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Why do meteors burn up in our atmosphere / why are they moving so fast?

  1. Dec 7, 2011 #1
    Hi,
    I have a question, that I kind-of know the answer to, but then have a follow-up question:

    Why do meteors burn up in our atmosphere?
    The answer must be that they are "slowing down" causing friction/air pressure that turns into heat and combusts.
    But then, why are they moving so fast in the first place? I heard our terminal velocity is around 700 mph and meteors are moving at 17,000 mph or something around that. Why are they moving so fast? Is the sun pulling them in and they just happen to get sidetracked into our planet? If there's a massive mass (the sun) attracting a relatively tiny rock (a meteor) and there's a relatively small rock (the Earth) in the way, why would the tiny rock be pulled towards the earth instead of going to the sun, and then why would the earth's gravitational pull outweigh the massive sun's? Are we simply in the way when a meteor enters our atmosphere?
    And then:
    What is terminal velocity? Gravity has a range and a speed? Does that mean if the earth is X big, it has a terminal velocity of 700mph that extends so and so far (our atmosphere), but the sun is 10000000X big so it has one of 7,000,000,000mph and extends as far as our solar system? Why do certain things get caught in orbit and certain things get pulled? Why would the momentum of the sun's pull not just blast a meteor straight through us? Does the gravitational pull get stronger as a object gets closer? I thought Newton said gravity was constant, but it wouldn't make sense as to why the gravitational pull is called weak in the kuiper belt. Can gravitational pull be graphed? Is it linear?

    Sorry, I'm just new to physics and the more I learn the more questions I have. I've only been studying it for a month or so now and I'm sure there's a few years worth of things to learn, if not a few decades. I find all these things so interesting and I'm just waiting until the school year starts to take a class and annoy the crap out of my professor with a million questions about how the universe works. Anyone who is patient enough, or finds teaching rewarding, and takes time to answer me, I am very grateful to.
     
  2. jcsd
  3. Dec 7, 2011 #2
    Terminal velocity is different for every object. It depends on the local strength of gravity, the thickness of the atmosphere, the falling object's mass, its shape and possibly other variables that are not occurring to me. TV is the limit of how fast an object can fall (purely by the force of gravity) before the strength of gravity equals atmospheric resistance to further acceleration.

    As for gravity - it decreases by the square of the distance (along with just about any other spherically radiating phenomena if I am not mistaken). So if you increase your distance from a source of gravity by a factor of two the gravitational force will be 4 times weaker.
     
    Last edited: Dec 7, 2011
  4. Dec 7, 2011 #3

    Drakkith

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    Staff: Mentor

    Mostly the Earth is simply in the way. The Earth's gravity is much smaller than the Suns, however meteors only get pulled in when they get very close to the Earth anyways.

    Terminal Velocity from wikipedia:

    The fluid is the atmosphere in the case of meteors. In space there is almost no drag. In fact, until you reach a sizeable fraction of the speed of light you can effectively think of it as a perfect vacuum for most purposes. A key thing to realize is that as an object falls into the atmosphere it's terminal velocity actually decreases as it descends because the atmosphere is thickening and drag is increasing. Meteors are moving much much faster than terminal velocity though.

    Gravity acts kind of like electric charge except that there is only one "charge" instead of two and everything attracts everything else, there is never a repulsion. The further away from an object you are the less it's gravity pulls on you. Gravity increases as the MASS of an object increases, not it's size. Mass is similar to weight, but mass does not change when you move to somewhere with less gravity such as the Moon.
     
  5. Dec 7, 2011 #4

    rcgldr

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    Homework Helper

    The heat is mostly due to air pressure (compression) caused by the very high speeds.

    Yes for the most part.

    Wiki article:

    http://en.wikipedia.org/wiki/Meteoroid
     
  6. Dec 7, 2011 #5
    You know, it did not occur to me that TV applies in liquids as well as gasses. Of course, it makes total sense in hindsight.
     
  7. Dec 7, 2011 #6
    Thanks guys. That clarifies a lot for me but I will look into Terminal Velocity and Fluid Dynamics some more, as it's not really clicking 100% for me. And I should of used "mass" instead of the "big", which was a silly simplification.
    To put it in real layman's terms:
    Space (the areas in between atmospheres) is a vacuum, meaning it lacks the nitrogen/oxygen/etc. that is in our atmosphere, so these things don't "get in it's way" or cause friction, leaving a meteor free to fly as fast as it's little heart desires (you put it as it has "no drag"). When it enters our atmosphere, the "drag" (caused by an ever-nearing denser atmosphere here) causes the (compression/friction/whatever) energy transfer, that slows it down and heats it up (just like slamming on the brakes of a car heats up the brakes/tires). They may not reach terminal velocity but they "want" to, sometimes burning up or crashing as meteorites.
    I read the wikipedia article, and it states how a skydriver reached much higher speeds in higher altitudes because of less drag. It's interesting that the (lack of) drag increases an object's speed at high altitudes, and then the gravity increases the speed as an object gets closer to the mass that's attracting it. Does this mean that the skydiver would heat up if he continued to dive all the way into an area of more drag?

    You'd think it'd be a simple linear progression that is strongest on the face of the earth and get's weaker and weaker. But I guess it's not, because the Earth's gravitational pull (or I guess now I should call it the object's Terminal Velocity) is actually strongest at a high altitude with much less drag (as the skydiver example shows in the wikipedia article).
    So, I was oversimplifying things by thinking it was just gravitational force. I had to incorporate the drag into my thought. It's still interesting to me. If the atmosphere was constant/smooth/even, the speed as something crashed into the earth would gain speed. Is there an equation that combines Mass, Distance and Gravitational Pull?

    And why don't physicist see gravity as a bonding force at the atomic level? Why do they explain (quark) connections by some "gluon" and not believe it can be gravity? I understand that the overall consensus is that something has to be massive to have a gravitational pull, but isn't a nucleus massive to an electron, and even at subatomic levels couldn't things be relatively massive to the other things, causing a gravitational pull?
     
  8. Dec 7, 2011 #7
    Okay, so stuff reaches terminal velocity cuz the drag force is a resistive force proportional to the velocity of the object moving through it. F= -bv, and b can be some function but usually it can be represented more simply. Terminal velocity is reached at around five or four tau- I forget. Tau is a time constant that is a function of mass and the proportionality constant of the drag force. Essentially, the drag force and mg oppose each other. When they are equal terminal velocity is reached- no acceleration.

    Gravity is too far weak on the atomic level so it is not considered. It is 4x10^36 times weaker than the electromagnetic force. The reason gravity is powerful on a large scale is because celestial bodies are massive and their electromagnetic forces are not strong. Subatomic particles have large electric charges in proportion to their masses. This along with the fact that the gravitational constant is way smaller than the electromagnetic constant.

    The nucleus is huge compared to the electron. However, remember that Fg=gm1m2/r^2. The fact that the electron has such a low mass only weakens the force of gravity between the nucleus and the electron. On the other hand, the ratio of electric charge between the electron and the nucleus is much higher.

    Sorry if my answer isn't very clear. I'm just a high school student with an interest in physics, but I'll be glad to answer any more questions you have!
     
  9. Dec 7, 2011 #8
    The difference in the force of gravity throughout a skydiver's decent is negligible. I believe the highest skydive was done at around 31Km. At this altitude, gravity isn't much lower than at the surface. However, the atmosphere is significantly thinner.

    Gravitational force is calculated with respect to the center of the Earth. The distance between the surface and the center of the earth (radius) is approximately 6,371 km. Moving 31km farther from the center isn't going to change that force appreciably.

    Skydivers never obtain high enough speeds to heat appreciably. When they jump, they are effectively starting with 0 speed with respect to the surface. In contrast, an object such as a meteor enters the atmosphere with a much greater speed.

    The gravitational force between two objects can be calculated with:
    [tex]F = G\frac{m_1 m_2}{r^2}[/tex]
    • F is the Force in Newtons
    • G is the gravitational constant : 6.67300 × 10-11 m3 kg-1
    • m1 is the mass of object one in Kg
    • m2 is the mass of object two in Kg
    • r is the distance between the center of m1 and m2 in meters.
    You may find this FAQ entry interesting:
    https://www.physicsforums.com/showthread.php?t=511172 [Broken]
     
    Last edited by a moderator: May 5, 2017
  10. Dec 8, 2011 #9
    Thanks !!

    So this explains why "pressures" are stronger the deeper you go? Is pressure caused by the stronger gravitational force, or are they very related?

    And this would also lead us to believing our core is super dense, thus pulling everything into it, and things get denser the deeper we go. Heck if we dug really deep there could be a whole lot of diamonds in there, huh? :wink:
     
  11. Dec 8, 2011 #10
    In Einstein's General Relativity, did he rewrite the rules for Gravity and the Gravitational Constant?
     
  12. Dec 8, 2011 #11
    And lastly, if the gravitational forces are so much weaker than electric at the atomic level: Why do electrons move in an "orbit"? If the electric forces were so strong, wouldn't the negatively charged electrons be super attracted to the positively charged protons and they would be clumped together?

    Is it an electrical orbit? That seems like the obvious answer. Have scientist recreated an electrical orbit, since it should be relatively easy to make? (assuming you just take something with a strong positive charge, stabilize it, and push something negatively charged into it's "orbit")
     
  13. Dec 8, 2011 #12
    The pressure at a point below the surface of the earth is caused by the weight of the matter above that point pushing down. Things get a little more complicated when going below the surface. The deeper you go, there is less mass pulling you toward the center. At the very center, there is effectively no gravity. However, there is still the weight of all the matter pushing toward the center. The pressure is greatest there.

    * Since the Earth's density isn't constant- it increases the deeper you go, gravity will increase with depth down to a certain point. (There's more mass per unit volume at the center than nearer the surface.) However you will reach a point where gravity will start decreasing until at the center it is effectively gone.
     
  14. Dec 8, 2011 #13

    Drakkith

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    Electrons do not occupy "orbits". The correct term is called an "orbital". There is a significant difference. See here: (It is MUCH different than what you envision it to be) http://en.wikipedia.org/wiki/Atomic_orbital

    Electrically charged particles will emit radiation when accelerated, which means that if the electrons were "orbiting" in the classical sense they would emit radiation, lose energy, and fall down into the nucleus. Which does not happen.
     
  15. Dec 8, 2011 #14
    You can view General Relativity as a modification of Newtonian Gravity. It's needed when dealing with very massive objects- such as black holes- and when precise calculations are necessary. GPS is one example oft cited:
    http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

    However, Newtonian Gravity is accurate enough to place satellites in orbit and send probes to other planets.
     
  16. Dec 8, 2011 #15
    Hmm.. so so so so so so interesting. So, it makes sense, that in the center "gravity would be, (or feel) gone" because you've entered into the point where gravity is trying to take you. You've made it there, now there is no gravity pulling you, right? But you're also saying that gravity would not continue to get stronger as we went further into the center? I would think that to get out of the very center of the earth would take incredible amounts of energy; more than anywhere else between the crust and the core, either because of the gravity or the pressure caused by matter "on top" of us, which seems pretty interchangeable, doesn't it?

    The picture you describe would almost make the earth hallow-ish, but it cannot be hallow or even have a spam that becomes less dense. Shouldn't the center of the earth be the most compact/dense part because of all the earth's mass is being pulled in on it?

    If were on the crust (like we are) gravity is pretty weak, but if we were halfway down, gravity would be stronger, but then if we were really really close to the center of the earth gravity wouldn't be incredibly strong (like the event horizon on a black hole)?

    I'm going to dedicate some more time to gravity it so I don't make silly mistakes or ask silly questions.
     
  17. Dec 8, 2011 #16
    Wow awesome, thanks!

    So is it right so say Newtonian Gravity isn't accurate enough to make the nano-second calculations needed in GPS, but Einstein's General Relativity is? Shouldn't we look for the calculation of gravity to be exact and not "accurate enough" or we might make errors in precise calculations? (planets are big and orbits are forgiving)
     
  18. Dec 8, 2011 #17
    Thanks, although with my knowledge today I can't comprehend an orbital, or really want to buy into quantum mechanics or accept the uncertainty principal... until I am forced to agree. I have a problem with a statement that starts "It is impossible to know..". I understand the problem with Bohr's model of an atom, that the electron would crash into the nucleus rather quickly, but to explain by taking something as complex, incomprehensible, and mysterious as quantum mechanics seems... out of order. The rest of physics is comprehensible; even general and special relativity seem nice and simple in comparison. Can anyone here really say they understand quantum physics and that is makes sense to them? Are we really supposed to use 2 different physics when things go from macroscopic to subatomic? Do any of us lean towards making things simpler, rather than more confusing? Couldn't there be a plethora of other reasons why an electron doesn't fall into the nucleus? Perhaps our conceptions of an atom are just ever-so slightly off and there's a missing piece to the puzzle? Don't mean to make uneducated and outlandish statements, I just have this belief, kind-of wonder if anyone else does.. and I like to discover things for myself.


    Anyway, thanks to all of you for your patience, time, and thorough answers. You've clarified some things that in retrospect were somewhat simple, but more than that you've clarified that fact that I need to read up more on this stuff and take some classes if I'm so interested. Thanks to all.
     
  19. Dec 8, 2011 #18
    I'm not sure if anyone touched on the "why are meteors so fast" question. They are fast because they orbit around the sun, and when you do that some fast speeds are involved. The earth orbits around the sun at ~60,000 mph for example.

    However, on top of that there is a concept called "escape velocity", which is how fast a rocket in orbit around the earth has to increase it's speed to in order to escape from earth's gravity. It's about ~24,000 mph. Any less, and the rocket would fly up up and away, but as it goes higher gravity would slow it down and would eventually get pulled back down (like a car going up hill). As it gets pulled down it speeds up again and the cycle continues. BUT, if it goes ~24,000 mph or faster, then it still slows down as it goes farther away, but it goes so fast away from the earth that by the time it slows all the way down, earth's gravity has dimished so much that it will never actually stop it...and the rocket will keep going up forever.

    This works the same in reverse. If a rocket (or meteor) started from rest far away from the earth, but close enough to get pulled in, it will accelerate to ~24,000 mph before it hits the earth's atmosphere. This means that all meters must be going at least that fast when entering earth's atmosphere.

    As for quantum physics...you are right in thinking that nobody truly understands it. If they think they do, then they don't. It 's a theory that doesn't make any common sense and nobody likes it. Nevertheless, it's not up to scientists to pick a theory that they like or not, it's up to them to pick one that explains natural phenomena. Quantum theory happens to be VERY good at this, whether we like it or not. It (or gravity) has some problems, mainly that the two don't work together. Figuring out how to make the two work together is pretty much the holy grail of physics.
     
  20. Dec 8, 2011 #19

    Drakkith

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    Gravity does not disappear in the center of the Earth. It is simply that gravity is pulling every point in your body equally in every direction, resulting in zero NET force. Initially as you dig down the force you feel increases because of the way the Earth's interior is. However about a third of the way down or so there starts to be so much matter behind you that the net pull starts to decrease at which point it becomes zero in the core.

    Yes, the core of the Earth is the densest part at 12.8–13.1 g/cm^3.
     
  21. Dec 8, 2011 #20
    Unless I am mistaken, aside from pure mathematics, there is no such thing as an exact measurement. There is always a margin of error.
     
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