likely interference than truly white noise.  Something in your microwave was emitting a radio signal in the frequencies your radio can receive.  You note that you didn't notice anything until you got close.  That because the interfering signal is weak.  And so it is with most EM radiation.  Circuits are designed to handle the weak signals we encounter most frequently.  They do what we want (them to do) despite the presence of interfering signals, until they get too strong. And some radios are built so as to be more susceptible than others.</p> <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

DrRocket · Dec 6, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move".</DIV></p><p>Keep in mind that motion is relative, and you cannot sense motion.  You can sense acceleration, relative to an inertial reference frame.</p><p>First, what is an inertial reference frame ?  An inertial reference frame is one that is moving freely in whatever gravitational field may be present, and in the case of no gravitational field, that means moving uniformly in a straight line.  In other words an inertial reference frame is one in which you feel no force.</p><p>You can't feel the motion of the earth because the only significant force applied to the earth is the gravity of the sun and the earth is in free fall around the sun.  An astronaut in the ISS doesn't feel any force because the ISS is in free fall around the earth and actually also in free fall around the sun.</p><p>A more precise notion comes from general relativity and basically an inertial frame is one in which the observer is in free fall relative to the sum total of all graviational fields that affect him -- he is following a geodesic curve in space-time.  The fundamental "take away" from this is that everything follows a free fall path unless there is an outside force imposed, but you sometimes have to look closely to understand what a free fall path is.  </p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> and last (keep in mind i'm not an electro-magnetic expert) but i was using my portable radio two days ago by my microwave while cooking something, and as i leaned in to the window to look at what i was cooking, the radio signal began to "white noise", it made me wonder why isn't all our radio signals always "white noise" if were exposed to all kinds of electro magnetic radiaton every day on earth <br />Posted by science_newb</DIV></p><p>As mee-n-mac noted,  radios are designed to isolate a meaningful signal from a somewhat noisy background.  There is indeed electromagnetic radiation across the spectrum all the time, but not normally at high levels.  You only hear the noise when there is no signal, as between channels in sets with analog tuners, or when the noise signal overpowers the intended signal.  But if your microwave was interfering with your radio, then something is amiss.  Broadcast radio sigjnals are of longer than microwave wavelength and should not pick up your microwave if it is operating properly.  So if the microwave was really interfering with the radio then there is something generating sub-harmonics that should not be happening or at least not ought be leaking out of the microwave oven itself. Your microwave is supposed to be designed to prevent the microwaves from being transmitted to the surround environment.  If your microwave has developed a leak, then that could be dangerous.  Did you feel yourself getting warm ?  </p> <div class="Discussion_UserSignature"> </div>

origin · Dec 7, 2008

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move". and last (keep in mind i'm not an electro-magnetic expert) but i was using my portable radio two days ago by my microwave while cooking something, and as i leaned in to the window to look at what i was cooking, the radio signal began to "white noise", it made me wonder why isn't all our radio signals always "white noise" if were exposed to all kinds of electro magnetic radiaton every day on earth <br />Posted by science_newb</DIV><br /><br />There is a transformer that steps the voltage up from 120 to the order of 3000 volts that could be what what making the white noise.  Microwaves usually leak at 1000 of times below what could be dangerous to people but that small leakage can interfere with cell phones.  So if your cell phone can be used when the microwave is on it is probably just the transformer emitting some RF. <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There is a transformer that steps the voltage up from 120 to the order of 3000 volts that could be what what making the white noise.  Microwaves usually leak at 1000 of times below what could be dangerous to people but that small leakage can interfere with cell phones.  So if your cell phone can be used when the microwave is on it is probably just the transformer emitting some RF. <br />Posted by origin</DIV></p><p>That is a good point about the transformeer.  If the transformer is generating much broadband RF, wouldn't you think that there is some arcing occurring ?  And if there is arcing, I would expect that the transformer is about to fail, the dielectric breakdown probably resulting in shorting out of the high voltage side.  That will probably just result in a breaker tripping, but it might be exciting for a moment or two.  What are your thoughts ?<br /></p> <div class="Discussion_UserSignature"> </div>

emperor_of_localgroup · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'><br /> Posted by science_newb</DIV></p><p><font size="2" color="#ff0000">i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?</font></p><p><font size="2">I also do not like the term "constant free fall" they use these days to explain weightlessness in outer space. The basic principle, IMO, is gravitational pull is balanced by the 'centripetal' force, these two forces are equal and act in opposite directions, that makes objects weightless. Centripetal force requires certain value of velocity to counter gravitational pull, if the velocity is even slightly lower, only then we'll have a 'constant free fall'. If  earth moves with one everlasting  fixed speed, we don't need the earth to have a 'constant free fall'. </font></p><p><font size="2">I'm sure constant-free-fallers have their own arguments. </font></p><p><font size="2"> This thread should be in Physics forum.<br /></font></p><p> </p> <div class="Discussion_UserSignature"> <font size="2" color="#ff0000"><strong>Earth is Boring</strong></font> </div>

DrRocket · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?I also do not like the term "constant free fall" they use these days to explain weightlessness in outer space. The basic principle, IMO, is gravitational pull is balanced by the 'centripetal' force, these two forces are equal and act in opposite directions, that makes objects weightless. Centripetal force requires certain value of velocity to counter gravitational pull, if the velocity is even slightly lower, only then we'll have a 'constant free fall'. If  earth moves with one everlasting  fixed speed, we don't need the earth to have a 'constant free fall'. I'm sure constant-free-fallers have their own arguments.  This thread should be in Physics forum.  <br />Posted by emperor_of_localgroup</DIV></p><p> </p><p>There is a reason that it is called constant free fall.  It is because a satellite IS in a state of constant free fall.  This is not a matter of semantics.  It is important.</p><p>The state of free fall experienced by a satellite is precisely the state that is required in general relativity for an inertial reference frame.  The satellite is in fact following a geodesic in curved space-time and is not influenced by any outside forces.</p><p>Even if you look a the problem in a classical Newtonian sense, what is happening is that a satellite is falling around the Earth.  It is being pulled downward by gravity but has a transverse speed that causes the trajectory to miss the Earth -- literally falling around it.</p><p>There is no centripetal force counterbalancing gravity.  In fact the only force exerted on a satellite is exerted by gravity. There are no strings, no electromagnetic forces, nothing but gravity in action.  Instead what you have is an acceleration directed radially inward and caused by gravity that causes a circular (or nearly so) trajectory.  The acceleration is manifested as a change in direction of motion, and that is what acceleration is if you look at it properly as a vector -- a change in the velocity vector in the form of a change in speed, a change in direction, or both.  <br /></p> <div class="Discussion_UserSignature"> </div>

emperor_of_localgroup · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> It is being pulled downward by gravity but has a transverse speed that causes the trajectory to miss the Earth -- literally falling around it.There is no centripetal force counterbalancing gravity.  In fact the only force exerted on a satellite is exerted by gravity. T<br /> Posted by DrRocket</DIV></p><p><font size="2" color="#ff0000">The satellite is in fact following a geodesic in curved space-time and is not influenced by any outside forces. </font></p><p><font size="2">No, I'm not rejecting the new interpretation of circular motion. I'm wondering why this dual interpretations of circular motion produce the same result? If you use moon's mass and its speed around the earth and use "gravity=centripetal" formula, you'll get a earth-moon distance of  240000miles. If you use mass of earth and its speed around the sun, and use the above equality, you'll again get  a sun-earth distance of 93000000 miles. It goes on and on... If a satellite is 'free falling', IMO, that's because it can't keep a constant speed for various reasons.</font></p><p><font size="2">Long time ago I read somewhere, I'm not sure if it is true, that NASA still use Newtonian gravitation and mechanics for their space flights instead of more 'accurate' Einstein's relativity. </font></p><p><font size="2">Btw, in good old mechanics, the force used against gravity is called 'centrifugal force', away from the center. </font></p> <div class="Discussion_UserSignature"> <font size="2" color="#ff0000"><strong>Earth is Boring</strong></font> </div>

nimbus · Dec 7, 2008

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The satellite is in fact following a geodesic in curved space-time and is not influenced by any outside forces. No, I'm not rejecting the new interpretation of circular motion. I'm wondering why this dual interpretations of circular motion produce the same result? If you use moon's mass and its speed around the earth and use "gravity=centripetal" formula, you'll get a earth-moon distance of  240000miles. If you use mass of earth and its speed around the sun, and use the above equality, you'll again get  a sun-earth distance of 93000000 miles. It goes on and on... If a satellite is 'free falling', IMO, that's because it can't keep a constant speed for various reasons.Long time ago I read somewhere, I'm not sure if it is true, that NASA still use Newtonian gravitation and mechanics for their space flights instead of more 'accurate' Einstein's relativity. Btw, in good old mechanics, the force used against gravity is called 'centrifugal force', away from the center. <br />Posted by emperor_of_localgroup</DIV>??<br /> <div class="Discussion_UserSignature"> </div>

science_newb · Dec 7, 2008

thanks alot to everyone who responded to my thread, and to dr rocket my microwave is very old and sucks, also i leaned in and put my radio on the window and there's a slight slit in between the grooves of the door which possibly allowed for EM radiation, any btw the sun is stationary right ?

origin · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The satellite is in fact following a geodesic in curved space-time and is not influenced by any outside forces. No, I'm not rejecting the new interpretation of circular motion. I'm wondering why this dual interpretations of circular motion produce the same result? If you use moon's mass and its speed around the earth and use "gravity=centripetal" formula, you'll get a earth-moon distance of  240000miles. If you use mass of earth and its speed around the sun, and use the above equality, you'll again get  a sun-earth distance of 93000000 miles. It goes on and on... If a satellite is 'free falling', IMO, that's because it can't keep a constant speed for various reasons.Long time ago I read somewhere, I'm not sure if it is true, that NASA still use Newtonian gravitation and mechanics for their space flights instead of more 'accurate' Einstein's relativity. Btw, in good old mechanics, the force used against gravity is called 'centrifugal force', away from the center. <br />Posted by emperor_of_localgroup</DIV><br /><br />Using equations for centripetal force (like a ball on a string) can be used to derive an <em>approximate </em>mass, distance or velocity of a orbiting body.  But it is not exact because gravity is not a string and consequently orbits are not circular.  A body in orbit is in fact falling towards the earth continuasly, if you don't think so, then imagine what would happen if the orbiting object were to suddenly put on the brakes and stop its forward motion.  It would continue to fall, but with no forwad motion.  </p><p>For some reason that reminds me of that terrible joke - what do you call a boomerang that won't come back? </p><p>A stick.</p><p>   </p> <div class="Discussion_UserSignature"> </div>

origin · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That is a good point about the transformeer.  If the transformer is generating much broadband RF, wouldn't you think that there is some arcing occurring ?  And if there is arcing, I would expect that the transformer is about to fail, the dielectric breakdown probably resulting in shorting out of the high voltage side.  That will probably just result in a breaker tripping, but it might be exciting for a moment or two.  What are your thoughts ? <br />Posted by DrRocket</DIV><br /><br />Is arcing necessary?  I know that when I drive by a certain transformer (I asume it is the transformer) near where I live the AM radio reception is terrible and it has been that way for years.</p><p>I know that arcing also affects AM reception - I tune to 1600 in the summer occasionally to check for thunder storms in the vicinity.</p> <div class="Discussion_UserSignature"> </div>

weeman · Dec 7, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move". and last (keep in mind i'm not an electro-magnetic expert) but i was using my portable radio two days ago by my microwave while cooking something, and as i leaned in to the window to look at what i was cooking, the radio signal began to "white noise", it made me wonder why isn't all our radio signals always "white noise" if were exposed to all kinds of electro magnetic radiaton every day on earth <br />Posted by science_newb</DIV><br /><br />Think about flying in a plane. Most commercial airliners fly at about 500mph when they're at their cruising altitude, yet you can't feel the plane moving; you can freely walk about the cabin with no effort at all. It's the same thing for Earth. We can't feel it moving because it moves at a constant velocity. </p><p>Earth's orbit is held in place by the Sun's gravitational pull. You are right about the idea of "constant free fall", just as the ISS and its astronauts are doing right now. </p><p>Additionally, Einstein argued that all the planets are actually traveling in straight lines; yet, due to the curvature of space, their paths appear to be pulled into circles. </p> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

DrRocket · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The satellite is in fact following a geodesic in curved space-time and is not influenced by any outside forces. No, I'm not rejecting the new interpretation of circular motion. I'm wondering why this dual interpretations of circular motion produce the same result? If you use moon's mass and its speed around the earth and use "gravity=centripetal" formula, you'll get a earth-moon distance of  240000miles. If you use mass of earth and its speed around the sun, and use the above equality, you'll again get  a sun-earth distance of 93000000 miles. It goes on and on... If a satellite is 'free falling', IMO, that's because it can't keep a constant speed for various reasons.</DIV></p><p>They produce the same result because they are describing the same phenomenon.  Both Newton's theory of mechanics and of gravity and general relativity are accurate within their domains of validity.  It is just that general relativity is accurate over a broader range of circumstances.  In circumstances in which gravitational fields are modest (they almost always are except in rare circumstances like black holes) and in which speeds are small compared to the speed of light, general relativity and Newton's theory agree very very closely.  General relativity is a little bit more accurate, but not enough to notice for most practical considerations.</p><p>But what is going on is relatively simple.  Let's neglect the drag that occurs from a very thin atmosphere in space, solar wind and other tiny effects.  In a vacuum satellites do in fact maintain a constant speed.  What they don't maintain is a constant velocity, and the difference is important.  Speed is a measure of how fast something moves, and it does not matter what the directin of motion is.  So a race car speeding around a circular track can do so at constant speed.  But velocity is a vector and measures not only speed but also the direction of motion.  If the direction changes then so does the velocity, even if the speed is constant.  So a race car going around the track at Indy at a constant speed of 200 mph is also changing velocity constantly because the direction changes.</p><p>Now Newton's equation for a body of constant mass,  F = ma tells us that force is equal to mass x acceleration.  Acceleration is a change in velocity, not necessarily a change in speed.  The driver of the race car traveling around the track at Indy, feels the car pushing on him towards the center of the track, and the car "feels" a similar force due to friction between the tires and the pavement.  That force, directed towards the center of the track is the result of the change in direction of his motion.  In the case of the Earth revolving around the sun, the force comes from gravity.  Similarly in the case of a satellite orbiting the Earth.  The speed of the Earth and the speed of the satellite are constant (I have idealized this to the case of a circular orbit, for an elliptical orbit the speed also changes a little bit as the satellite gets closer and farther from the Earth, just like going up and down hills).  What changes is the direction of motion as the satellite describes a circular path.  And what is happening to maintain an orbit is that as the satellite "falls" it also has a tangential component of velocity that causes it to curve outward from the center of the Earth, and it literally falls around the Earth.</p><p>"Centrifugal force" and "centripetal force" are examples of unfortunate terminology.  They often serve to confuse rather than elighten.  They are not really forces at all, but are simply the result of Newtonian mechanics and changes in the direction of motion, even when speed is constant.  </p><p> [Long time ago I read somewhere, I'm not sure if it is true, that NASA still use Newtonian gravitation and mechanics for their space flights instead of more 'accurate' Einstein's relativity. Btw, in good old mechanics, the force used against gravity is called 'centrifugal force', away from the center. <br />Posted by emperor_of_localgroup</DIV></p><p>Think about the situation of a satellite.  There is nothing whatever acting on the satellite to pull it away from the center of the Earth.  No strings, no electromagnetic forces, nothing.  Gravity is pulling the other way.  The ONLY force involved is gravity.  </p><p>Everybody uses Newtonian mechanics for almost all applications.  There is nothing special about NASA in that regard.  That is because it is sufficiently accurate for most things.  It is certainly sufficiently accurate for calculating the orbits of satellites.  In fact even in many astrophysical models, say the motion of galaxies about one another, Newtonian mechanics is used.  It is far far easier to perform the calculations to solve Newton's equations than it is to solve the Einstein equations in most situations.  And they work just fine for most things.</p><p>Orbital mechanics models for the solar system and such rely exclusively on Newtonian mechanics, and often on approximations to Newtonian mechanics at that.  Quite a few models look at only two bodies at a time and put together trajectories using patched conics.  It turns out to actually be a bit difficult to model the interactions among three or more bodies, even in a Newtonian setting.  In fact there are no close form solutions for three or more bodies and one must rely on simulations.  The problem would be even more difficult, and not more enlightening, if formulated in terms of general relativity.</p><p>You only need general relativity if the speeds are an appreciable fraction of the speed of light, if gravitational fields are extremely high, or if you need an extraordinary degree of accuracy.  For most purposes Newtonian gravity suffices.  <br /></p> <div class="Discussion_UserSignature"> </div>

Mee_n_Mac · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Is arcing necessary?  I know that when I drive by a certain transformer (I asume it is the transformer) near where I live the AM radio reception is terrible and it has been that way for years.I know that arcing also affects AM reception - I tune to 1600 in the summer occasionally to check for thunder storms in the vicinity. <br />Posted by <strong>origin</strong></DIV><br /><br />I suspect like all microwaves I've seen recently, the OPs has some digital (control and) display.  My first guess would be the clock and/or other digital circuitry is leaking RF out.  There may also be a switching power supply in the box as well.  These are notorious for being sources of EMI.</p><p> </p> <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

Aaupaaq · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move". and last (keep in mind i'm not an electro-magnetic expert) but i was using my portable radio two days ago by my microwave while cooking something, and as i leaned in to the window to look at what i was cooking, the radio signal began to "white noise", it made me wonder why isn't all our radio signals always "white noise" if were exposed to all kinds of electro magnetic radiaton every day on earth <br /> Posted by science_newb</DIV></p><p>Gravity, is indeed one of the mysteries of this known space.  In my definition, gravity is three forces in one.  Scientists say that I'm not elite on basic physics and science, but I know about them, and do not believe them, so I came up with my own interpretations of this gravity.</p><p>First, characteristics of this gravity, is attraction and repelling properties that everyone knows about.  The second, is the weight of the non magnetic elements.  And the third is cell to cell pull or atom to atom pull of all visible objects, resulting in matter, chaotic they might seem, but I think its very organized somehow.  If it wasn't organized, we would have not seen any solar system, but lines of matter going everywhere, instead of rotations and revolutions and systems. </p> <div class="Discussion_UserSignature"> We always walked on water, like skating! </div>

michaelmozina · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>You only need general relativity if the speeds are an appreciable fraction of the speed of light, if gravitational fields are extremely high, or if you need an extraordinary degree of accuracy.  For most purposes Newtonian gravity suffices.  <br /> Posted by DrRocket</DIV></p><p>This is often overlooked by many folks.  Newtonian calculations tend to work quite well for ordinary space travel in our solar system.    Even when a theory is replaced by another more accurate theory, the old theories tend to have a "domain" of accuracy.  </p> <div class="Discussion_UserSignature"> It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. - Kristian Birkeland </div>

MeteorWayne · Dec 8, 2008

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>This is often overlooked by many folks.  Newtonian calculations tend to work quite well for ordinary space travel in our solar system.    Even when a theory is replaced by another more accurate theory, the old theories tend to have a "domain" of accuracy.  <br />Posted by michaelmozina</DIV><br /><br />AFAIK, the only place where Einstein is really required for refinement within the solar system is for calculating Mercury's orbit. One of the proofs of relativity (I forget which one) invloves the precession of Mercury's orbit. In that case, the strong gravity of the sun at that distance does make it necessary to include in doing long term calculations. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

MeteorWayne · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Posted by Aaupaaq</DIV><br /><br />MOD HAT ON***</p><p>That had better be your last post about your nonscientific ideas concerning gravity in this thread.</p><p>Wayne</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

Aaupaaq · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>MOD HAT ON***That had better be your last post about your nonscientific ideas concerning gravity in this thread.Wayne <br /> Posted by MeteorWayne</DIV></p><p>You sure sound a lot like my late grandpa, but sure thing.  Let me tell you one last thing, research will confirm my tellings to yous, of mysteries we're talking bout. </p> <div class="Discussion_UserSignature"> We always walked on water, like skating! </div>

DrRocket · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>AFAIK, the only place where Einstein is really required for refinement within the solar system is for calculating Mercury's orbit. One of the proofs of relativity (I forget which one) invloves the precession of Mercury's orbit. In that case, the strong gravity of the sun at that distance does make it necessary to include in doing long term calculations. <br />Posted by MeteorWayne</DIV></p><p>The phenomenon to which you are referring is the calculation of the precession of the perihelion of Mercury.  http://en.wikipedia.org/wiki/Tests_of_general_relativity  Newtonian calculations are off by 43 arc seconds per century (out of 5600 arc seconds per century).  That is not bad accuracy, but general relativity provides a precise prediction, accurate to within observational error.   This feat is a triumph for the accuracy and validity of general relativity, which in approach is quite different from Newton's theory of universal gravitation, but it does not take Newtonian calculations off the table for any but the most extreme situations, or cases where the required accuracy is most demanding.  </p><p>Newton is still good enough for guvm't work.  <br /></p> <div class="Discussion_UserSignature"> </div>

MeteorWayne · Dec 8, 2008

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Posted by DrRocket</DIV><br /><br />Thanx Dr Rocket, I didn't recall the details off the top of my head and didn't have time to research it yet. STill thawing out and processing my meteor obs from this morning. Brrrrrr! <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

emperor_of_localgroup · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>appreciable fraction of the speed of light, if gravitational fields are extremely high, or if you need an extraordinary degree of accuracy.  For most purposes Newtonian gravity suffices.  <br />Posted by DrRocket</DIV><br /><br /><font size="2">Good post , drRocket. It covers the basics of circular motion and centripetal force. Yes, I'm aware of the fact that change in direction causes centripetal acceleration. I view centrifugal force as 'reaction' force and is observed when no string is attached. As some of the posters are indirectly suggesting non-existence of centripetal force, I'd like to remind them</font></p><p><font size="2">a) Your head moves away from the center as you try to turn around a sharp curve with high speed. No string is attached to your head.</font></p><p><font size="2">b) Stuntmen can ride bikes round and round on a vertical wall. No string there either.</font></p><p><font size="2">Our practical experiences tell us if anything moves in a circle, it experiences a centripetal force, caused by the change in direction. IMO, an elliptical orbit can be approximated as 'piece-wise' circular, that's why an object in elliptical orbit requires varied speeds in order to stay in orbit. </font></p><p><font size="2">IMO the path of an object as it enters a gravitational field of a massive object has some resemblance with the path of an  electron as it enters a strong magnetic field.  The electron eventually get trapped into a circular path. And then we can use "Magnetic force=centripetal force".  I have this weird thought may be that's how all the planets got trapped in orbits around the sun once they unknowingly entered sun's gravitational field. Just a thought.</font></p><p><font size="2">Also good to know, Newton lives on.</font></p><p> </p> <div class="Discussion_UserSignature"> <font size="2" color="#ff0000"><strong>Earth is Boring</strong></font> </div>

derekmcd · Dec 8, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>IMO the path of an object as it enters a gravitational field of a massive object has some resemblance with the path of an  electron as it enters a strong magnetic field.  The electron eventually get trapped into a circular path. And then we can use "Magnetic force=centripetal force".  I have this weird thought may be that's how all the planets got trapped in orbits around the sun once they unknowingly entered sun's gravitational field. Just a thought.Also good to know, Newton lives on.  <br /> Posted by emperor_of_localgroup</DIV></p><p>I believe electrons are actually deflected by magnetic fields where the trajectory is determined by the Lorentz force.  What you are referring to Columb's law where opposite charges attracts and like charges repel.  It is Columb's law that binds electrons to protons.</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

vandivx · Dec 11, 2008

<p> <font size="0" color="#ff0000"><strong>CAUTION! - the ideas presented in this post do not adhere </strong></font><font size="0" color="#ff0000"><strong>to </strong></font><font size="0" color="#ff0000"><strong>or follow the official textbook material and accordingly are not recommended or advised reading for those who are still in the learning stages of the physical science. You have been WARNED!</strong></font><font size="0"><br /></font></p><p><font size="0"> </font></p><p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>"Centrifugal force" and "centripetal force" are examples of unfortunate terminology.  They often serve to confuse rather than elighten.  <font color="#ff0000">They are not really forces at all</font>, but are simply the result of Newtonian mechanics and changes in the direction of motion, even when speed is constant.   <br /> Posted by DrRocket</DIV></p><p> How so?</p><p> In my books those two are certainly forces - the <font color="#333399">centrifugal force</font> is the one-to-one equivalent of the <font color="#333399">inertial force</font> (or the <font color="#333399">reaction force </font>in Newton's laws) and the <font color="#333399">centripetal force</font> is the one-to-one equivalent of the acting force F as in F=ma (or the <font color="#333399">action force</font> as it is called in Newton's laws). These action and reaction forces are certainly forces if anything is. They arise in any mechanical rotating sytem but only in it (as opposed to orbits under gravitational influence as explained below). </p><p> The terminology is not unfortunate or confusing at all if one keeps strictly to mechanical rotating systems, its purpose is simply to let us know that we talk about the special case of circular (rotating) accelerated motion as opposed to linear one. It is a motion distinct enough to warrant its own special terminology (i.e., acceleration can easily be maintained indefinitely as opposed to linear acceleration).</p><p> ~~~~~~~~~~~~~~~~~~~~~~~~~~~</p><p>Any confusion arises from misapplication of these forces to gravitationally bound system but then the confusion is due to misapplication, not because the terms are not in some way not kosher. </p><p>When it comes to orbits under gravitational influence as opposed to mechanical rotating sytem, these two forces - action and reaction Newtonian forces - are not present or acting and that is where some confusion may come from. The reason is that gravitation is not the classical mechanical Newtonian force. Where the mechanical force acts on the body directly in the push/pull fashion and alters its motion that way by working against and overcoming its inertial resistance, the gravitation is a force which acts directly on the body's inertial properties so to speak, altering them and thus generating the acceleration of the body without it being mechanically pushed/pulled around. That is also why it is (correctly) said that gravitation is (really) not a force, at least not in the sense of the classical Newtonian force.</p><p>It might seem odd calling gravitational force non-Newtonian given that Newton was the one who 'discovered' it. That has to do with the fact that as he admited himself, he didn't understand how gravitation works, that is what is the machinery behind it. I call the gravitation non-Newtonian force to distinguish it from the classical Newtonian force acting in the F=ma formula. Perhaps there is a better way to make the distinction and I am open to ideas in that regard. </p><p> ~~~~~~~~~~~~~~~~~~~~~~~~~</p><p> The gravitation doesn't work against the body's inertia in the sense of overcoming it but rather it alters the inertia of a body itself, so that the body moves in accelerated fashion on its own. Centripetal (pulling) force takes no part in the Moons orbital motion (for example) simply because there is no centrifugal (pull resisting) force to begin with and vice versa - that's because gravitation is not a mechanical Newtonian force as pointed out and is really not a force at all in a strict sense. </p><p>In a sense the gravitational 'force' is the very opposite of the mechanical Newtonian force - the Moon is in a freefall towards the Earth's surface which it approaches at the rate of 9.8 m/second squared and if there were no gravitation and we wanted to achieve the same result, we would have to do some hard pushing to make it move that way. Now if we were instead somehow able to adjust the Moon's inertial properties the same way that gravitation does it, then the Moon would have acceleration as its normal and natural motion without us having to push it. But then instead we would have hard time holding it still suspended in the space above the Earth. In short the Moon's natural motion with such tweaked inertial properties would then want to accelerate all the time in some direction (of course the inertia of the Moon could be put into imbalance only in a certain direction at a time in respect to its body) as opposed to the usual way of staying put, moving with whatever velocity it had, if there were no other bodies in close proximity to influence it gravitationally.  </p><p> ~~~~~~~~~~~~~~~~~~~~~~~~~~</p><p>That's how gravitation works and that's why it behaves differently from the classical Newtonian forces (F=ma) and why the forces of action and reaction (or centri-petal/fugal when it comes to rotation) don't apply when it is the gravitation that acts. That is also the reason why in the GR the gravitation is not a force. In the classical Newtonian context also, the gravitation is not a force (should not be taken as such) if one wants to be strict with terminology employed, given that one is using today's insight into gravitation.</p><p>Of course, in Newton's time there were no such insights and one equated ma=G[(mm)/r*r] without scruples. The danger is that confusion can develop with such usage which mixes the two kinds of forces freely. </p><p>As explained, the F=ma force achieves acceleration by acting against the body's inertial properties - that is it seeks to overcome them. In contrast the gravitation instead acts to alter these inertial properties themselves so as to achieve the acceleration of the body in question without forcing it in any way. In short it is like the difference between hand pushing a recalcitrant stuck-in-place ox (oxen?) to make it go someplace instead of suspending a carrot on a stick and dangling it in front of its nose which it then naturally follows of its own will without any pushing being necessary. Alas we can only wish we were able to influence a body's inertia the same way we can influence the ox's because then all our problems would be solved. Like Sartre or some other philosopher said, quoting from memory of a quote - if a man has ten cows, all his problems are solved. </p><p> </p><p> Constructive criticism is welcomed (welcome?) </p> <div class="Discussion_UserSignature"> </div>

how does earth stay "in place" in space?

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