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Space Suit: 1949

wired.com

51 pointsbootload18 comments

> Ross and Smith ended their paper by estimating their suit’s weight. It would, they calculated, have an Earth weight of 150 pounds. On the moon, however, where gravity pulls with about 15% as much force as on Earth, their suit would weigh only about 25 pounds.

This kind of audience-confusing shortcut always makes me jump. It would still weight 150 pounds on the moon, but it would exert 15% of the force as generated on Earth. The distinction is important, even for the uneducated, because it means that although you appear to weight less statically, you have exactly the same inertia.

It would still have a mass of 150 pounds. But it would weigh 15% of what it would on Earth.

The problem is that it makes people believe that the suit would feel like a 25 pound suit on the moon. And that isn't the case. In terms of gravity (so, strictly downward force), it's true, but in order to move around (to push forward the whole thing, so to speak) it still feels like 150 pounds, because that's what its mass is.

That's absolutely true, I was just talking about the use of words: Weight is the downward force, not the mass :)

I think the sentence could be more clear in the article, perhaps something like:

> On the moon, however, where gravity pulls with about 15% as much force as on Earth, their suit would weigh the [earth] equivalent of about 25 pounds.

would be better.

You go on to make the mistake you were trying to correct:

> It would still weight 150 pounds on the moon

No, it would have a mass of 150 pounds everywhere.

> even for the uneducated

This is really uncalled for.

> You go on to make the mistake you were trying to correct

Sorry, non-native english speaker, so I hastily mixed it up.

>> even for the uneducated

> This is really uncalled for.

What I wanted to say is "someone without knowledge of such physics laws", and I apologize if I sounded rude in any way.

Isn't pound the name of both the mass and force units? In that case it would have the mass 150 pounds, but weigh 25 pounds.

When I studied physics in highschool, we were taught to measure mass in kilograms and to measure weight in newtons. While this seemed a little strange at the time, it had the effect of forcing you to be clear which one you were talking about and it avoided this kind of confusion.

As I recall, the mass term "slug" is often used rather than pound-mass to remove this ambiguity. A slug = 32.174049 lbm (i.e. multiplied by the earth's gravitational acceleration constant without any units).

But, yes, the terminology among other things can introduce ambiguity and the potential for error when using Imperial units for these types of calculations. (In fact, I seem to remember it sometimes was easier to convert into metric and back again at the end.)

The general point here though is that the local gravitational constant changes the weight (downward force on the object) but not its inertia (a property of mass and other physical properties).

It seems you're correct (http://en.wikipedia.org/wiki/Pound_%28force%29). It's hugely confusing though. Newtons and Kilograms are much clearer, they're unambiguous even without context.

When I was a kid (in Canada) we still did a part of our science/physics education in Imperial units (gravitational FPS), and we used the slug as a mass (inertia) unit, reserving pound for force.

Now that's one I missed completely. I actually checked fr.wikipedia.org[0] beforehand to be sure, but it made no mention about the force unit, whereas en.wikipedia.org does.

[0] http://fr.wikipedia.org/wiki/Pound which guides one to http://fr.wikipedia.org/wiki/Livre_(unit%C3%A9_de_masse), this one failing to mention the force unit, except as a secondary connection (http://fr.wikipedia.org/wiki/Livre-force) down the page.

Aren't they using the term "pounds-force" when explicitly talking about force? I've heard that term used.

A paradigm example of why geeks are said to have poor social and/or communication skills.

The distinction (between weight and mass) may be important to nit-picking geeks but ordinary people, educated or otherwise, will understand perfectly what was being communicated: that the seemingly heavy suit would be much less heavy when worn on the moon.

It's a far cry from a nitpick, because disparity between inertia and gravitational pull makes all the difference when trying to slow down or come to stop.

Suppose a moon explorer needs to slow down rapidy, for example upon encountering an obstacle.

On Earth, the 150 pounds of suit's inertia is countered by 150 pounds worth of gravity, which provides for some good traction, through friction.

On the Moon, however, the 150 pounds of suit's inertia is countered only by (apparent) 25 pounds of gravitational pull, which gives much lower friction.

Suddenly the haples moon explorer finds it much harder to come to hald -- the distinction between inertia and gravitational pull, while apparently a nitpick -- has real effects.

Every Earthian has an intuitive expectation of gravitational pull, and thus possible friction, in relation to mass, and so we tend to skip the distinction in casual conversation. On the Moon, this relation is much different and should be taken into account.

It would be less confusing in metric.

The mass would be 68 kg both on Earth and the Moon.

The weight on Earth would be about 668 Newtons. (9.8226 x 68)

The weight on the Moon would be about 110 Newtons. (1.625 x 68)

http://www.gutenberg.org/files/19141/19141-h/19141-h.htm Edison's Conquest of Mars

aka "Space Suit: 1898"

Ctrl+F "the air-tight suit".

The British Interplanetary Society is still around and active... http://www.bis-space.com/

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