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Gravitation Questions

  1. Oct 12, 2004 #1
    This is my first post here, so I will introduce myself. I'm Allen, just a 16 year old interested in physics and eager to learn more.

    So I was reading Brian Greene's The Elegant Universe and a few ideas and examples involving gravity dawned on me. I am sure that they have been addressed before, and I am also sure that there are theories pertaining to them. Anyway, without further ado, here is my proposition put forth for your analysis.

    The law of conservation of energy, as well as Newton's third law can be summed up together to say (which I have labeled as my golden rule of physics, no matter what experiment I am working): Everything must be accounted for, everything. So, in theory, the following statement would be true. The total amount of gravitational force exerted by an object must equal the amount of gravitational force simultaneously acting upon said object. Also, it is well known that the amount of gravity is proportional to an object's mass. Keeping these two things in mind at all times, I'll continue with an idea.

    (I brought this next topic up to my science class last year and two people grasped it. I was one, and the teacher was the other)

    The mass of the Earth, is constant, unless something enters or leaves the atmosphere. Some may argue that overpopulation and industrialism must make the mass rise. However, for every bit that someone grows, or for every building that is constructed, something must be taken from the environment. The Earth's mass never changes. My class was dumbfounded, everyone argued that overpopulation MUST have an effect on the mass of the Earth. Well anyways, that is what got me thinking that everything must be accounted and compensated for.

    After reading Greene's book, I began to think more about gravity, something must be missing, and I want to find out what that something is. I began thinking of a new way to describe gravity, and I am still working on a theory. I do, however, have an example that I would like to share with you all, and get your feedback.

    (This is hard to type out, and to keep my thoughts organized, so bear with me, voice any questions you have and I would be glad to answer them. Trust me, when verbally announced, the following actually makes sense.)​

    Say, for example, you have a splendid new shiny bowling ball. It is a beautiful bowling ball indeed. The bowling ball has mass, therefore it has a certain amount of gravity proportional to said mass. So why then, don't things with a smaller gravity than your new bowling ball become attracted to it? Because, my dear, the whole of the ball's gravity is being exerted directly on the Earth. The Earth has a huge amount of mass, therefore it has a proportionately huge amount of gravity to exert. Although 100% of the ball's gravity is being used upon the Earth, only a miniscule fraction of the earth's gravity is being used on our shiny spherical buddy. So, the Earth is cancelling out all gravity exerted by the bowling ball, and the bowling ball is cancelling out a tiny fraction of the Earth's sweet gravity. Where does the cancelled gravity go, one might ask? That is what I will next attempt to explain.

    So here we are, with our bowling ball in hand, prepared to launch it by cannon, into the earth's orbit. If we do this, how can we expect gravity to react? Well, everything must be accounted for. So, still, since the earth is amazingly larger than the ball, all of the ball's gravitation, even whilst in orbit, will be cancelled out by the earth (after all, it is still in the Earth's orbit). What about the Earth's tiny fraction of gravity, you might ask? Well 100% of that exact small fraction of gravitational force would, in theory, still be going towards the cancellation of your bowling ball's gravitational force, to keep it in orbit.

    So what would happen then, if we launched the ball even further, to just outside of the earths gravitational field entirely? Well, according to my theory, the bowling ball, would then attain it's own gravitational field (although it would be a small one, compared to that of the Earth). Where would it get this newfound gravitational force? And what about the tiny fraction of the Earth's gravitation that was being used to cancel the bowling ball's out? Well, according to theory, that exciting new gravitational field that the bowling ball has, was "borrowed" from that of the Earth. HUH?!!!?

    Well think of it this way, it is sort of like a "gravitational displacement". The earth has lost a bowling ball's amount of mass, and therefore has also lost a bowling ball's amount of gravitational force. While the ball was on the planet, and inside the planet's gravity, Earth's gravitational field was static, not changing, for the field, as a whole, had not lost or gained anything. Now as soon as the ball left the field, it packed it's bags and took it's gravity with it. So now, the Earth's gravitational field as a whole is smaller, because of the lost (or, no longer cancelled) gravity.

    So, this raises an interesting question about gravityas well as mass. If gravity is proportionate to mass, why then would the total gravitational field of the Earth not change while the bowling ball is in orbit? After all, the ball isn't directly part of the planet's mass any longer. Also, is it not gravity directly that brings objects together? Or, is it merely the cancellation of gravity between two objects that brings them together so that the gravitational field may remain static?

    Well, It is late, and I am tired. So take my two cents and please give me feedback. I am just a sophomore in highschool interested in this sort of thing. I am guessing beforehand that I am way out of line with this theory, however helpful critisism would be greatly appreciated. Also, I am just 16 years old, If this whole theory has already been stated and addressed, so be it, I would be glad because I thought of it on my own.

    Cheers everyone,
  2. jcsd
  3. Oct 12, 2004 #2
    Sorry to reply to my own post, but I also apologize for the lengthiness of the post before this.
  4. Oct 13, 2004 #3
    Every object in the universe attracts every other object in the universe with a force proportional to the product of their masses and divided by the square of the distance between them.

    So, your bowling ball does attract the other smaller balls you postulate. It's just that the force is so small due to their small masses you'd need very sensitive instruments to see it. The earth doesn't suck up all the gravity in the bowling ball. In fact, your bowling ball exerts a gravitational force on Jupiter, albeit very very tiny.
  5. Oct 13, 2004 #4
    Actually the Earth is not as isolated as you think.

    All the light that we can see from the sun and stars is electromagnetic radiation which carries energy. The energy of the sun is used by our plants and coverted into mass and food. The light of the sun excites the atoms on the surface of the oceans which in turn radiate their own photons out to our eyes so we can see them.

    This all adds energy and mass to the earth.

    Nevertheless geologists have shown up that Earth exists in a dynamic energy equilibrium, where the energy taken in is approximatly matched by the outgoing energy. This will vary in the short run but in the long run it is at equilibrium like you said. Overpopulation won't increase the mass of the Earth, but that doesn't mean that the mass of the Earth isn't changing, however slight.

    Oh by the way, as Einstein showed with E=mc^2, mass and energy are equivalent. So you need to consider both.

    The rest of your post lacks a clear purpose because you need to make some better definitions.

    For example, you said:

    To which I would reply, "Was it ever?"

    You need to define what is considered in the System of the Earth, and what is not. If you define the bowling ball as part of the system of the Earth, then it doesn't matter where it is, it contributes to the Earth's mass. It could be at the other side of the galaxy and it would still contribute. It could be sucked into a black hole, but still, according to the definition, it would contribute to the Earth's system.

    So you need to clearly define the systems you are working in, to track the energy inputs and outputs from those systems.

    Here's an encyclopedia thing that includes the formal definition of a system. The concept of a "system" is extremely important when talking about thermodynamics.

    Last edited: Oct 13, 2004
  6. Oct 13, 2004 #5
    Thank you for feedback, as I expected, I was completely wrong. However, I have some questions now.

    The reply from subodei, "was it ever?" raises an interesting question. If the ball, which was spawned and created on and from the Earth was never a part of the mass, why does the Earths gravitation increase when they work together? Is mass a "communty effort" of particles?

    With Einstein's special relativity, gravity is just an indention in the fabric of space, made by anything that has mass. I used plastic wrap, pulled tightly over the edges of a hula hoop to try and discover on my own how this works (in a 2d fashion). I used many different sizes of marbles to try and demonstrate different masses. I placed the largest marble on, first, and of course it went straight to the center, and made an indention. Next, I placed a small marble on the plastic and observed the bending of the wrap and observed what I saw. While closer to the outside of the circle (not on the circle's edge, because the hoop prevented bending), there was an indention change in the center, but not much. However, the indention made by the smaller marble was deep. As the small marble approached the center due to "gravity of the large marble", I saw that the indention made by the small marble itself grew smaller and smaller, however the overall indention grew.

    I guess this was a good example of what I was saying with the "borrowing" of gravity from one mass to another. When the marbles were close together, the gravitational pull was stronger (depth of indention). When they were further apart, the pull of the stronger marble was weaker (less mass than when touching?). However, the total area affected by the pull never changed, while the small marble was outside of the larger's indention, the total area of indention for the larger was smaller. As the smaller was pushed into the larger's area of indention, the total area of indention grew. Once the area of the smaller marble's indention was completely inside that of the larger marble, the total area was static, not changing, even though the depth changed. What does this mean? Is there one single total amount of gravity in the universe? Like a gravity bank, loaning itself out to it's single investor, mass.

    I find myself constantly thinking about this sort of thing. Only after I started reading Brian Greene's book, have these thoughts had any real meaning.
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