Gravity

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I feel like such an idiot asking this. If this is a really dumb question, please just be informative and not mean.

Okay, here is my question. Why is there gravity? I was thinking that it has to do with the earth moving at about 66,630 miles an hour and it's also spinning at 1,707 mph. If you spin something around really fast then you can make another object stick on it. I was doing this with a ball and that's why I thought about this question. The moon travels at 2,288 mph so maybe that is why the gravitational pull isn't as great. Isn't it weird how we don't feel like we are moving this fast through space?

Physics really.

Gravity is the force generated by mass. It's kinda hard to explain, but if something has mass, then it has gravity. The Earth has a large mass, so we feel the pull towards the center of that mass. The moon has a quarter of the mass, so the Apollo astronauts weighed less-one sixth-than on Earth. Go to Jupiter, and you'll weigh about a ton.

Other dense masses like nreutron stars have enormous mass for a small size, and have a large force of gravity. Black stars are so dense in mass that gravity becomes so great that not even light can escape that force of gravity.

We all also generate gravity, but the force is so small as to be non-existant.

As for orbiting objects as the moon, it's a balance between centrifugal-flinging out- and centripital-gravity- forces. I once saw Mr. Wizard demonstrate this concept. He made the concept extremely understandable.

It has nothing to do with spinning.

Every object has a gravitational pull, the bigger the object the more gravity it has.

It has nothing to do with movement? But, if mass were the sole cause of a gravitational pull, then why don't objects pull things to them on earth. I mean, a boulder doesn't just attract things here on earth. If the boulder was moving though and something was on it, then that object would cling to it.

As everything around you is moving at the same high speed through space, it will appear to you that you are either standing still or moving at "normal" human speeds around the world. The problem is that the observer (you) are not in a fixed location. I think it is close to impossible to have any truely fixed point in the universe.

By the way

Force due to gravity = G x Mass of object one x Mass of object two / (distance between objects)squared

If it were in an isolated system, without any other masses to cause gravitational effects, the boulder would slowly attract things to it, but it would be barely noticeable and slow because it's so small relative to something like a planet/moon. And no, if it were moving, something would not cling to it just because it's moving. The chapstick tube and quarter on my desk aren't going to suddenly come to each other, either, because there are so many other forces to overcome (friction, moments, etc) and their gravitational attraction is extraordinarily small (think along the lines of the force a virus could push against you).

Using the previously mention equation, I'll give you an example. Say that you have two 1 liter water bottles with a mass of 1kg apiece sitting on a table 1 meter apart from each other.
The gravitational force between the two is F = (G*m1*m2)/r^2 = (6.67428*10^-11 m^3/kg-s^2)*(1kg)*(1kg)/(1m)^2 = 6.67*10^-11 Newtons.

Now let's assume that the coefficient of friction of each water bottle is an underestimate of 0.1, this means that whatever the force is that the bottle pushes against the table, a minimum of 10% of that force is required to push it along the table. So for a 1kg bottle, it pushes against the table approximately 10 Newtons, and therefore a minimum of 1 Newton of force is required to move it along the table.

So 1/(6.67*10^-11) = 14982889540, meaning a force 14982889540 greater than the gravitational force between the two bottles is required to push them together.


Mind you, this is a simplified equation assuming that bodies of mass are points, and the radius is in reference to the radius between the two centers of of each mass (so no, pushing the two bottles together doesn't mean the radius goes to zero and causes the gravitational force to approach infinity).

I once did an estimate of the gravity field of the planet mars acting on a human on earth, it is weak. It is about as strong as that of another person at about 1 meter distance.

The thing is that the inverse square term for the distance makes the effect short ranged. This is common for almost all forces except for friction where the distance has to be zero (I am considering classical physics only).

Just to address the "why" of gravity, although it must be accepted really as a simple fact, if we took general relativity as a sufficient account of gravity (which we probably can't), then gravity is simply a warping in the geometry of spacetime caused by mass. Why mass warps spacetime is another story, but in the relativistic framework gravity is not a force exactly but is reduced entirely to geometry.

In fact it is quite nonsensical, because all movement is defined relative to an inertial reference frame.

actually we don't know 'precisely' what gravity is. It's a weak force, but what generates it (Geordi's gravitons notwithstanding..., we're not 100% sure yet. Looking forward to the day we can turn it off, redirect it, and push back with it.

If Einstein is right that day won't come any time soon. According to General Relativity gravity is curvature of the space-time manifold produced by mass or its energy equivalent.

ruveyn

Friction exceeds the gravitational attraction of small to huge things. Mountains might be the smallest thing with a measurable effect. You still wouldnt notice.

Mass-energy is what curves space. And things do "pull" each other gravitationally here on Earth. That is how Michell and Cavendish measured the Gravitational Constant, with a torsion scale and heavy metal spheres. Gravitation is 10^43 times weaker than the electromagnetic force which is why you can pick up a weight which the entire earth is pulling against. You are stronger than Earth's gravitational pull. Whereas, if each of us had imbalanced charges in only one percent of our body mass we would pull or repel each other with a force equal to the weight* of Mt. Everest. Fortunately we are electrically neutral except for the trivial charge we acquire shuffling across a carpet during the winter.

See http://en.wikipedia.org/wiki/Cavendish_Experiment

ruveyn

*weight is a force. Mass is a measure of inertia i.e. how much matter there is in a body.

Do you think Cavendish had AS or some other form of autism. I do think he had it.

I have to say that I do admire Cavendish for his experiment which gave us the first way to estimate G. I had a go at the experiment at school in A level physics and it was a hard one to do even with modern equipment.

I know that most forces have an inverse square distance term in the equation, can any one think of a force with either an inverse or inverse cube distance term in the F = blagh Blagh equation.

We do have F = BIL = BeV which is a force equation which does not have an inverse distance term in the equation. But can we have a F = Blagh / distance or F = Blagh / (distance x distance x distance) equation.

The weak nuclear force and the strong nuclear force fall off much faster than 1/r^2 which is why they are not felt outside the nucleus.

ruveyn