Stoopid basic physics question / just checking (rusty brain).

[polygonum]

But it is not weightless, is it?

[The Natural Philosopher]

The final nail in the coffin of the concept of weight as a physics defined quality.

[djc]

+1 in Physics World things have mass, and having mass are subject to gravity. In Everyday World things have weight, and having weight they fall down.

[Lawrence]

comet

weighless

other

applied

There are places where the fields cancel, about 9/10ths of the distance to the moon for example.

I don't understand the last point you are trying to make. Weight is because of your local grav field. Other forces,like rocket engines or drag cannot contribute to drag, only to the net force.

[Lawrence]

,

gravitational

cannot tell

doesn't

Why do you say that? Just because you don't or won't understand.

[John Rumm]

In fact it is constantly accelerating toward the centre of the earth... (but getting no closer)

[polygonum]

I do agree. Which rather says that the earlier statment " The ISS is still weightless..." is wrong.

[Andy Champ]

Things have changed then. Even the most fancy balances I used at Uni were balances - illuminated readouts and all.

Well B***r me, things HAVE changed.

"Sartorius analytical balances are lab weighing instruments that feature the highest accuracy for analytical weighing processes" ... "Their world's first monolithic weigh cell meets unique prerequisites for ensuring high measurement accurracy. " [sic]

And yet it's still called a BALANCE! Andy

[Tim Streater]

Yes it is, as is everything in it. Why else d'ye think they float around inside?

[Tim Streater]

The earth's and moon's fields happen to balance at that point, yes, but that leaves the sun and all the other planets and the rest of the galaxy and the other galaxies, all of which pull on you.

... which you have to resist one way or another to experience weight.

If I take a rocket out into space and then turn it off, I'm in free fall and am weightless. So I'll float about the cabin. This is true wherever I am in the universe, and regardless of my distance from any other mass (such as a planet). Other masses will pull on me and I'll experience a net acceleration in some direction based on the sum of those forces. But I'll still be weightless. That acceleration might be large if I happen to be close to some large mass. Between the galaxies it'll be quite small - but not zero.

Now let's suppose I turn the rocket on again. The rocket engine supplies a force to the rocket, and thus to me and to everything else in the cabin. Thus, guess what - I have weight again. If the rocket motor can accelerate the rocket+contents at 32 ft/sec/sec, I'll weigh just as much as I do on Earth. If the rocket is even more powerful and can supply 2G,

3G or more, then I better have a good suit, just like the fighter pilots need. As soon as I turn it off, weightlessness returns.

[Lawrence]

,

gravitational

cannot tell the

surroundings in order

doesn't

No it isn't. It is reactionless. It is accelerating towards the centre of its orbit just as everything else in it is.

[Lawrence]

whatever

weight.

wherever

experience a

forces. But

supply 2G,

You misunderstand (i suspect on purpose) the concept of weight. It acts on you whether you can feel it or not. What you feel is the reaction to your weight. Your weight acts down on you. You feel the reaction pushing up as long as there is something there to do the pushing.

[Reentrant]

Have you read "Neutron Star" by Larry Niven? If you happen to in close orbit around a supermassive object the tidal force difference between your head and feet could tear you in half.

(You did say "wherever in the universe".)

[Tim Streater]

You might want to giggle for "Roche Limit" I Think WInky has an article about it.

[Tim Streater]

Of course it's accelerating. That's what being in free fall means. It just happens to have enough forward speed so it keeps missing the planet.

[The Natural Philosopher]

I've had the effect around the mother in law.

[Steve Firth]

That's becau'se mas's wa's a Johnny Come Lately Concept. People used weight for centuries before the concept of mass was determined and mass was firstly made equivalent to weight in a 1g field and then was re-defined so that weight on earth is now expressed as mass in a 1g field with the old unit of weight being defined as the current unit of mass.

[Andy Champ]

If you read it carefully you'll find that the tidal force between head and feet led to an unpleasant feeling. It was the tidal force between bow and stern of the ship that was the killer.

Also that (as he didn't realise at the time) after such a close pass you'd keep the spin.

Andy

[newshound]

I can assure you that "kgf" and "tonf" are very widely used in mechanical engineering design calculations. Although the purists like to see the proper SI units it's often easier to "sanity check" kgf because we all recognise that ballast comes in 25 kg bags, etc. Modern design tools like MathCAD are brilliant because they understand units so you don't have to do any conversions (although you have to watch out because a MathCAD "ton" is is a "short" ton, i.e. 2000 pounds).

[geoff]

Apples, apples, get yer apples 'ere

4d a newton