If you were writing a recipe, that would be used by people living in
different gravity fields (lets say Earth, orbit and Moon), what
measurements woud you use to make sure the quantities are the same
everywhere?

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Request for Question Clarification by shiva777-ga on 08 May 2003 12:05 PDT

I am posting this as a clairication request because while I have
confidence in my research abilities, my self-confidence in the area of
physics is far from absolute. :-)
 
I just finished up some research on this and I came to a conclusion on
it based on reading a lengthy Usenet discussion on it that you can
read for yourself at:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&threadm=ue03672rt9l613%40corp.supernews.com&rnum=25&prev=/groups%3Fq%3Dmeasurements%2Bspace%2Bdifferent%2Bgravity%2Bpound%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26safe%3Doff%26selm%3Due03672rt9l613%2540corp.supernews.com%26rnum%3D25

If it's acceptable as an answer let me know and I will post it as
such. If others have comments or opinions please share.

After reading this Usenet discussion in its entirety, my answer to
your question is that you would not have to change anything in a
recipe no matter where you were. Recipes use "proper weight" which is
measured by mass. A cup is *always* 8 ounces. There will *always* be 2
tablespoons (or 6 teaspoons) in an ounce. It doesn't matter whether
you're on earth, on mars or in outer space.

Physicists determine weight by using the formula:
"weight = mass * gravitational_acceleration"
but for recipes this would make no sense.

As an illustration, in a somewhat heated exchange in that discussion
thread someone jokes about how big a Mcdonalds 1/4 pounder would have
to be when they open up their first franchise on the moon if they are
using the physics formula rather than "proper weight" which is used in
the food industry, and every other industry that sells things by
weight.

Mass is used in recipes and not true weight. 

Well, I think this just about took my brain to it's limit. :-)  If you
are satisfied with this as an answer, let me know and I will post it
as such. And if there are any objectors please post away. This is one
of the more interesting questions I've had the pleasure to research on
here.

thanks,
-shiva777

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Clarification of Question by spacebloom-ga on 08 May 2003 14:43 PDT

Thanks shiva777 for your work so far.
The usenet discussion is interesting, although, for someone like me,
it does not offer an answer. In fact, it is very hard to determine who
speaks with authority in that thread, so I will not take that as an
answer.

Lets forget about the definitions and formulas.  
Here are few examples that hopefully will illustrate what I do NOT
know.

case A: I am on Earth, recipe calls for 1 pound of sugar. Easy. I will
use a scale to separate 1 pound of sugar from a 10 pound bag.

case B: I am ready to lift off and know I want to make a pie in the
space, I check out the recipe find out I need 1 pound of sugar, using
a scale while still on Earth I separate 1 pound of sugar form a 10
pound bag, put it in a bag. When I am floating around, I locate the
bag and know it is 1 pound of sugar.

case C: I am in space floating around when I get hungry for an apple
pie. I open a recipe and it calls for 1 pound of sugar. There is a 10
pound bag onboard. How do I separate 1 pound from it? That I do not
know.

Leaving aside other solutions (such as dividing the volume into 10
equal parts) how do I measure the weight, whether it is called "proper
weight", mass, or just weight. Would I need a special scale, one that
would work in space? Would people living on Moon have different
scales?
I hope this clarifies what I am trying to find out. I wish I had a
ticket to go to space and do a few of hands-on experiments for myself
(seeing is believing). As I do not have the required $20 million or so
I am relying on Google Answers for now.

Thanks.
Spacebloom

Hello -

The key concepts to distinguish here are the differences between
weight and mass.  Mass is the quantity of stuff that we have, and
weight is the effect of gravity upon that mass.  Weight changes with
gravity (so that you weigh much less on the moon than here on earth),
whilst mass is independent of gravity (we will here of course ignore
things that are really massive like blackholes and things moving near
the speed of light).

The mass of ingredients can be measured on earth by comparing their
weights using a balance -  a balance measures the force produced by
gravity acting on a given mass (the weight) - this is calculated as
mass*acceleration due to gravity.  Since a balance involves the
comparison of two weights which "balance" - the forces on either side
must be the same.  Therefore, the mass is the same, regardless of the
acceleration due to gravity.  Thus, a balance scale would work just as
well on the moon (with the same standard masses) as it would here (ie
one kilogram would be one kilogram - one pound of sugar would be 2.2
kilograms).  Now, most scales that are used are not balances but
actually measure weight (usually by measuring the amount a spring or
other material is compressed by the force of gravity acting upon the
mass).  These devices would need to be adjusted for the different
gravity on different planets/moons.

An approach that has been used on space missions is to measure the
rate of ocillation of a spring attatched to the mass of interest
(inertial balance)- if the properties of the spring are known, the
response to a particular mass can be calculated, and thus by measuring
the rate of ocillation, the mass can be determined.
A nice explanation of inertial balances from NASA can be found here
(search for inertial balance):
http://quest.nasa.gov/space/teachers/liftoff/spacelab.html
A brief description of this can be found here:
http://history.nasa.gov/SP-401/ch6.htm

So, your original question:
What to use to measure quantities that would be the same everywhere?
Mass.

So, looking at your cases:

Case A and B are as you describe.

Case C - you could use an inertial scale to measure 0.45 kilograms (1
pound worth) of sugar.

and I would suggest a Case D - if on Mars, you could use the same
balance scale used on Earth to measure 0.45 kilograms of sugar.

Let me know if you have further questions.

synarchy

Google search for references: measure mass space

Synarchy-ga, I appreciate your willingness to leave out the formulas
and explain this in plain language. Thank you. Spacebloom-ga

Well Spacebloom...this is a mind bender. Using the formula
 
weight = mass * gravitational_acceleration, 

I suppose you would need to figure out the gravitational pull of
wherever you were at and an adjustable scale that you punch this into.

I think that I will leave this to one of my more learned colleagues
here. I'm interested to find out what they come up with myself!

-shiva777

Synarchy-ga provided good and comprehensive answer.

 I want to add few tangential comments on this issue,
 which is clear and simple from the point of view of
 physics, but debated not only on the Usenet but between 
 teachers of physics as well. The issue is how to explain this best.
 Students (and so public) on both sides of Atlantic have
 long standing problem with this question, but there is
 more confusion in the USA, due to it's use of customary
(imperial) system of units.

 In the SI (metric) units the mass and the weight (which is force)
 are measured by different units (in principle).
 In customary units, both are mesured by 'pounds' which is fine 
 as long as you are stationary on Earth surface but breaks down
 in the orbit and on other planets.

 An advice:  before venturing into space, you
should master enough physics to clearly understand the difference
between force and mass. That only became clear with Newton's
discovery in 1687. Only after that date were physicists able to
predict
how much different masses will 'weigh' in different gravity fields 
and orbits. (Aparently many students get stuck in pre-18 century
physics).

 There is another difference between Europe and USA:  US recepies
 use volume measures, (a cup of this, spoon of that). In Europe they
 mostly use mass (20 grams of this, 2 grams of that). Both types of
cookbooks remain valid in space, but to use American cookbook would be
the simple practical solution, since cup is a cup anywhere, and can be
measured the same way anywhere.

With a European recepie, gram remains the gram everywhere, but the
spring
scales (which include the modern electronic scales) will not show the
correct
number of grams (as  Synarchy-ga explained).

The reason for this 'incorrect reading' is that these scales have
scale
calibrated in grams, while they really measure the weight - the force.
This is the root of the confusion: The SI (metric) unit of force is 1N
(one Newton) but only physicists use it. The general public in Europe
 is as unaware of Newtons as American public is of the difference
between
'force pound' and 'mass pound'. You go to market to buy a kilogram of
bread (or even a 'kilo' of bread) - where kilo really means a
'thousand'
(1 kg = thousand grams) and grocer will measure it on a spring scale
reading in kilograms (or "kilos").
So, while 
 the SI (metric) units the mass and the weight (which is force)
 are measured by different units in principle, the common practice,
 even in metric countries covers up this diffrence by marking
the reading of the weight-measuring instruments (spring scales)
in grams and kilograms, instead of in Newtons. That marking or
calibration makes an assumption that the field of gravity g(r)
 has the particular value, the value it has on Earth surface.
value, namely g(r)=9.81 m/ s *s.

 So - to make it simple: when getting ready for a trip to space,
you not only brush up on physics, but you also take a simple
accelerometer (which can be small 'inertial balance' with known mass,
or a pendulum, like a grandfather's clock) 
In addition to common variables, like temperature and barometric
preasure
(and amount of oxygen in your atmosphere..) you alway want to know
what
the acceleration (or intensity of graviational field) is going to be
and is.
It affects you well being: e.g. during liftoff it can get high, 
often  (inproperly) described in popular press or sci-fi as let's say
5g.
 Strictly,  symbol g means intensity of gravitational field, it is not
a unit.
 g(E) = 9.81 m per second squared -
   meaning value of field at the surface of   Earth is about 9.81 m/
s*s.
 At higher altitude g(r) will decrease as r - the altitude - is
increasing and
 it will also decrease as your angular velocity(the speed in orbit)
increases.


 So, in proper SI units we should say that during liftof we had
 g(r)=50 (m/ s*s) - 5 times the usual g(E) of 9.81 m/ s*s
 Such high g  is quite uncomfortable, I am told.

In free orbit it - the g(r), will drop to zero,
 with all the inconveniences of weightlessness.

 When you know the value of your g(r) you can allways correct the
reading
of a spring scale by multipying the reading of a spring scale
by the factor  g(E)/g(r).
  The *g(E) will convert back to Newtons
  the /g(r) will convert to what the mass is in your local gravity
filed g(r).

 Of course, that will not work if g(r)=0  (you get 0/0) and balances
scale
 will not work either.
 then you resort to inertial balance....

Is this all perfectly clear?

hedgie

Thanks for the additional comments. Greatly appreciated. As an aside I
grew up in Europe and although I have live in Canada for more than 13
years I still have trouble with pounds, feet and miles. In my question
I used pounds, mostly because all the Americans I know will not
understand the metric terms. As the saying goes, when in Rome do as
Romans do. I should have known better.
---
The reason I wanted to leave out the formulas is simple. They do not
figure in our normal lives. Nobody thinks of the physics when using a
scale. Most of us instinctively know what I kilogram should feel like.
The same for volume. We know what a cup is or half a liter of beer. I
was imagining what normal life would be like when we live in space and
this issue came to my attention. I had initially sidestepped the issue
by using volume measurements, thinking it would be easier, since a
liter on Earth will still be one liter on Mars. (as hedgie-ga
suggested) But volume measurements are often not practical. That's why
Earth recipes use both volume and weight.
---
Anyway in my imaginary environment I wanted to publish a space
cookbook but did not want to go to the trouble of localizing the
recipes. Now I know I dont need to. It all is perfectly clear! I will
just use kilograms and liters and publish my recipe book solar system
wide. Thanks.
Spacebloom