OT: I'm confused about gravity

[Indy Jess John]

It is. Didn't you recognise it as a wind-up?

Jim

[Norman Wells]

No, I think you're right in the second place.

[Indy Jess John]

You can:

Jim

[Norman Wells]

That's cheating! It's not an actual inverse square, just a silly song. It could have been about unicorns, or even three lions.

Yes, I know lions exist, and maybe even in threes, but you wouldn't get three of the things for just 99p.

Not good ones anyway.

[newshound]

There is no paradox.

Either you genuinely don't understand the very clear explanations of Newtonian gravity as explained in the links (if not always in the posts), which seems unlikely to me, or you are, in this thread at least, just trying to provoke responses by making ridiculous statements; which is my definition of trolling.

TNP's philosophical digression into what we mean by by knowledge is another distraction, valid in its context but applying to all of science.

[newshound]

Wasn't it someone asking questions at the end of a Bertrand Russell lecture whose model was based on "turtles all the way down".

[Norman Wells]

Only one per world, but of course there has to be more than one:

"There was, for example, the theory that A?Tuin had come from nowhere and would continue at a uniform crawl, or steady gait, into nowhere, for all time. This theory was popular among academics.

An alternative, favored by those of a religious persuasion, was that A?Tuin was crawling from the Birthplace to the Time of Mating, as were all the stars in the sky which were, obviously, also carried by giant turtles. When they arrived they would briefly and passionately mate, for the first and only time, and from that fiery union new turtles would be born to carry a new pattern of worlds. This was known as the Big Bang hypothesis."

[Pamela]

Oh! :)

[Cursitor Doom]

Just the Germans being bloody-minded over Brexit. They've taken it very personally for some reason; it's a part of their national character that's normally suppressed; you don't see it until you've lived among them for a good while and this mask they wear slips. I've suggested this chap and his German wife move back to Blighty, since she can retain her German nationality no problem and since they're both retired, there's no careers/jobs to worry about. It really shouldn't be necessary, though! :-/

[Bill Wright]

I don't see any contradiction between Newtonian gravity and my belief that a mass such as a mountain cannot simultaneously have gravitational force in two directions at right angles, ie towards an observer at ground level nearby and towards the centre of the earth. It follows from that that the total downwards gravitational force felt by a person near a large mass will be less than then felt by an observer in space, where the force of the large mass and of the earth will simply add.

It seems to me that some of the opposition this idea, which is simple and self-evident, is actually trolling. There have been appeals to authority that the writer knows well that very few people could challenge because of the nature of the material.

Bill

[Pamela]

Before Newton, Muslims would say "If the mountain won't come to Muhammad then Muhammad must go to the mountain". Were they referring to gravitational forces?

I appeal to the Qu'ran. There is no god but Allah; Newton and Einstein are His messengers. So they say.

[The Natural Philosopher]

Yesp. Just the sort of thing expected of you.

Another quote that you dont even begin to understand

[Fredxx]

I don't think they are recognised as His Messengers:

[Martin Brown]

Gravity wave already has a very specific meaning. A density wave propagating in the atmosphere or ocean caused by gravitation.

The waves on the sea are gravity waves.

A gravitational wave is the result of two big masses combining. Every moving object leaves some gravitational disturbance in its wake. But to really thrash things around it takes a black hole merger where a decent proportion of their combined mass can be radiated away on combining.

[Tim Watts]

That's completely correct and it's why maths has vectors (a thing with a magnitude *and* a direction).

Vectors can be expressed as polar (literally a magnitude and a direction measured with angles, 2 for a 3D vector)

or they can be expressed as cartesian aka (x,y,z) where the x is the magnitude in the x direction, y for y, z for z.

So with the mountain, you might have a force due to gravity of:

(0.001, 0, -1000)N

-z for straight down, and the mountain being to the right of you along the x axis.

In polar form, this would be a vector of something like

(1000.000001, angle-of-buggerall, 0)

(magnitude being pythagoras - sqrt (1000^2 + 0.001^2)

So very quickly you realise the effect of the mountain is in fact bugger all.

[mechanic]

OK we have the references to proofs of this 'center of gravity/mass' idea which we can follow in the case of spherical earth-like bodies, but how general is this idea? Apparently the proof was later (than Newton) generalised to oblate spheroids but what shaped bodies follow this idea and which don't? Presumably it's a question of how easy is it to do the integration in the proof, but the idea won't work for some simple solid bodies like plane sheets or toroids.

[The Natural Philosopher]

Ther whole issue of the concept of a 'center of gravity' is that is the center which resolves all the forces of gravity to an effective point.

AIUI - and its a LONG time ago - the attracting of an object in 'far field' will always be effectively the same as for a point mass at the C of G. For ANY object shape.

It gets more complicated in 'near field' cases . Like standing next to a mountain.

'Near field' only works for sphericals. C of G only works for far field or sphericals.

[soup]

The rope in a tug of war WILL dip in the middle but it is by a VVV small amount in fact with the unevenness of the diameter of the rope I would hazard it is immeasurable.

I've installed a lot of overhead catenary

Not in my experience a string will not droop visually if pulled at the two ends

[Bill Wright]

Don't do any critical fixings work then.

Bill

[soup]

Eh?