"The only instance where mass and weight are different."

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wtfxcore
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"The only instance where mass and weight are different."

Postby wtfxcore » Sun Oct 09, 2011 6:28 pm UTC

So, this topic comes from my middle school sister's science homework.

The question is, "What is the only instance where your mass and weight are different?"
The answer that her teacher gave her is "Anywhere other than Earth."

My argument is, technically on Earth, your mass and weight aren't the same either.
Mass is measured in (kilo)grams, weight is measured in Newtons.

I know I may be being overly critical, but it bugs me.
On Earth, my mass could be, let's say, 50kg. So my weight on Earth would be roughly 9.8 TIMES 50kg.
Those aren't the same numbers, correct? I know that, informally, we use weight and mass interchangeably. But that doesn't mean technically that mass and weight are the same!

What do you think? Do you agree with me, or am I just overthinking things?

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Re: "The only instance where mass and weight are different."

Postby Dr. Diaphanous » Sun Oct 09, 2011 6:39 pm UTC

Wouldn't your weight and mass also be different*, even on Earth, when you are accelerating or decelerating, such as in a lift or in free fall? Maybe the question is asking something to do with that.

*I suppose the question is using "different" to mean "not in a 9.8N/kg ratio"
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Re: "The only instance where mass and weight are different."

Postby Tass » Sun Oct 09, 2011 6:39 pm UTC

wtfxcore wrote:So, this topic comes from my middle school sister's science homework.

The question is, "What is the only instance where your mass and weight are different?"
The answer that her teacher gave her is "Anywhere other than Earth."

My argument is, technically on Earth, your mass and weight aren't the same either.
Mass is measured in (kilo)grams, weight is measured in Newtons.

I know I may be being overly critical, but it bugs me.
On Earth, my mass could be, let's say, 50kg. So my weight on Earth would be roughly 9.8 TIMES 50kg.
Those aren't the same numbers, correct? I know that, informally, we use weight and mass interchangeably. But that doesn't mean technically that mass and weight are the same!

What do you think? Do you agree with me, or am I just overthinking things?


In SI indeed mass and weight are proportional but not equal anywhere. As you say they have different units.

But he might have a point (not really but you could make an argument for it) if you are using the units pounds weight and pounds force, both are commonly just called pounds, and there is only a one to one relationship when the gravitational acceleration is g.

Anyway it is still not true on Earth in accelerated frames.

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Re: "The only instance where mass and weight are different."

Postby doogly » Sun Oct 09, 2011 8:27 pm UTC

Yeah, you can find elevators on earth. This is a sloppy question.
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Re: "The only instance where mass and weight are different."

Postby EdgarJPublius » Mon Oct 10, 2011 12:20 am UTC

Also consider that the force of gravity acting on you varies depending on altitude and latitude.

So, even on Earth, standing perfectly still on solid ground, Mass != Weight.
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Re: "The only instance where mass and weight are different."

Postby yurell » Mon Oct 10, 2011 12:34 am UTC

Not to mention the presence of groundwater, which is why we can use gravity detectors to map the Earth with satellites.
In fact, mass and weight have entirely different dimensions, so they can't be said to be equal. Your mass is your resistance to acceleration, your weight is the force necessary to stop your acceleration. So your sister could probably argue to get her marks back, if she lost any.
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Re: "The only instance where mass and weight are different."

Postby curtis95112 » Mon Oct 10, 2011 4:38 am UTC

If the teacher is reasonable, she could get her points back by claiming that the gravitational acceleration isn't exactly g on most places on Earth.

But anyway, bad question. Mass doesn't equal weight. They're different concepts and the question isn't helping the students.
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Re: "The only instance where mass and weight are different."

Postby Diadem » Mon Oct 10, 2011 4:42 am UTC

Not to mention the terrible phrasing. "everywhere but x" is not a single instance.

Q: "Who was the only person that didn't write Hamlet?"
A: Everybody who wasn't Shakespeare.

Madness!


Also the answer is wrong for another reason as the one already given as well. If I'm on a spaceship accelerating at 1 g my mass equals my weight (for a certain definition of equals), but I'm not on earth.
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Re: "The only instance where mass and weight are different."

Postby Zamfir » Mon Oct 10, 2011 8:46 am UTC

Diadem wrote:Q: "Who was the only person that didn't write Hamlet?"
A: Everybody who wasn't Shakespeare.

There were versions of Hamlet before Shakespeare. [/pedantic]

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Re: "The only instance where mass and weight are different."

Postby AvatarIII » Mon Oct 10, 2011 9:33 am UTC

Diadem wrote:Not to mention the terrible phrasing. "everywhere but x" is not a single instance.

Q: "Who was the only person that didn't write Hamlet?"
A: Everybody who wasn't Shakespeare.

Madness!


Also the answer is wrong for another reason as the one already given as well. If I'm on a spaceship accelerating at 1 g my mass equals my weight (for a certain definition of equals), but I'm not on earth.


yeah this was my complaint too.

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Re: "The only instance where mass and weight are different."

Postby curtis95112 » Mon Oct 10, 2011 9:48 am UTC

If your sister has any intention of going into a STEM field, it might be beneficial to find a new teacher if possible or to get someone competent to privately tutor her. The knowledge you're exposed to during your formative years is very important.
While you can't generally tell the competence of a teacher from a single question, that question is horrible enough to warrant suspicion.
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Re: "The only instance where mass and weight are different."

Postby Aelfyre » Mon Oct 10, 2011 9:55 am UTC

Does your teacher have a particularly bad sense of humor? Maybe the answer they are looking for is "At a Catholic church." :)
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Re: "The only instance where mass and weight are different."

Postby Gigano » Mon Oct 10, 2011 10:42 am UTC

Aelfyre wrote:Does your teacher have a particularly bad sense of humor? Maybe the answer they are looking for is "At a Catholic church." :)


*groan*

The pun is good, but it's just so amazingly lame. :')

OT: I agree with what has been said here: mass and weight aren't the same concepts, have different units and weight is too much dependent on environmental factors for it to be ever considered a constant quantity such as mass. Tell the teacher, via your sister, never to ask that question again and suggest reading up on same basic physics.
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Re: "The only instance where mass and weight are different."

Postby dainbramage » Mon Oct 10, 2011 12:09 pm UTC

Yes, the teacher is completely wrong. The correct answer would be "the only instance where mass and weight are different is all the fricking time". Bad science teachers seem to be very common (I had a terrible physics teacher... I think my favourite moment was getting docked marks in an exam for not putting units on a dimensionless constant). Hopefully your sister's bright enough to know when they're wrong, and/or look up the material herself. If you haven't already done so, you should probably make sure your sister understands that they're different.

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Re: "The only instance where mass and weight are different."

Postby Tass » Mon Oct 10, 2011 1:44 pm UTC

dainbramage wrote:I think my favourite moment was getting docked marks in an exam for not putting units on a dimensionless constant.


Wow. Which constant? And what units did the teacher want?

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Re: "The only instance where mass and weight are different."

Postby mfb » Tue Oct 11, 2011 5:29 pm UTC

wtfxcore wrote:The question is, "What is the only instance where your mass and weight are different?"

My answer: The universe.
May be expanded for models with multiple universes.

The question is really strange...

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Re: "The only instance where mass and weight are different."

Postby curtis95112 » Wed Oct 12, 2011 9:48 am UTC

Tass wrote:
dainbramage wrote:I think my favourite moment was getting docked marks in an exam for not putting units on a dimensionless constant.


Wow. Which constant? And what units did the teacher want?


Maybe an equilibrium constant?
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Re: "The only instance where mass and weight are different."

Postby wtfxcore » Mon Oct 24, 2011 12:09 pm UTC

Anyway, it's been a while since I've checked the thread.

Thank you guys for all the responses- I was getting so fired up that day and I wanted to make sure I wasn't in the wrong!

My sister is 13 and chose to take that opportunity to argue with me (one of her favorite pastimes.) She said "I think my teacher knows more than you do" - even though I'm in school for Secondary Physics Education, no big deal. She said that her science teacher previously worked for NASA, so I couldn't possibly be right.

Oh, youngins. Her dream careers at this point are lawyer, marine biologist, and professional cheerleader.
I still highly suspect that one of us was switched at birth.

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Re: "The only instance where mass and weight are different."

Postby dainbramage » Wed Oct 26, 2011 7:42 am UTC

Tass wrote:
dainbramage wrote:I think my favourite moment was getting docked marks in an exam for not putting units on a dimensionless constant.


Wow. Which constant? And what units did the teacher want?

Gamma in relativity. Which I guess isn't a constant, but eh. It was the same 3 times for the question, so I calculated it once at the start then reused it. I'm not sure what units she wanted, just that "everything has to have units". Trying to show her that it didn't have units through dimensional analysis only resulted in "if you keep pestering me, I'm going to dock you marks". There were a few other cases with the same teacher, such as a question where I gave a more detailed answer than they required, and lost marks for it (A diagram of two parallel wires with currents being run through them. Question: why are they pulled together? I wrote about how the current in one wire produces a magnetic field which interacts with the current in the other wire. The correct answer was "because the currents are parallel"). I can't remember the rest, but there were several other cases similar to that. And it wasn't just exam stuff, she was consistently wrong about a lot of stuff when teaching it. That is, when her teaching involved more than spending the first 15 minutes of class googling whatever she was meant to be teaching for that day. Oh, and she always pronounced de Broglie as "de-bro-glee".


In retrospect I should have gone to the head of curriculum at my school (who was nice and always reasonable), since this occurred in the final year of our secondary education where every mark you do or don't get is important for university entry, plus her teaching would have put the whole class at a disadvantage come the end-of-year exams which are universal rather than set by the school. The random docking of marks also cost me my first place in physics, which is extremely important with how the system worked (basically, marks are split into internal, which are done by the school, and external, which are sat by everyone. Internal marks are scaled by external, such that the highest rank internally gets the top mark from that school in the external exam, and the second rank will get approximately the second highest mark in the external exam).

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Re: "The only instance where mass and weight are different."

Postby Tass » Thu Oct 27, 2011 6:01 am UTC

dainbramage wrote:Gamma in relativity. Which I guess isn't a constant, but eh. It was the same 3 times for the question, so I calculated it once at the start then reused it. I'm not sure what units she wanted, just that "everything has to have units". Trying to show her that it didn't have units through dimensional analysis only resulted in "if you keep pestering me, I'm going to dock you marks". There were a few other cases with the same teacher, such as a question where I gave a more detailed answer than they required, and lost marks for it (A diagram of two parallel wires with currents being run through them. Question: why are they pulled together? I wrote about how the current in one wire produces a magnetic field which interacts with the current in the other wire. The correct answer was "because the currents are parallel").


Ouch. :shock: :evil:

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Re: "The only instance where mass and weight are different."

Postby Macbi » Thu Oct 27, 2011 4:34 pm UTC

dainbramage wrote: Oh, and she always pronounced de Broglie as "de-bro-glee".
Aww... dammit. How is it supposed to be pronounced?
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Re: "The only instance where mass and weight are different."

Postby eSOANEM » Thu Oct 27, 2011 5:09 pm UTC

It's pronounced "de broy" (with broy rhyming with boy).
my pronouns are they

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Re: "The only instance where mass and weight are different."

Postby thoughtfully » Thu Oct 27, 2011 6:25 pm UTC

and Euler is pronounced oiler :)
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Re: "The only instance where mass and weight are different."

Postby Dopefish » Thu Oct 27, 2011 6:46 pm UTC

Hum, I knew it wasn't "de-bro-glee", but I thought it was de-broy-lee. Good to know theres apparently no lee part and the entire end of his name is just there to throw people off. :P

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Re: "The only instance where mass and weight are different."

Postby doogly » Thu Oct 27, 2011 8:15 pm UTC

Lah-tech is also acceptable in certain circles.
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Re: "The only instance where mass and weight are different."

Postby Tass » Fri Oct 28, 2011 11:01 am UTC

thoughtfully wrote:and Euler is pronounced oiler :)


Of course. How else would you pronounce it?

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Re: "The only instance where mass and weight are different."

Postby yurell » Fri Oct 28, 2011 11:15 am UTC

Many people say You-ler
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Re: "The only instance where mass and weight are different."

Postby Zamfir » Fri Oct 28, 2011 12:18 pm UTC

Euler and de Broglie are different cases. Euler has just the natural pronunciation in German, but "de Broglie" is not obvious to French speakers either. It's like Lancastershire.

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Re: "The only instance where mass and weight are different."

Postby jmorgan3 » Fri Oct 28, 2011 12:34 pm UTC

Tass wrote:
thoughtfully wrote:and Euler is pronounced oiler :)


Of course. How else would you pronounce it?

Out of curiosity, how do you pronounce "Euclid"?
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Re: "The only instance where mass and weight are different."

Postby cpt » Fri Oct 28, 2011 1:22 pm UTC

jmorgan3 wrote:
Tass wrote:
thoughtfully wrote:and Euler is pronounced oiler :)


Of course. How else would you pronounce it?

Out of curiosity, how do you pronounce "Euclid"?


I believe that this one actually is "You-clid". Reinforced by the way every single person I have ever heard has pronounced "Euclidean"

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Re: "The only instance where mass and weight are different."

Postby Tass » Fri Oct 28, 2011 2:11 pm UTC

jmorgan3 wrote:Out of curiosity, how do you pronounce "Euclid"?


Its been a long time since I've had to pronounce it as part of an english sentence. Every dane I know pronounces it (in danish talk) as "øvklid". Hmm I don't know how to write that so the pronounciation is clear to an english speaker, I should really learn the phonetic alphabet one of these days. I guess the wowel as in french "neuf" followed by a v or w, and ending with the soft danish d. I know it is wrong but You-klid would sound terribly out of place in a danish sentence.

I guess if I actually had to say euclid while speaking english I would stumple on it because the danish pronunciation I am used to would sound awfully out of place in an english sentence. Unless I had thought about it before hand and remembered to say you-klid.

Euler we pronounce as "øgler", except with more force on the first syllable, "oiler" comes pretty close in this case though again the force on the first syllable is more than the spelling indicates. So, yeah, "eu" gets pronounced differently in the two names. (Even though curiously I would use the same 'Ø' to write it phonetically in danish. Danish is almost as irregular as english.)

Unsurprisingly we get the german right but the greek wrong. Closeness of language probably have a lot to do with it.

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Re: "The only instance where mass and weight are different."

Postby Zamfir » Fri Oct 28, 2011 2:48 pm UTC

You sure the Greek one is wrong? Pronunciation of dead languages is fraud with uncertainty, but what I was taught as best guess sounds a lot like "eu as in neuf, followed by a w". Perhaps a bit closer to "a as in english bane, followed by w".

So euclides would then be something like "aw-clee-des", with the a from bane and the last e from bed. Or perhaps "aou", using the same long "a", but the ou from you.


That's not how it is pronounced here, just what they taught me in school as how Greek was perhaps pronounced, somewhere around 400 BC or so, in the neighbourhood of Athens.

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Re: "The only instance where mass and weight are different."

Postby eSOANEM » Fri Oct 28, 2011 5:56 pm UTC

Zamfir wrote:Euler and de Broglie are different cases. Euler has just the natural pronunciation in German, but "de Broglie" is not obvious to French speakers either. It's like Lancastershire.


No, even though it is the shire of Lancaster, it's spelt Lancashire so its pronunciation is fairly obvious. Better examples in English would Cholmondeley (pronounced chum-lee) or possibly Leicester (less-ter).

cpt wrote:
jmorgan3 wrote:
Tass wrote:
thoughtfully wrote:and Euler is pronounced oiler :)


Of course. How else would you pronounce it?

Out of curiosity, how do you pronounce "Euclid"?


I believe that this one actually is "You-clid". Reinforced by the way every single person I have ever heard has pronounced "Euclidean"


I believe this is the standard pronunciation of a greek "eu" in English, because [eʊ̯] (the sound Zamfir is describing) is a very hard diphthong for most English speakers to pronounce and mutates into "you" fairly naturally by simply making it a rising diphthong and a couple of small movements to make it fit the usual phones.
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Re: "The only instance where mass and weight are different."

Postby Zamfir » Fri Oct 28, 2011 9:42 pm UTC

eSOANEM wrote:
Zamfir wrote:Euler and de Broglie are different cases. Euler has just the natural pronunciation in German, but "de Broglie" is not obvious to French speakers either. It's like Lancastershire.


No, even though it is the shire of Lancaster, it's spelt Lancashire so its pronunciation is fairly obvious. Better examples in English would Cholmondeley (pronounced chum-lee) or possibly Leicester

Doh! I meant to use Leicester, but mixed them up somewhere in between.

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Re: "The only instance where mass and weight are different."

Postby gmalivuk » Sat Oct 29, 2011 7:52 pm UTC

Tass wrote:
jmorgan3 wrote:Out of curiosity, how do you pronounce "Euclid"?
Every dane I know pronounces it (in danish talk) as "øvklid".
...
Euler we pronounce as "øgler"
The point being, both begin in English with "eu", so it's natural for an English speaker unfamiliar with one or the other to be inclined to pronounce them beginning with the same sound.
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Re: "The only instance where mass and weight are different."

Postby tomandlu » Sun Oct 30, 2011 7:34 am UTC

wtfxcore wrote:What do you think? Do you agree with me, or am I just overthinking things?


In fairness to the teacher, although inaccurate for the all the reasons others have pointed out, as an introduction to the difference between weight and mass, it strikes me as a reasonable approximation.
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Re: "The only instance where mass and weight are different."

Postby thoughtfully » Mon Oct 31, 2011 5:07 am UTC

tomandlu wrote:
wtfxcore wrote:What do you think? Do you agree with me, or am I just overthinking things?


In fairness to the teacher, although inaccurate for the all the reasons others have pointed out, as an introduction to the difference between weight and mass, it strikes me as a reasonable approximation.

It would be far better stated as "the only instance when mass and weight are the same". I don't think I'd quibble much with that.
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Re: "The only instance where mass and weight are different."

Postby Soralin » Mon Oct 31, 2011 12:30 pm UTC

Even then it doesn't really fit, they're different units, weight has units of force, not mass. The only way it really fits is by people colloquially and incorrectly using units of mass when they mean weight, or vice-versa.

I'd say something like this might be a better introduction:

http://www.projectrho.com/rocket/misconceptions.php
Mass Is Not Weight

There is a difference between weight and mass. An object's mass is always the same, but an object's weight depends upon what planet it is sitting on. A brick with a mass of one kilogram will have a weight of 9.81 newtons (2.2 pounds) on Terra, a weight of 1.62 newtons on Luna (0.36 pounds), and a weight of zero newtons (0 pounds) on the International Space Station. But in all cases it's mass will be the same: one kilogram. (Chris Buzon points out that if the object is moving at relativistic velocities relative to you, you will measure a mass increase. But this is not noticeable at ordinary relative velocities.)

The practical consequence is that if you are in a spacesuit on the Space Station, you cannot move everything by tapping it with your pinky finger (you may start it moving at a rate of one millimeter per week, but that is close enough to "cannot" for government work). The Space Shuttle may be floating next to the station with a weight of zero, but it still has a mass of 90 metric tons. If it is stationary and you pushed on it, there will be very little effect (in fact, about the same effect as if the Shuttle was sitting on the tarmac at Cape Kennedy and you gave it a shove).

And if it is moving slowly on a collision course with the station, and you are in between, the fact that it has zero weight will not prevent it from crushing you like a bug despite your attempts to stop it. It takes just as much energy to stop an object as it took to start it moving.

Sorry, but your orbital construction crews will NOT be able to manually manipulate multi-ton girders like they were toothpicks.

The other factor to consider is Newton's Third Law. If you push on a girder, there will be both action and reaction. Since the girder has more mass, it will start moving a microscopic amount. But since you have less, you will start moving in the opposite direction with much more velocity. This renders many common tools unusable in the free fall environment, such as hammers and screwdrivers.

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Re: "The only instance where mass and weight are different."

Postby Tass » Mon Oct 31, 2011 1:37 pm UTC

Soralin wrote:
And if it is moving slowly on a collision course with the station, and you are in between, the fact that it has zero weight will not prevent it from crushing you like a bug despite your attempts to stop it. It takes just as much energy to stop an object as it took to start it moving.


Of course things are somewhat easier to move when they have no weight given that you can apply force over time. If "slowly" here is something like 10cm/s then you should be able to stop it you push against it with all your strength while slowly squatting, but if you reach the end of your range of motion and it is still moving considerably then you are in big trouble.

Soralin
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Re: "The only instance where mass and weight are different."

Postby Soralin » Mon Oct 31, 2011 6:10 pm UTC

Tass wrote:
Soralin wrote:
And if it is moving slowly on a collision course with the station, and you are in between, the fact that it has zero weight will not prevent it from crushing you like a bug despite your attempts to stop it. It takes just as much energy to stop an object as it took to start it moving.


Of course things are somewhat easier to move when they have no weight given that you can apply force over time. If "slowly" here is something like 10cm/s then you should be able to stop it you push against it with all your strength while slowly squatting, but if you reach the end of your range of motion and it is still moving considerably then you are in big trouble.

Well let's see, it's listed as 90 tons. Lets say that you stand between the two of them, and can start pushing against it and the station when it's 2 meters away, and at 0.5m away is when you start getting crushed. at 10cm/s, it would traverse that distance over a period of 15s. If you're able to apply constant force to bring it to a stop over that distance, then it would take 30s to travel the whole 1.5m, before coming to a stop. Which means you have to provide 0.1m/s of acceleration to a 90,000kg object over 30 seconds. So, 0.003333 m/s2, of acceleration to a 90,000 kg object, is 300N. Or approximately the amount of force of holding a 30.6kg mass above your head on Earth. Although not quite as easily, as you have to keep providing that force as it's lowering down against you.

So at that speed, that might actually be doable, although there's still the potential for doing some damage to you if you can't apply that much force that whole time, or if it's moving a little bit faster than that, etc (say if it was moving at 0.2m/s, and you had 1m to apply the force in, you'd have to apply a constant 1800N over 10s, or the equivalent of holding up a 183.7kg object on Earth). You might even get a bit of extra time from the station below you getting pushed back a bit, although the ISS is about 450 tons, so it wouldn't move as much. Also make sure the place you're standing is well reinforced, your feet getting pushed through the floor might prevent you from getting crushed, but you probably don't want to depressurize the station.


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