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Watsisname
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13 Dec 2016 00:32

Your explanation of the origin of centrifugal force is correct, but there is more subtlety behind it, as gnargenox explains.  An xkcd comic also said it well.

Centrifugal forces appear when you formulate the laws of motion in certain accelerated or non-inertial frames of reference.  What you really mean when you say "there's no such thing" is that they are "fictitious" -- but this has a very specific meaning in physics.  It means the forces can be made to vanish by adopting an appropriate frame of reference, where the accelerated motion of the body is produced by a force which is actually moderated by an interaction -- like a normal force, or tension.  

Usually when people are learning about laws of motion and forces, they try to emphasize physics in inertial frames of reference, and any other non-inertial forces are just "illusions" which can be readily explained by inertia + 'real' forces.  This is common teaching and it often leads people to dismiss things like centrifugal force.  But I think that is too narrow.  Physics with fictitious forces is just as valid as physics without them. In fact, we analyze problems in noninertial frames and call upon fictitious forces as if they are real all the time.  For example, we do it when modelling atmospheric motions for weather forecasting (Coriolis force is a fictitious force, but we use it in the equations!  Why?  Because it's way easier!)

There's a philosophical side to this as well.  Suppose you are in a sealed crate which is being swung around in a large slow circle. If you have no knowledge of the world exterior to your crate, how do you know you are being pressed against the side because it is moving in a circle and your body is trying to follow a straight line?  How do you know the crate isn't simply sitting on its side on the surface of the Earth? What experiment could you perform inside the crate that would unambiguously distinguish between these two situations?

This very logic (it's actually the essence of the Equivalence Principle) leads us to conclude that gravitation is a fictitious force, too!  What you "feel" as gravity only appears because you are in an accelerated frame -- the surface of the Earth with respect to coordinates which are in freefall.  And freefall is the most inertial frame of reference there is!  If you are in a frame of freefall, the force due to gravity disappears as far as any localized experiment can tell!  

So every time you do physics and say there is a gravitational force F=mg acting on something, you are actually invoking a fictitious force.  Calling upon fictitious forces can be a very useful way of describing nature.  It just depends on your perspective. =)

-edited (hopefully) for clarity
 
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13 Dec 2016 08:21

just learned in physics class how to caculate the gravity force between 2 masses.
i started to caculate crazy stuff,
i caculated the gravity force between me to an ant that located in los angeles.
and even hypoteticly caculated the gravity force between me and some ameba that located on the moon :lol:
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13 Dec 2016 08:43

Great explanation Watsisname
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14 Dec 2016 03:56

Fun fact:  If you stand on top of the great pyramid of Giza, the gravitational force on you due to the Pyramid is equal to the gravitational force on you due to the Moon.


Coincidence?  I THINK NOT.
 
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14 Dec 2016 04:13

Watsisname wrote:
Fun fact:  If you stand on top of the great pyramid of Giza, the gravitational force on you due to the Pyramid is equal to the gravitational force on you due to the Moon.

Isn't that surprising?  Imagine we remove Earth, so you float in space and have this huge pyramid blocking almost your entire view on one side and that tiny moon on the other side, yet the moon has equal pull?
NIL DIFFICILE VOLENTI
 
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14 Dec 2016 08:26

It's pretty neat.  I actually didn't know that until I tried playing around with the calculations.  And to be fair, the 'equality' of the two depends on things like the distance of the Moon in its elliptical orbit, and precisely finding the force due to the pyramid not easy given its shape.  Actually, it's an impressively difficult math problem -- one must consider the force due to each infinitesimally thin square slab of the pyramid, accounting for the average distance from the top vertex of the pyramid to any point in that slab, and integrate over the height of the pyramid.  But if you work it out and consider when the Moon is at periapsis, the two forces come out to be well within a factor of 2 of each other.  Which is pretty amazing, considering I could easily imagine them being many orders of magnitude different!

You can also convince yourself that this is true by just considering the center of mass of the pyramid (or guessing where it must be).  The pyramid is 139 meters high, 230 meters wide at the base, and weighs 5.9x109 kg.  You might guess the center of mass is around 1/3 of the height, or about 90 meters below you if you stand on top.  Now compare mass/distance of the pyramid to mass/distance of moon.  The moon is 1.2x1013 times more massive, but 4.0x106 times farther away.  The ratio of the masses is very close to the square of the ratio of distances, so the forces are close to equal!
 
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14 Dec 2016 11:37

what is the mass of the pyrmide? lets caculate that :d
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14 Dec 2016 12:39

Gravity is also influenced by energy levels around it and that includes heat.

Temperature is a measure of energy density, and an energy density of any type will contribute to the stress-energy tensor, a quantity that determines the spacetime distortion. So all things being equal, a clock in a region of higher temperature, higher energy density, will run a little more slowly compared to a clock that is in a lower temperature region, and a little more slowly still compared to the minimum energy density or absolute zero.

Gravity is related to temperature in two ways, and the two of them work in opposite directions. Energy adds to gravity. Heating things just a little bit, like bringing a pot of water from 0 °C to 100 °C, the heat would contribute a very negligible amount of gravity, something in the order of 5 x 10^-3 nanograms. But the water would simultaneously expand (assuming it was liquid water to start with and not solid). This expansion would decrease its gravity. Where you see significant influences from temperature is in very hot objects such as stars.

Gravitational anomaly map of Earth
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14 Dec 2016 20:58

The gravitation-producing mass-energy equivalence is also important in cosmology.  It is not only the density of matter which causes the universe's expansion rate to change with time, but also the density of energy (for example in the form of light, relativistic particles, or even a cosmological constant).  This is shown in the Friedmann equations, which describe the expansion rate, and its time derivative (the acceleration), in relation to what the universe is made of.

Spacer wrote:
what is the mass of the pyrmide? lets caculate that :d

Multiply the volume of the pyramid by the density of the rock that it is built from.  You should be able to find the formula for the volume of a square pyramid on google. =)
 
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15 Dec 2016 11:27

Ran across this Black Hole in a cluster of stars above the Circinus Galaxy while exploring last night. The first I've run across with an asteroid belt orbiting it, but as usual it was a binary with a giant star. Even a few scorched Selenas circled it. So I just had to touch it! Once I landed and miraculously wasn't speghettified, I saw the familiar blue shifting of light as it fell into the gravity well I stood in. And I wondered how light could be going faster than light to be blue shifted so much. And I also remembered, as light passes a rotating object it is sped up on the side that rotates in the same direction and when it passes a rotating object on the side opposite of the rotation light is slowed down. Not quite sure how this happens.

Hopefully in this clip below you can see the blue, shiney asteroids orbiting seem to jump and flicker, leaving behind a double image. Again, I don't understand the mechanisms of this. Can anyone elaborate?

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15 Dec 2016 12:44

Gnargenox wrote:
And I also remembered, as light passes a rotating object it is sped up on the side that rotates in the same direction and when it passes a rotating object on the side opposite of the rotation light is slowed down. Not quite sure how this happens.

The light still locally moves with a speed c through the space, but around a rotating mass the space itself is also being dragged around.  This is a general relativistic effect known as 'frame dragging', where the angular momentum of an object causes a further change to the space-time metric. For a rotating black hole this is given by Kerr's metric, where at a particular distance outside the event horizon the frame dragging is severe enough to prevent light (and anything else) from being able to move against the hole's rotation.  But all rotating masses (while not described by Kerr's metric exactly) exhibit this effect to some degree -- even the Earth causes some frame dragging, which was directly measured by the Gravity Probe B experiment.  Juno should also demonstrate the effect as it orbits Jupiter.
 
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17 Dec 2016 01:28

Watsisname, funny thought i just had. if i will stand on titan with no helmet (lets say we can breate there)
will the air smell like farts? its methane and ethane all over there.  :lol:
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17 Dec 2016 01:40

Nope!  Methane and ethane are both odorless. :)  The smelly stuff in a fart is mostly hydrogen sulfide and other sulfur-containing compounds.
 
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17 Dec 2016 02:03

Watsisname wrote:
...and other sulfur-containing compounds.

That's right. Also people think that's the odor of Sulfur, but Sulfur is odorless!
(as you said, it's the hydrogen sulfide that smells like egg or... yeah, farts)
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17 Dec 2016 02:23

All my life was a lie!!
All this time i thought its the methane that smell like that.
Thanks for clearing it up!
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