Is gravity cumulative

[Halsafar]

When an object is falling, assuming only gravity is pulling on it. Does gravity accumulate on the acceleration, like after 2 seconds in the air is the object accelerating at 9.81*2 or is it still accelerating at 9.81 but has a velocity of 9.81*2??

I believe it has to be cumulative or else -- things seem to float funny in my physics simulator.

[NoteMe]

The gravity is a constant. It is only the speed that changes.




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[wossname]

The gravity is a constant. It is only the speed that changes.
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NOOOOOOO!!!!!!

Wrong!

Gravitational force decreases with the square of the distance!!

The distances would have to be quite large though, you would have to take this into account if you are writing a 3d space game.

Hence "Gravity Well". If gravity was constant then it would be the Gravity Ramp

F = (G * (m₁ * m₂)) / r²

Where
# F is force due to gravity
# G is the gravitational constant (6.67x10^-11)
# m₁ and m₂ are the masses of the 2 objects
# r is the distance (in metres) between the objects' centres

[szlamany]

It is only the speed that changes.

The acceleration changes as the gravity increases as well.

[NoteMe]

NOOOOOOO!!!!!!

Wrong!

Gravitational force decreases with the square of the distance!!

The distances would have to be quite large though, you would have to take this into account if you are writing a 3d space game.

Hence "Gravity Well". If gravity was constant then it would be the Gravity Ramp

F = (G * (m₁ * m₂)) / r²

Where
# F is force due to gravity
# G is the gravitational constant (6.67x10^-11)
# m₁ and m₂ are the masses of the 2 objects
# r is the distance (in metres) between the objects' centres

If you are dropping something from a building, you can't see any difference. But of course the gravity on a geostationary satelite is not the same as for you sitting on your toilet. Not on Mars either.

[NoteMe]

The acceleration changes as the gravity increases as well.

gravity IS the acceleration. But the gravity don't change unless you ar calculating a space trip. The diffrence on gravity on Mount Everest and where you are sitting now is next to nothing. But since it is a bit diffrent they have made it a constant for calculations on the earth surface, and that is 9.81*******

[szlamany]

You can have many, many points of gravity - the acceleration in my opinion is more the aggregate of all of the gravity points.

[NoteMe]

You can have many, many points of gravity - the acceleration in my opinion is more the aggregate of all of the gravity points.

You can have more then one acceleration on an object too...just think about when you decompose the gravity accelerations when you calculate them.

[NoteMe]

Just to add to that. Gravity is a force. And it is forces that makes objects change direction or speed. As long as it works on the object that is. But when it stop to work on the object, then the object will have the same speed and direction untill an other force starts working on the object.


And yes of course you can have many forces working on one object. And we say that the gravity force is a constant force on the earths surface .


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[Halsafar]

Well after all this lovely discussion I have a few words and still need some explaining.

Yes the force of gravity is equal to the mass of two objects divided by the square of the distance that seperated them.

A little demo I read stated:
70kg kid, at sea-level on earth -- Fg = 685N
A boeing jet at 40,000ft -- Fg = 683N

The difference is less than 1%. This is why when stated two objects dropped or thrown horizontal from the same height will hit the ground at the same time.



NOW, once again my true question.
I drop a ball down, it has a velocity of 0 and an acceleration of 9.81. 1 second into the fall, it has a velocity of 9.81 and an acceleration of _____?

[NoteMe]

9.81m/s²

[Halsafar]

So acceleration due to gravity is constant, but cumulates on velocity, an obvious statement in the end.

Thanks guys.

[NoteMe]

YEah, I guess you can say it that way, even if I wouldn't have used the word cumulative. Never used that for physics..


But you are right. the speed of a dropping object will always be:


v_y = v_0y - gt


Now give me all your points........J/K




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[Halsafar]

You know, I give everyone rep who answers my posts nicely...since this rep started.
Since it has started, I have not even given out enough points to give the first person I gave points to more points...You Noteme where like the third person I gave points to, so I still can't give you any...
I guess no more than 10 people help me on these forums.

[Phenix]

Halsafar,
A habit you should get into is including units with your dimensions. I think it will help you with "what accumulates on what as the seconds tick by" so to speak.

Don't say

I drop a ball down, it has a velocity of 0 and an acceleration of 9.81. 1 second into the fall, it has a velocity of 9.81 and an acceleration of _____?

Instead, say

I drop a ball down, it has a velocity of 0 m/s (or meters per second) and an acceleration of 9.81 m/s² (or meters per second per second ) . 1 second into the fall, it has a velocity of 9.81 m/s and an acceleration of _____m/s²?

Also, you forced me to check the acceleration due to gravity at 40,000 feet with that boeing jet post. I found a similar post. It refers to the force of gravity on the 70kg kid who happens to be in a jet at 40,000 feet. Not the force (or weight) of the jet.
http://www.glenbrook.k12.il.us/gbssc...les/u6l3c.html

First, observe that the force of gravity acting upon the student (a.k.a. the student's weight) is less on an airplane at 40 000 feet than at sea level.

Acceleration from gravity at 40,000 feet is approximately 9.79 m/s². So that jet will still be alot heavier than the kid whether the jet is at sea level or at that altitude.

[NoteMe]

Bare in mind that it is not only "altitude" that makes a diffrence. Since the earth is slightly pear formed the gravity is also diffrent on the northpole and Sahara. But again this is minimal.



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[wossname]

Bare in mind that it is not only "altitude" that makes a diffrence. Since the earth is slightly pear formed the gravity is also diffrent on the northpole and Sahara. But again this is minimal.

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Indeed. Interestingly enough this is for 2 reasons. Since the earth bulges outwards around the equator (due to centrifugal force from the earth's rotation) you are slightly further away form the earth'd C Of G. Thus making you lighter at the equator. but ALSO, the centrifugal force affects you too and so it makes you lighter still . I think the difference in weight is about 1 pound in the average man.

[|2eM!x]

when something hits the ground, does its force gets squared?

i remember something like that from a physics class. something to keep in mind i guess

[szlamany]

Indeed. Interestingly enough this is for 2 reasons. Since the earth bulges outwards around the equator (due to centrifugal force from the earth's rotation) you are slightly further away form the earth'd C Of G. Thus making you lighter at the equator. but ALSO, the centrifugal force affects you too and so it makes you lighter still . I think the difference in weight is about 1 pound in the average man.

But it's not the center of the earth that matters - it's the mass attracting the object. The gravitational force is not the same at one location on the earth as the other (even when at the same elevation). It's determined by the mass that is under that point.

All of this might be minimal - but meaningful when testing theories.

[wossname]

But it's not the center of the earth that matters - it's the mass attracting the object. The gravitational force is not the same at one location on the earth as the other (even when at the same elevation). It's determined by the mass that is under that point.

All of this might be minimal - but meaningful when testing theories.

I agree

I'ts interesting to note that a pendulum held near to Mt Everest leans slightly toward the mountain. They have confirmed this with lasers and stuff.

Also. You cut a planet in half (pretend its cold rock all the way through) and stand at one edge of the flat bit. It would not feel like you were on a flat plane. you would have to lean backwards outwards! To your eyes it would look like a flat plane (which it is) but if you walked towards the centre of the earth, the slope would get less and less. Keep walking through and you be going uphill again!!

[NoteMe]

My last 2 cents....


the planets is round because of the gravity..





PS: Your half planet Wossy would be morphed to a sphere eventualy...

[szlamany]

My last 2 cents....


the planets is round because of the gravity..





PS: Your half planet Wossy would be morphed to a sphere eventualy...

But the moon is egg shaped with the heavy butt end hanging towards us - that's why it's rotation keeps that end facing us at all times.

It would never be round even if liquid because the earth force works against it's own gravitational desires!

[NoteMe]

But the moon is egg shaped with the heavy butt end hanging towards us - that's why it's rotation keeps that end facing us at all times.

It would never be round even if liquid because the earth force works against it's own gravitational desires!

And Newton 3rd law. The moon has the same force at us, as we have on it. That is why the wather in the sea is high sometimes, and low sometimes. That is the moon working on us..


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[szlamany]

And Newton 3rd law. The moon has the same force at us, as we have on it. That is why the wather in the sea is high sometimes, and low sometimes. That is the moon working on us..


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And you really start appreciating the "gravitational lines of force" when you realize that the high-tide is on the side of the moon, and the opposite side of the earth from the moon at that same moment.

The low tides are on both perpendicular sides at that same moment.

That whole rubber sheet image starts making more mental sense!

( I really should be doing some work today! )

[NoteMe]

And you really start appreciating the "gravitational lines of force" when you realize that the high-tide is on the side of the moon, and the opposite side of the earth from the moon at that same moment.

The low tides are on both perpendicular sides at that same moment.

That whole rubber sheet image starts making more mental sense!

( I really should be doing some work today! )

I am starting to find this thread really interesting actualy..


Actualy I read somewhere that the moon is slowing down slower and slower. Not much, but it will be pretty catastofic if it does. But I guess that will take some million years anyway.


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[wossname]

But the moon is egg shaped with the heavy butt end hanging towards us - that's why it's rotation keeps that end facing us at all times.

Surely you jest? You don't seriously think that? What is it hanging from?

The moon always faces us because of angular velocity. By chance, its angular velocity is almost exactly equal to its orbital frequency. So we only ever see one side of it. It takes hundreds of thousands of years to (from our earth perspective) rotate on its axis and show us its other side.

It has nothing to do with being egg shaped! An object always rotates around its centre of gravity, shape has absolutely nothing to do with it.

[szlamany]

Surely you jest? You don't seriously think that? What is it hanging from?

The moon always faces us because of angular velocity. By chance, its angular velocity is almost exactly equal to its orbital frequency. So we only ever see one side of it. It takes hundreds of thousands of years to (from our earth perspective) rotate on its axis and show us its other side.

It has nothing to do with being egg shaped! An object always rotates around its centre of gravity, shape has absolutely nothing to do with it.

Sorry wossy - this is fact - I'm not making it up. 30 years of astromony magazine and a keen interest in what's above...

The moon is so mis-shapen and so incredibly close to the earth that we have a "rotational" tidal effect of keeping the "mass-ive" end facing us.

There is no chance in the universe that would make angular velocity exactly equal the orbital frequency.

[szlamany]

Gravity acts both ways so the Earth has been creating tidal bulges on the Moon and has slowed it's rotation down so much that it rotates once every orbital period. The Moon keeps one face always toward the Earth.

I found this - poorly written and vague to so the least, but gets the basic point across.

Wossy, you asked what's it hanging from? Basically it's hanging on the earth - it wants to fly straight in the solar system and can't get that string cut from mother earth.

If you want to see the site that this came from, which has a really neat explanation of the dual-high tide situation and how the sun also effects the tides, check this out...

Earth/Lunar gravity info

[szlamany]

I am starting to find this thread really interesting actualy..


Actualy I read somewhere that the moon is slowing down slower and slower. Not much, but it will be pretty catastofic if it does. But I guess that will take some million years anyway.


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Check out the site I posted in my last post - it's got a neat explanation of the moons orbit enlarging to conserve angular momentum - the end result will be pretty amazing.

[azteched]

The moon's orbit is known as "tidally locked" - the change of (axial spin) angular momentum is a very slow, gradual one.

[NotLKH]

But the moon is egg shaped with the heavy butt end hanging towards us - that's why it's rotation keeps that end facing us at all times.

Sorry wossy - this is fact - I'm not making it up. 30 years of astromony magazine and a keen interest in what's above...

The moon is so mis-shapen and so incredibly close to the earth that we have a "rotational" tidal effect of keeping the "mass-ive" end facing us.

From http://spacekids.hq.nasa.gov/osskids/animate/moon.html

The Moon's shape is unusual. It is slightly egg-shaped, with the small end of the "egg" pointing toward Earth.

Hmmmm, is it Extremely misshapen, with its heavy butt pointing towards us, or is it just slightly, with its small end facing the us...

Well, I don't know, but I think Nasa trumps your thirty years.

[szlamany]

From http://spacekids.hq.nasa.gov/osskids/animate/moon.html


Hmmmm, is it Extremely misshapen, with its heavy butt pointing towards us, or is it just slightly, with its small end facing the us...

Well, I don't know, but I think Nasa trumps your thirty years.

Actually the pointy end of the egg has more mass (measured from the "center" of the egg) - I was wrong in my statement about the "heavy" end - that's me thinking about breakfast!

[wossname]

An object ONLY revolves around its C of G. If you say that the moon hangs towards us because of gravity, then you are saying that it is rotating around a point eccentric to the C of G, thus letting the C of G come closer to the earth. This is impossible.

To add to an earlier point, it is sufficient to treat planetary gravity as being r distance from the planet's centre of gravity rather than the planets surface. (Although this does not apply where r is less than the planet's radius.)

[NotLKH]

Imagine, if you would, a pipe with weights on either end.
One Weight masses 10 KG, the other Masses 100.

Now imagine the point whereby they balance on a fulcrum.
Drill a hole through that point, and insert an axle, such that the pipe is supported halfway down the axle via a ledge that is highly lubricated.
Clamp it secure with a sleeve, again Highly Lubricated.

Now, Build a harness, tuning fork shaped, such that the axle is connected to the tines of this harness. The Weighted Pipe can revolve freely around the axle, thru the tines, without anything obstructing its revolution.

Since the Axle is located in the center of this pipe/weights cumulative mass, this pipe will certainly revolve about the axle. One Side will NOT pull down and hang, with the other side up.

Now, Grab the handle of this "Tuning Fork" shaped harness, and rotate in circles, arms outstretched.

I believe, at first, the Pipe and the weights capping each end will have a tendancy to revolve smoothly about the axis.

However at some point, one of the weights will be closer to you, the center of this entire systems rotation, while the other weight will be farthur away.

The One Furthur away will have more centripital acceleration applied to it than the one closer to you.

I believe if the more massive weight is on the far side, while the less massive weight is closer, the weights will lock in this position, due to this now uneven system of forces.

still, this is a purely mechanical system, all centers of rotation physically locked in place. Could such a thing actually happen via non-"physical" gravitational "connections"

[NotLKH]

Ahhh, the wonders of a well placed smoke break...

The center of mass is the center of mass is forever the center of mass...

But it is NOT necessarily the center of angular momentum!


The system's center of mass will remain where it is and cannot shift to the center of momentum, {Work would be required, the potential energy of the entire system would have to shift, expending energy, and where would this come from???} so the center of momentum in this system will be off center the center of mass, locking its relative rotation in place.

Nothing could change this short of cataclysmic impact or megatonnage explosive force.

[NotLKH]

For it to freely revolve about its center of mass, the center of mass would have to move towards the center of momentum.

But this would then be pushing it into a higher orbit.
Which would then greate a more unbalanced system of momentum, pushing the center of momentum furthur away from the center of mass,

Which would require the center of mass to move yet again towards the center of momentum, ...

A forever acclerating "Dog-Chasing-Tail" phenomena, expanding ever increasing energy, while yet building up more and more potential energy, all under the hypothesis that this is a closed system. No Energy input, so it can't change its energy state.

Its stuck.

[Something Else]

But what happens if the bar is 10 times as long from the axle to the smaller weight?

Wouldn't the center of the angular momentum now be on the lighter weights side, thus keeping its heavy side always pointing towrds the swinger?

[NotLKH]

Well, of course, if you don't consider that your center of mass has now also shifted.

Place that axle back at this new center of mass, and it will pivot back to where the more massive weight is still pointing away.


-Lou

[Halsafar]

Very interesting results to the thread I posted. I am glad, I have always been a space nut but I never learned the physics behind it all. I have been doing research until the ends of the night.

Did you know the mass of Jupiter actually cause's Saturns near perfect orbit to bend inward as it pass's Jupiter. Now I saw this on Space TV which quoted the Nasa website article it came from, I thougt Jupiter was a very hot gas planet, can it have a c of g? or even a very large gravitational force?

I also read on an observation website also quoted from Space TV showing an area of stars, 500 light years away that are emitting lots of xrays, they believe it is a new type of stage of a star never seen before - a process it goes through just before collpasing under its own gravity. How can something even collapse under its own gravity? I heard this was the final star stage, then a black hole.

Black holes, as far as I can research -- they have yet to be seen and proven. Stephen Hawking has many theories an explanations on the subject which are fascinating; Besides what other logical explanation could there be for something that collapse's under its own gravity.

[azteched]

Well, you can't *see* a black hole, by definition, because you see by means of electromagnetic radiation (e.g. visible light), and a black hole is something from which light cannot escape: that is:

Escape velocity > c

In fact you can easily approximate the maximum radius for a black hole to form from a given mass, by considering the above condition. The max. radius is known as the Schwarzschild radius.

A star doesn't just collapse to a black hole immediately - there are various stages prior to black holes, for stars of different masses and radii. In neutron stars, for example, the mass is held apart from collapse by "quantum mechanical pressure", coming from the fact that certain particles can't occupy the same quantum states (Pauli exclusion principle).

Anyway, it's interesting.