stupid physics questions

[Darkflame]

ah, got it now. Thats quite clear - I can rest easy.

[Absolute_Zero]

Gödel's theorems are about mathematics and have nothing to do with physics. E.g. Gödel's theorems are about infinity and infinite number systems while the universe is decidedly finite.

what makes you say that the universe is decidedly finite? and in what ways is it decidedly finite?

http://en.wikipedia.org/wiki/Olbers%27_paradox

[guythp]

Ah, I'm glad this thread has come back as I wanted to settle an argument I had with my girlfriend a while back (not a serious argument, but neither of us could see the others' point of view).

My question is this (and I hope someone can back me up on this); In the Disney movie 'Up', the main characters' house is floated into the air by balloons, which he had inflated (presumably during the night) previously and once he released the balloons out of the chimney, it lifted the house up. I argued that the house should have already been lifted once enough balloons were inside the house, whether released out of the chimney or not, as the lifting force would be already be there. She said that they would be pulling up until they'd been released. Who is correct?

Am I missing something really obvious and am I being really, really stupid? (I know it's a fantasy, animated fictional film, but the point of it bothered me. Similarly to when cartoon characters step off a falling rock/phone booth/platform as it hits the ground and therefore suffer no injuries).

[Dio]

You were right. Treat the house as a rigid body and a force applied inside it has the same magnitude of lifting effect as a force applied outside.

You shouldn't let physics in films get to you though, or really there is no point in watching anything that isn't a rom-com, and not even many of those.

[pogo]

Thy don't come out of the chimney. They come from behind the house under a tarpaulin which opens to let them rise, and it's not until that happens that the strings quite visibly lift the house. So it makes sense, as long as there are enough balloons. The real implausible bit is how did he get them all done in a night?

[guythp]

Dio, thank you.

pogo - I'm sure they do come out of the chimney. He definitely cuts a few strings in the fireplace in order to drop altitude and a couple of the trailers I've quickly checked in panicking doubt show the fuckers coming out of the chimney....

*edit* (having just youtubed'd a trailer)
No, you're right pogo. Arse-giblets.

That said, the initial "discussion" I had, with subsequent arguments, stands that I'm right. Ha ha!

[pogo]

It is all just cartoon physics. One oddity is how come the balloons are strong enough to rip the house off its foundations, severing metal pipes and cables at the same time, then only cause the house to slowly and unevenly rise. There's no way he could steer it like he does. And it only gets less plausible later on. So although they explained how he suddenly got the balloons doing their thing there are lots of other problems if you think too hard about it.

[Dio]

I'm reminded of Kelly Johnson's quote when reviewing a proposal for a competitor to the SR-71, that relied on being hot-air-balloon-lifted to 70k feet - "According to my calculations, the balloon would need to be somewhat over a mile wide. Gentlemen, that's an awful lot of hot air."

[Darkflame]

It is all just cartoon physics. One oddity is how come the balloons are strong enough to rip the house off its foundations, severing metal pipes and cables at the same time, then only cause the house to slowly and unevenly rise..

For months before hand he had pre-weakened the foundations - a bit of accelerated rusting here, carefully cuts here etc.
Do Disney have no-prizes now they own marvel?

[pasquinelli]

Gödel's theorems are about mathematics and have nothing to do with physics. E.g. Gödel's theorems are about infinity and infinite number systems while the universe is decidedly finite.

what makes you say that the universe is decidedly finite? and in what ways is it decidedly finite?

http://en.wikipedia.org/wiki/Olbers%27_paradox

assuming there's no way to resolve that paradox, the universe must either be finite in some way, or the universe must not be static. how can you decide?

[zip accident]

If Gravity bends space, do electric charges bend space for charged particles?

What about magnets? do they bend space for iron an moving conductors?

I bet there's some physics and maths to be done on these paradigms.

Pity you can't patent ideas, these/this must be worth a bundle.

Quite woke me up, I had to ask the world.

[gravy]

If Gravity bends space, do electric charges bend space for charged particles?

Gravity bends spacetime not just space. If electric fields bent spacetime for charged particles there'd probably be some measurable effect, like a difference in muon decay time for muons in a high electro-magnetic field.

[zip accident]

If Gravity bends space, do electric charges bend space for charged particles?

Gravity bends spacetime not just space. If electric fields bent spacetime for charged particles there'd probably be some measurable effect, like a difference in muon decay time for muons is a high electro-magnetic field.

Yes, spacetime.

I think it's just a way of looking at things, a paradigm.

It applies to gravity, why wouldn't it work for all field effect forces? If attraction and repulsion can't always be described in terms of curved spacetime, then why not, how not?

There are three macroscopic forces, and three geometric dimensions, could be a coincidence ... So far we're short a few microscopic forces to make up the numbers for string theory I believe, if forces do correlate with dimensions.

I must try again to sleep now.

[Dio]

Gravity doesn't bend space. Mass does.

There was a fascinating article in New Scientist a couple of months back that postulated that what we currently view as the bending of spacetime is simply the second law of thermodynamics at work (since gravitational clumping causes a lower entropy state).

... three macroscopic forces? The strong and weak forces are not macroscopic - a macroscopic force needs an extremely light (i.e. probably massless) force boson.

[pogo]

The reason masses bend space but charges or magnets don't is because of the way gravity works. Gravity acts on everything, even things without mass, as e.g. even light has it's path changed by gravity. In extreme cases gravity can be too strong for light to escape, as in a black hole. So all things moving in space follow curved paths due to gravity, independent of their masses (providing they're not massive enough to curve space themselves). These curves can be thought of as straight lines in a curved universe.

Another way to think of space curvature is as a convenient way of describing things. It's because gravity works on everything that we can think of space as curved. But we can't go outside of space and measure the curvature. We can only measure it by placing masses or lighting lights and seeing how they're affected. So the curvature of spacetime and the universality of gravity are two ways to describe the same thing.

This is related to Einstein's Equivalence principle, that if something (a person, say) experiences a force they've no way to tell if it's due to gravity or due to acceleration as they are identical. This comes from general relativity from where we also get spacetime curvature. It relates to curvature as the geodesic lines (straight lines) in spacetime correspond to a particle moving under gravity, in orbit around a mass for example. To move it out of orbit orbit you need to supply a force, which is a force to overcome gravity or a force to move it off the straight line (which is Newton's 1st law, although this is well outside the bounds of Newton's physics).