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Tue 14 Nov, 2006 10:48 am
Why are the Earth and Plants ROUND
why do you think they are?
Gravity attracted particles in the solar nebula into planets uniformly. Because the force is uniform and exists between all particles in a planet it acts as a squeezing force to hold it all together just as if you were clenching a fistfull of sand with your fist.
Conservaton of Energy.
Spheres are the most efficient three dimensional shape for enclosing a given space.
Soap bubbles tend to form spheres due to surface tension. Planets (and stars) tend to form spheres due to gravity. The forces are different, but the underlying physics which makes them form spheres and not cubes or pyramids, is conservation of energy.
(the reason I said 'tend to' form above is because external forces can distort the basic predisposition to form a sphere. Large bubbles are distorted by many things, and stars and planets are rarely perfect spheres. But under ideal conditions, they would be spheres.)
No, it is not conservation of energy. That is completely different.
Actually, its clear evidence Einstein was right when he said "God does not play dice with the universe" ... as anyone can see, the game is marbles.
timberlandko wrote:Actually, its clear evidence Einstein was right when he said "God does not play dice with the universe" ... as anyone can see, the game is marbles.
This is why you should hang out at A2K, Portslade.
You don't get this kinda wisdom just anywhere!
stuh505 wrote:No, it is not conservation of energy. That is completely different.
The underlying physics which makes things form spheres and not cubes or pyramids, is conservation of energy.
If you think it's something different, then please specify and explain.
Well, there are a myriad of reasons, most of which can be ignored because we assume them to be true due to our innate conceptions of reality...but the fundamental reasons why planets are roughly spherical are Newton's laws of motion 1-3 + Newton's law Gravitation.
If a planet were composed of a set of uniform classical particles, a non-spherical shape would imply that there were some particles that have a non-zero net force. This force will move the particle into a direction that makes the planet more spherical, and result in a reduction of the magnitude of the net force.
It is provable (although I will not do the proof) that every force will move particles into a state where there is less than or equal net force than before. Therefore it will converge to a local minima in terms of net force.
The global minima of net force would be a sphere because in a spherical configuration there is zero net force (according to Newton's first and third laws). Therefore, given enough time, any collection of uniform particles will form a roughly spherical shape.
Gravition + law 2 explain planetary differentiation and allow us to assume roughly uniform size and density of particles at any given radius.
Conservation of energy has absolutely nothing to do with any of this. To demonstrate that, assume a hypothetical universe where the law does not exist. Assume that in this universe, the strength of gravity increases logarithmically as a function of time.
Note that, although energy is no longer conserved, the planet will continue to heat up over time, but will converge to a spherical shape even faster than in our actual universe.
stuh505 wrote:Well, there are a myriad of reasons, most of which can be ignored because we assume them to be true due to our innate conceptions of reality...but the fundamental reasons why planets are roughly spherical are Newton's laws of motion 1-3 + Newton's law Gravitation.
If a planet were composed of a set of uniform classical particles, a non-spherical shape would imply that there were some particles that have a non-zero net force. This force will move the particle into a direction that makes the planet more spherical, and result in a reduction of the magnitude of the net force.
It is provable (although I will not do the proof) that every force will move particles into a state where there is less than or equal net force than before. Therefore it will converge to a local minima in terms of net force.
The global minima of net force would be a sphere because in a spherical configuration there is zero net force (according to Newton's first and third laws). Therefore, given enough time, any collection of uniform particles will form a roughly spherical shape.
Gravition + law 2 explain planetary differentiation and allow us to assume roughly uniform size and density of particles at any given radius.
Conservation of energy has absolutely nothing to do with any of this. To demonstrate that, assume a hypothetical universe where the law does not exist. Assume that in this universe, the strength of gravity increases logarithmically as a function of time.
Note that, although energy is no longer conserved, the planet will continue to heat up over time, but will converge to a spherical shape even faster than in our actual universe.
I will agree that there are multiple factors in the physical universe which lead to spherical planets (and spherical objects), but I believe that you are incorrect when you says that Conservation of Energy is not one of those factors.
So my original answer stands: Conservation of Energy. I'll try to defend it with online sources later, but I don't have time right now.
Also, I don't believe you can postulate a Universe in which Conservation of Energy does not exist, and have even the slightest hope of understanding the physics of such a universe (much less to use it as an extrapolated example to support your argument).
Whenever this question comes up...roundness of planets...."shot towers" always come to mind, as in the old way of making lead bullets.
Because God made a right balls up of creation.
Ok, I'm not as educated or intelligent as the rest of you, but don't they form circles because of entropy? As the spinning gas that will make a planet rotates, and contracts under the force of gravity, it will conserve it's angular momentum, but radiate energy. Following this line of thought, won't it form a sphere because in this state it has the least gravitational potential energy overall, compared to a cube or pyramid or whatever, plus it has already radiated energy, following i-don't-know-which law of entropy, that entropy always tends to increase, and thus it goes into the lowest energy state, a spere. Thus other shapes are unstable because they are not the lowet energy state.
I don't know, but that is my humbly uneducated guess...?!!!?
Quincy wrote:Ok, I'm not as educated or intelligent as the rest of you, but don't they form circles because of entropy? As the spinning gas that will make a planet rotates, and contracts under the force of gravity, it will conserve it's angular momentum, but radiate energy. Following this line of thought, won't it form a sphere because in this state it has the least gravitational potential energy overall, compared to a cube or pyramid or whatever, plus it has already radiated energy, following i-don't-know-which law of entropy, that entropy always tends to increase, and thus it goes into the lowest energy state, a spere. Thus other shapes are unstable because they are not the lowet energy state.
I don't know, but that is my humbly uneducated guess...?!!!?
Yes. I think you're making the same basic argument I am, except you're calling it Entropy and I'm calling it Conservation of Energy. I'm not sure which terminology is more accurate.
quincy / rosborne,
you are both confusing the reason that the solar system is a disc in the ecliptic plane with the reason that planets form spheres.
stuh505 wrote:quincy / rosborne,
you are both confusing the reason that the solar system is a disc in the ecliptic plane with the reason that planets form spheres.
No I'm not.
Conservation of Angular Momentum causes the solar system disk, and Conservaton of Energy is the reason spheres form instead of cubes or other shapes.
you keep saying that, and yet I provided a very clear counterexample...
Actually planets and stars are round due simply to the way gravity works. Smaller bodies, such as comets or asteroids, may and do in some instances exhibit irregular shapes, but as a body's mass and density increase, so increases the gravitational force pulling equally on all the body's mass toward the point which is the body's center of gravity. Everything "falls" toward the center; the only shape that can form, given sufficient mass, is a sphere. Of note is that with their lower mass, therefore lower gravity, both our Moon and the planet Mars have steeper, higher mountains and chasms/canyons than does Earth. The Egyptians would have had an easier job building pyramids on Mars; the lower gravity would permit smaller bases and steeper sides relative to height than is the case here on Earth.
Planetary rotation will to some extent distort the shape of a massive rotating body, centrifugal force causing an equatorial bulge; the Earth, for instance, is a few dozen kilometers larger around its equator than it is around its poles, due to rotation around the planet's axis.
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