1458: Small Moon

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mathmannix
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Re: 1458: Small Moon

Postby mathmannix » Thu Dec 11, 2014 2:38 pm UTC

slinches wrote:
AEB wrote:
wumpus wrote:A better question is "so where is the planet it is orbiting"? I don't recall a planet or more importantly a blazing star nearby.


You mean Alderaan? They blew it up.

Wasn't it orbiting (or at least performing a gravitational assist maneuver around) a gas giant in the Alderaan system?

You're probably mixing its first shown location (next to the remains of the rocky planet Alderaan) with its last location (shown or otherwise) in the first Star Wars movie: orbiting a gas giant, Yavin. Yavin had an actual moon (Yavin 4), which the rebels had a base on (the jungle moon, filmed in Guatemala, with the Mayan-esque pyramid). The Death Star was going to destroy the base, but had come out of hyperspace on the opposite side of the gas giant, so for some reason it had to slowly orbit it, giving the rebels time to blow it up before it could wipe out the base. (Not that the empire believed it could be blown up; that was Darthenschmirtz's fault for installing a self-destruct button!)
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Re: 1458: Small Moon

Postby Quizatzhaderac » Thu Dec 11, 2014 3:30 pm UTC

Neil_Boekend wrote:What bugs me about the statement is that mass doesn't matter in that way.
Then it's a good thing the article didn't mention mass.
Sir Lunch-a-lot wrote:I am pretty sure that the Canada Arm 2 could not lift a space-shuttle on earth.
220, 000 pounds of force is enough to lift the orbiter if the orbiter was in Earth's gravity. The arm is able to exert 200,000 pounds of force. These two facts are provided in proximity, not to suggest the arm actually operates on Earth, but to provide a frame of reference about the amount of force involved.
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Re: 1458: Small Moon

Postby Neil_Boekend » Thu Dec 11, 2014 4:04 pm UTC

Quizatzhaderac wrote:
Neil_Boekend wrote:What bugs me about the statement is that mass doesn't matter in that way.
Then it's a good thing the article didn't mention mass.
Sir Lunch-a-lot wrote:I am pretty sure that the Canada Arm 2 could not lift a space-shuttle on earth.
220, 000 pounds of force is enough to lift the orbiter if the orbiter was in Earth's gravity. The arm is able to exert 200,000 pounds of force. These two facts are provided in proximity, not to suggest the arm actually operates on Earth, but to provide a frame of reference about the amount of force involved.

You have a different article than I do. The article that started the discussion was this one.
At the end of that:
The entire 55-foot robot arm assembly is capable of lifting 220,000 pounds, which is the weight of a space shuttle orbiter.

It was designed to work in space. There it can handle a million kg. It would just need to do that very slowly.
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Re: 1458: Small Moon

Postby HES » Thu Dec 11, 2014 4:18 pm UTC

To me, pounds of force is my hitman/bodyguard's hourly rate
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Re: 1458: Small Moon

Postby Pfhorrest » Thu Dec 11, 2014 6:08 pm UTC

Neil_Boekend wrote:
The entire 55-foot robot arm assembly is capable of lifting 220,000 pounds, which is the weight of a space shuttle orbiter.

It was designed to work in space. There it can handle a million kg. It would just need to do that very slowly.

kg is a unit of mass. Pounds is a unit of force. Weight is also a measure of force, not mass. It is saying that it can apply 220000 pounds of force, which is how much force the mass of a shuttle in Earth's gravity applies downward, i.e. How much it weighs. In different gravity, that 220000 pounds of force could lift different masses, because different masses weight differently in different gravity. In zero gravity it could lift arbitrary mass, as you say... But still only apply 220000 pounds of force to di it.
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Re: 1458: Small Moon

Postby slinches » Thu Dec 11, 2014 8:56 pm UTC

Pfhorrest wrote:Pounds is a unit of force. Weight is also a measure of force, not mass. It is saying that it can apply 220000 pounds of force, which is how much force the mass of a shuttle in Earth's gravity applies downward

Actually, the pound in engineering can be either a mass or force unit depending on the context (F=m*a becomes F=m if a==1). And in this case, they're mixing in the non-technical term "weight" which only makes it more confusing. Because of issues like this it's common practice (but still less common than it should be) to use lbm and lbf for pounds mass and pounds force respectively in technical documents where the intent of lb (or lbs) might be ambiguous.

As for which is intended in the article, I did a few quick hand calcs and there's no way that the arm is designed for a 220000 lbf load. The stresses are an order of magnitude too high for aerospace grade materials and that's without considering the weight of the arm itself. Instead, it's likely designed to achieve some predefined performance criteria when maneuvering a mass of up to 220000 lbm while in orbit.

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Re: 1458: Small Moon

Postby ps.02 » Thu Dec 11, 2014 11:07 pm UTC

slinches wrote:Because of issues like this it's common practice (but still less common than it should be) to use lbm and lbf for pounds mass and pounds force respectively in technical documents where the intent of lb (or lbs) might be ambiguous.

And then there's kilograms as a force. I thought that was an artifact of people not understanding the difference, or of wanting to appear to use the metric system, while not really understanding it. But today I looked it up and learned that the kilogram-force is actually a thing - it is ~9.81 N. So I guess it's for people who like to use the metric system, but don't really want to have exact powers of 10 as conversion factors? A strange combination, to my mind. Anyway that makes me one of today's luck 10000, or perhaps today's lucky 9807.

It's not really on-topic for this What-If, but Randall has used kg as a unit of force in previous installments - I'm not sure how many, but I've noticed it at least once. I wish he'd use N, really, but kgf would be better than kg. At least it acknowledges that you know that mass and force are not the same thing.

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Re: 1458: Small Moon

Postby Neil_Boekend » Fri Dec 12, 2014 9:15 am UTC

I can't remember ever having seen kilogram force in actual use, although I do know of it's existence. I assume that it is an archaic unit and no longer in use.
It's also idiotic.
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Re: 1458: Small Moon

Postby Diadem » Fri Dec 12, 2014 10:10 am UTC

ps.02 wrote:And then there's kilograms as a force. I thought that was an artifact of people not understanding the difference, or of wanting to appear to use the metric system, while not really understanding it. But today I looked it up and learned that the kilogram-force is actually a thing - it is ~9.81 N. So I guess it's for people who like to use the metric system, but don't really want to have exact powers of 10 as conversion factors?

I guess you don't have a background in physics, or you'd have recognized 9.81 as earth's gravitational constant. This makes the definition entirely logical: The weight of an object in kg is the same as its mass in kg. And you probably use this all the time, without even realizing it. For example when you say something like "he can bench press 100 kg".

I never use American units, but as far as I know the situation is the exact same for pound force and pound mass, isn't it? There's the same conversion factor there. A pound mass = 0.453592 kg and a pound force = 4.44822162 N, according to Google. That's a factor 9.81 difference too.
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Re: 1458: Small Moon

Postby Neil_Boekend » Fri Dec 12, 2014 11:33 am UTC

I am an engineer so I usually use 10 N for the gravitational force exerted on a kg on the surface of Earth. Makes for easy sanity checking of my calculations.
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Re: 1458: Small Moon

Postby orthogon » Fri Dec 12, 2014 2:50 pm UTC

Diadem wrote:
ps.02 wrote:And then there's kilograms as a force. I thought that was an artifact of people not understanding the difference, or of wanting to appear to use the metric system, while not really understanding it. But today I looked it up and learned that the kilogram-force is actually a thing - it is ~9.81 N. So I guess it's for people who like to use the metric system, but don't really want to have exact powers of 10 as conversion factors?

I guess you don't have a background in physics, or you'd have recognized 9.81 as earth's gravitational constant.

To be fair, little g isn't really 9.81, it's 9.81N/kg=9.81m/s2=32.2 ft/s2. Someone used to imperial units may well not recognise the number 9.81. And anyway, it's not a constant, either!

Diadem wrote:This makes the definition entirely logical: The weight of an object in kg is the same as its mass in kg. And you probably use this all the time, without even realizing it. For example when you say something like "he can bench press 100 kg".

But those barbells and things that you buy are really masses. What he can actually do is put 100kg of mass on the bench press in his local gym, and proceed to lift it chestily, by exerting whatever force that turns out to require.
Diadem wrote:I never use American units, but as far as I know the situation is the exact same for pound force and pound mass, isn't it? There's the same conversion factor there. A pound mass = 0.453592 kg and a pound force = 4.44822162 N, according to Google. That's a factor 9.81 difference too.

Well, again, it's not a scaling factor, it's a quantity with dimensions: 9.81N/kg, representing the gravitational field strength at the Earth's surface. So it's always going to work out at 9.81N/kg if you convert it to SI units. But in imperial units, it's 1lbf is the weight in normal gravity of 1lb; leaving things in imperial units we have g=1lbf/lb. The point is that lbf and lb are not the same unit and can't be cancelled. They've just been chosen for convenience when dealing with normal gravity. It's a bit like the way one fluid ounce of water has a mass of 1 ounce, i.e. the density of water is 1oz/floz. Unfortunately in the US the "fl" part is normally omitted, which is even more confusing, since the density of water would be 1oz/oz where the "oz" top and bottom aren't the same unit and can't be cancelled.
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Re: 1458: Small Moon

Postby serutan » Fri Dec 12, 2014 3:26 pm UTC

Took me a while to realize that this was a nod to the beginning of the Pluto Approach mission for New Horizons.
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Re: 1458: Small Moon

Postby ps.02 » Fri Dec 12, 2014 5:32 pm UTC

Neil_Boekend wrote:I can't remember ever having seen kilogram force in actual use, although I do know of it's existence. I assume that it is an archaic unit and no longer in use.
It's also idiotic.

Here's one citation: some guy on the Internet was exploring the ramifications of a hypothetical scenario in which the earth expanded at a constant rate over time:
Randall Munroe, http://what-if.xkcd.com/67/ wrote:After five years, gravity would be 25% stronger. If you weighed 70 kg when the expansion started, you'd weigh 88 kg now.

This threw me at the time because, as I said, I didn't realize kg as a unit of force was actually a thing; I thought kg always referred to mass. And I figured that guy on the Internet would know the difference.

Diadem wrote:I guess you don't have a background in physics, or you'd have recognized 9.81 as earth's gravitational constant.

Oh, certainly I recognised g. I know that that's what a kgf represents: the force of gravity on 1 kg of mass. It still seems bizarre to me, as though someone had grown up in the Imperial system in which pounds are routinely used for both force and mass (so it's easy to confuse the two), and then late in life tries to use the metric system, but without noticing that in SI, these two different things have two different units.

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Re: 1458: Small Moon

Postby orthogon » Fri Dec 12, 2014 6:00 pm UTC

ps.02 wrote:
Neil_Boekend wrote:I can't remember ever having seen kilogram force in actual use, although I do know of it's existence. I assume that it is an archaic unit and no longer in use.
It's also idiotic.

Here's one citation: some guy on the Internet was exploring the ramifications of a hypothetical scenario in which the earth expanded at a constant rate over time:
Randall Munroe, http://what-if.xkcd.com/67/ wrote:After five years, gravity would be 25% stronger. If you weighed 70 kg when the expansion started, you'd weigh 88 kg now.

This threw me at the time because, as I said, I didn't realize kg as a unit of force was actually a thing; I thought kg always referred to mass. And I figured that guy on the Internet would know the difference.

I suppose we were intended to read it as "you would weigh [the equivalent of] 88kg now". I'd hazard a guess that even the most dedicated SI fanatic wouldn't actually, in a dinner party conversation, express their weight in Newtons. In any case, body mass is really the underlying quantity we're concerned about, and weight under normal gravity is a proxy for it. For example, if one of the All Blacks' front row is heading towards you at 8m/s, your first concern is going to be how much momentum he has when he hits you; how much force you'll be crushed by when he's lying on top of you is your second.
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Re: 1458: Small Moon

Postby mathmannix » Fri Dec 12, 2014 6:04 pm UTC

This is why mass in expressed in kilograms, and weight in pounds. Problem solved.

EDIT: And force in Newtons (or sometimes foot-pounds.)
I'm serious. Mass is used in physics/chemistry/math equations, for people that actually care about using terms correctly. Weight is used for measuring yourself on a scale and deciding you need to lose a few pounds (and not by changing elevation or riding an elevator). Force is ... eh, see mass.
Last edited by mathmannix on Fri Dec 12, 2014 6:05 pm UTC, edited 1 time in total.
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Re: 1458: Small Moon

Postby Kit. » Fri Dec 12, 2014 6:04 pm UTC

ps.02 wrote:It still seems bizarre to me, as though someone had grown up in the Imperial system in which pounds are routinely used for both force and mass (so it's easy to confuse the two), and then late in life tries to use the metric system, but without noticing that in SI, these two different things have two different units.

It has nothing to do with the Imperial system. The simplest way to measure masses here on the Earth's surface is by comparing their weights to the weights of some standard masses. And then you are having pound weights, kg weights and so on.

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Re: 1458: Small Moon

Postby Quizatzhaderac » Fri Dec 12, 2014 6:13 pm UTC

orthogon wrote:But those barbells and things that you buy are really masses. What he can actually do is put 100kg of mass on the bench press in his local gym, and proceed to lift it chestily, by exerting whatever force that turns out to require.
In this case the force is the first thing you decide: you pick/add the barbells to produce that force. If one constructed a bench press with springs instead of barbells, one wouldn't care how much mass the springs have so much as the force required to workout.
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Re: 1458: Small Moon

Postby orthogon » Fri Dec 12, 2014 6:27 pm UTC

Quizatzhaderac wrote:
orthogon wrote:But those barbells and things that you buy are really masses. What he can actually do is put 100kg of mass on the bench press in his local gym, and proceed to lift it chestily, by exerting whatever force that turns out to require.
In this case the force is the first thing you decide: you pick add the barbells to produce that force. If one constructed a bench press with springs instead of barbells, one wouldn't care how much mass the springs have so much as the force required to workout.

Fair point, and machines like that are quite common. It would be really interesting if we lived in an environment where gravity varied significantly. We'd all treat weight and mass as distinct. And we'd bench kilonewtons, not kg.
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Re: 1458: Small Moon

Postby Pfhorrest » Fri Dec 12, 2014 6:47 pm UTC

But only a few kilonewtons at best, considering a single kilonewton is the weight of a large man.
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Re: 1458: Small Moon

Postby Coyoty » Fri Dec 12, 2014 6:53 pm UTC

chenille wrote:
Coyoty wrote:"That's no moon... It's a Kardashian!"

Only two letters away from a Star Wars and Star Trek crossover.


You should see her Kardashian neck trick.

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Re: 1458: Small Moon

Postby ps.02 » Fri Dec 12, 2014 9:24 pm UTC

orthogon wrote:I suppose we were intended to read it as "you would weigh [the equivalent of] 88kg now".

That's always the problem with stating something inaccurate in order to sound more accessible, though, isn't it? I mean, if you say "nucular", a lot of people will assume you just didn't know any better, when the truth may be that you know what the prescriptivists say, but believe your audience will be more receptive to hearing it "wrong".

I'd hazard a guess that even the most dedicated SI fanatic wouldn't actually, in a dinner party conversation, express their weight in Newtons. In any case, body mass is really the underlying quantity we're concerned about

Indeed. Mass is what you're really talking about - and the fact that it is a lot easier to measure indirectly (gravitational force) than directly (inertia) doesn't change this. So it's entirely plausible to me, and I've always subconsciously believed, that those who express "body weight" in kilograms are really using "weight" to mean "mass", as opposed to using kilograms to measure force.

So yeah, the guy on the Internet talking about how your number of kilograms changes with a change in gravity threw me for a loop. It really did sound to me like he didn't quite understand the difference.

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Re: 1458: Small Moon

Postby PinkShinyRose » Sat Dec 13, 2014 9:55 pm UTC

ps.02 wrote:
I'd hazard a guess that even the most dedicated SI fanatic wouldn't actually, in a dinner party conversation, express their weight in Newtons. In any case, body mass is really the underlying quantity we're concerned about

Indeed. Mass is what you're really talking about - and the fact that it is a lot easier to measure indirectly (gravitational force) than directly (inertia) doesn't change this. So it's entirely plausible to me, and I've always subconsciously believed, that those who express "body weight" in kilograms are really using "weight" to mean "mass", as opposed to using kilograms to measure force.

So yeah, the guy on the Internet talking about how your number of kilograms changes with a change in gravity threw me for a loop. It really did sound to me like he didn't quite understand the difference.

It also only works if you use spring based scales as opposed to balance scales. And it becomes annoying when you want to calculate your BMI.


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