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[Putnam]

we're talking mass, not weight

[Sergarr]

I'm going to poke in with a completely irrelevant point: The ball on the shelf does indeed weigh less then the ball on the floor, as its closer to the center of the earth. Its a trivial amount, but the distance from the source of gravity does indeed affect the weight. Similarly, the falling ball will weight more with every second it falls, even though the amount is extremely tiny.

not really

center of the earth is not the source of gravity

[Putnam]

At the scale of a ball, it certainly is. If you want to take the Moon into account, it's still not that far off.

[Draxis]

not really

center of the earth is not the source of gravity

Pretty sure it is if you're above the surface of the Earth.

[Il Palazzo]

A stretched spring doesn't have more energy, though - I'm assuming you're speaking of a spring stretched to the point of distortion - because once you stop stretching it there's no energy there. And potential energy doesn't increase mass. If I put two similarly massive balls on shelves of different heights, the ball on the higher shelf isn't going to weigh more. And, actually, it's not going to weigh more as it's falling - there hasn't been any energy added to the system of the ball itself. It's simply attracted to the earth.

You're missing the point. The ball itself as an isolated system doesn't have more mass. But the system of Earth plus ball does. The change of position of the ball in the gravitational field of the Earth (and vice versa) requires energy. By imparting that energy to the system you increase its total mass (the rest mass of the system seen as a whole).

I don't know what you mean by the spring. Once stretched past the equilibrium point by any amount it has elastic potential energy. If you stretch too much and distort the material so that there is no more restoring force - then there is no elastic potential energy (it went into changing the shape of material, so in this sense it's still there but in a different form).


This isn't news, guys. It's what E=mc^2 says. In fact, forget the c^2 - it's just a unit conversion factor. You can choose units where it's equal to 1, so that you just get E=m.
The energy content of the system determines its mass. More energy in the system means it's more massive. Hot objects are heavier than cold, bound objects are lighter than their constituent parts, a box full of light weighs more than an empty one etc.

[Sergarr]

not really

center of the earth is not the source of gravity

Pretty sure it is if you're above the surface of the Earth.

well yes it is, but:

In the context of "ball falling", it is very important to distinguish between an apparent source of gravity and the real sources of gravity.

It makes the difference between infinite force in the center vs zero force in the center.

[Cryxis, Prince of Doom]

Has anybody ever seen or read 'Ender's Game'?
It's got a concept that is a bit of tangent to the conversation

[Frumple]

... I rather imagine something approaching a majority of the folks down here have read ender's game. Quite possibly a majority of the forum in general. It's a pretty well read text, especially among english speakers that are even marginally interested in sci-fi.

Remembering any of it (besides maybe the bit about down and the (pre-?)teen xenocide and the homosexual not-so-under tones and maybe a bit else) is a different question.

[Cryxis, Prince of Doom]

Let's just skip over that second bit...
I was going to ask about the engines in the space ships, the on ones that generate a force field around the ships and any matter that hits the force field is converted into energy to power the ship to relatavistic speeds. Is that even remotely possible in real life?

[Sergarr]

Let's just skip over that second bit...
I was going to ask about the engines in the space ships, the on ones that generate a force field around the ships and any matter that hits the force field is converted into energy to power the ship to relatavistic speeds. Is that even remotely possible in real life?

It's similar in concept to how turbojet engines work and yes, there has been proposals about this sort of thing.

Main problems are 1) you first need to reach high enough velocity for this thing to start working and 2) you need working fusion reactors to use matter (because it's mostly hydrogen).

[Sheb]

What you have in mind is a Bussard Ramjet I think. There are plenty of problems related to the rather low density of matter in space, but it's theoretically possible.

[Cryxis, Prince of Doom]

What you have in mind is a Bussard Ramjet I think. There are plenty of problems related to the rather low density of matter in space, but it's theoretically possible.

That sounds about right

[Scoops Novel]

A pair of binary stars entered the solar system 70000 years ago! I thought of this! I THOUGHT OF THIS BRAH

http://www.bbc.co.uk/news/science-environment-31519875

My take away:

While this is the closest flyby detected so far, Dr Mamajek thinks it's not uncommon for alien stars to buzz the Sun. He says a star probably passes through the Oort Cloud every 100,000 years, or so.

But he suggests an approach as close - or closer - than that made by Scholz's star is somewhat rarer. Dr Mamajek said mathematical simulations show such an event occurs on average about once every nine million years."

How fast where they going? Could you keep a planet in tow? And how noticeable are they from earth?

[10ebbor10]

They passed at 0.8 Light years. Meanwhile, Pluto sits at 0.0006 light years from the Sun.

Keeping a planet in tow is rather unlikely, though it might have send a few comets our way, which will reach us in about 2 million years. As for noticeability, at closest pass it would have a magnitude of 10.3. This means that it might have just been visible with binoculars. Not very spectacular.

[Il Palazzo]

How fast where they going? Could you keep a planet in tow? And how noticeable are they from earth?

You wouldn't see them. The system is made of a red dwarf with a brown dwarf sub-stellar companion. At the closest approach (which was ~1ly) they'd be many times (~40) dimmer than human eye sensitivity (10 apparent magnitude when they were passing by).

Their velocity is mostly in the radial direction, at 83.1km/s plus 3km/s tangentially. That's about twice as much as the Voyagers'.

The picture included in the BBC article is a bit misleading as it uses the logarithmic scale to depict the distances in the solar system. The Oort cloud is way, way farther than one could think - it extends almost halfway to the Alpha Centauri.

Here's the relevant paper:
http://arxiv.org/abs/1502.04655