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| Thread: A hoax called gravity. |  This thread is pages long: 1 2 3 4 5 · «PREV |
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Minion
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posted July 04, 2008 09:07 PM |
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I think it is strong nuclear force, but it is not of that much significance. And I'll add that all other forces in the universe are ultimately derived from these four fundamental forces, that alcibiades meantioned.
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TheDeath
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posted July 04, 2008 09:09 PM |
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No particle actually "exists" it's just some form of field or wave (because they are invisible) in the shape of spheres/whatever.
I think all forces have been combined recently in a theory, except gravity which still causes trouble 
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alcibiades
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posted July 04, 2008 09:48 PM |
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Quote:
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Hehe, please read Alchibiades post. It is almost right on top of yours. Can't miss it 
I understand, but I want a "why" not just "because it's how we observed until now".
I mean, what's gravity and why it pulls objects with higher mass with a higher force?
I don't think you have understood that it is mass that causes gravitation. The greater the mass, the greater the gravitational field it has.
But if we are talking about gravity on a planet like Earth, it has massive gravitational field. Unlike a pen. If you look from Earths perspective, it doesn't matter if an object weights 10kg or 1000kg. It's mass is huge enough to give any object the same acceleration. The same is not true with an object with the mass of a Moon, and distance matters too of course.
To answer the question why two masses attract each other, the best answer is probably because! - or more elaborate: Because nature made it so. Now this may seem like a very dissatisfactory explanation, but some things we just have to take for facts. I bet you, if you were all children, you would have much easier learning Newtonian mechanics! You doubt that? Well, let me give you an example:
When I was a kid, I had a wooden train. It consisted of a number of wagons that could be put together. In the end of each wagon there was a magnet, one was red, the other green. I pretty soon figured out that the red magnets attracted the green magnets, allowing me to stick the wagons together. However, if I tried to put a green and a green magnet together, they would detract each other, pushing the wagons apart.
I'm pretty sure most of you have had experiences like the above. The above is an example of the magnetic force, something we're all pretty familiar with. We all know that there are two magnetic poles: A positive pole and a negative pole, which attracts the other part but detracts its own part. However, if I were to question you: Why is it so? I guess most of you would be pretty clueless! None-the-less, this is not something we wonder much about in our everyday lives: We long since accepted, that so is the nature of magnets!
Now, mass works pretty much the same way as magnets, with one crucial difference: The is no such things as "mass poles", i.e. there is no negative mass and no positive mass. All massive objects attract each other! Why is it so? We don't know, we just have to accept that this is an inherent property of mass, in fact, when we feel massive objects as having weight, it is a reflection of this very property.
This is a very crucial step in learning, really: We have to accept a fact without understanding why. All we can say is: We can observe that it is so.
Now, once we've accepted the above, let's move on and take a closer look at the actual formulas. Newton's general law of gravitation looks like this:
Notice, that both objects, m1 and m2, are subject to this equal force, which is directed towards the other body.
Notice, that we can write the formula:
F = m * GM/r^2
When we compare to Newton's second (I think it is) law: F = m * a, we see that all objects on the surface of the Earth is subject to an acceleration of:
g = GM/r^2
Where M = mass of Earth of r = radius of Earth. r is actually not constant, if you're on a mountain, you'll feel less gravity, and hence, you're weight (but not your mass!) will be lower. Remember, that you weight is the force gravity causes your mass to exhert on the ground below you. Those details aside, we see that the accelerationfrom gravity on Earth is (almost) constant ~ 9.82 m/s^2.
Notice that this force will give your body an acceleration towards Earth. Earth similarly experiences an acceleration towards you:
e = Gm/r^2
but since m <<<<< M, e <<<<< g, hence this acceleration is very small. Furthermore, since the number of objects are approcimately equally distributed on Earth, these pulls even out. On the other hand, the gravitational pull from the Moon is significant, causing the tides of the ocean, as well as the gravitation from the sun, which keeps the Earth in it's orbit!
I will repeat from earlier, that it is only the acceleration a body experiences that determines how fast it moves. Since all bodies experience the same gravitational acceleration on surface of Earth, all bodies fall equally fast.
However, the objects will gain different Momentum. Momentum is defined as: P = m * v and is also known as "amount of movement". This is what Death talks about when he says that a feather that hits you with 50 km/h will do less damage than a train that hits you with 50 km/h!
(One of) the reasons we're hurt when we're hit by a car is because of a principle which is the preservation of Momentum. This means that when a car hit's you (and if we imagine the car stops when it hits you) it transfers it's momentum to you. Now as your mass is much less than that of the car, to gain the same momentum you need a much higher speed, which essentially means you gain a lot of energy of movement, which is not healthy for you! On the other hand, if you're hit by something which is much lighter than you, you hardly feel it, unless it's moving really, really fast!
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Minion
Legendary Hero
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posted July 05, 2008 10:49 AM |
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Thanks again for the comprehensive elaboration 
However that is Newtonian mechanics, and as we know that is not entirely accurate way to model gravitation. It is accurate enough for most calculations yes, but to be closer to the truth you need (tadaa!) general relativity. So maybe a brief look that way too?
I am especially looking forward to examples with calculations 
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"These friends probably started using condoms after having produced the most optimum amount of offsprings. Kudos to them for showing at least some restraint" - Tsar-ivor
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Lord_Woock
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posted July 05, 2008 12:20 PM |
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Look, people, it's all really simple.
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alcibiades
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posted July 05, 2008 05:19 PM |
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That sort of ends the discussion doesn't it?
But to remain on topic, yes, Newtonian Mechanics are only approximations under "classical" conditions, i.e. v << c etc. I won't go into relativistic stuff. I've had courses in specialized relativity, which is pretty bad in itself. I have only briefly browsed generalized relativity and I wouldn't ever try teaching it real life, much less here. 
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