# How can we determine "mass" in space

#### anirut

##### Legend
OK, if an object of unknown weight (and mass) is taken to space, in zero-gravity environment, how could it's MASS be determined?

I don't think there'd any be any "correct" answer, but let's see the possibilities.

Cheers!

#### dextor

##### Rookie
Quite impossible, as "zero-gravity" doesn't really exist, everything pulls on everything else in space, albeit decreases with distance.

Physicists calculate mass of distant planets based on the wobble in the wavelength of starlight caused by its gravitational pull on its star.

#### YULitle

##### Hall of Fame
I would build a rotating device with a basket on one end and a "fish scale" of sorts in between the basket and the pivot. The make sure that it can spin at a constant rate of speed. Then, after calibrating it with objects of known mass, you could use it to measure the mass of anything that could fit in the basket.

#### raiden031

##### Legend
When I was a teen learning physics, the whole idea of mass drove me insane. They said weight is the force exerted on an object by the earth's gravity, and that mass was "the amount of matter in an object". Then why the hell did they use a scale that relies on earth's gravity to measure both???

Back to the topic, the only answer I can give the OP is:

Density = mass / volume

So if you know the density and volume of an object, then you can calculate the mass.

#### NotAtTheNet

##### Semi-Pro
all too complicated... bring it to earth and weigh it...

#### dropshot winner

##### Hall of Fame
You could accelerate the object to a predefined speed and then measure the energy on impact.

#### TenniseaWilliams

##### Professional
With objects of normal size, you could get a good idea of mass by simply pushing against the object to see how easily it could be moved.

#### cucio

##### Legend
Piece of cake. You just disintegrate it, measure the energy released and divide it twice by the speed of light. Just don't try this at home unless supervised by an adult.

#### jrod

##### Hall of Fame
Piece of cake. You just disintegrate it, measure the energy released and divide it twice by the speed of light. Just don't try this at home unless supervised by an adult.

Nice Albert...I always thought you were one to have your cake and eat it too.

#### Bud

##### Bionic Poster
OK, if an object of unknown weight (and mass) is taken to space, in zero-gravity environment, how could it's MASS be determined?

I don't think there'd any be any "correct" answer, but let's see the possibilities.

Cheers!
Mass (weight as we know it) of any object will always vary depend on where/which planet/heavenly body it's located on. The larger the planet, the more mass (weight) a similar object exhibits.

Now, if you want to know this mystery objects mass (or weight) as it relates to Earth... just take some basic measurements and know the objects composition.

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#### NotAtTheNet

##### Semi-Pro
Mass (as we know it) of any object will always vary depend on where/which planet/heavenly body it's located on. The larger the planet, the more mass a similar object exhibits.

Now, if you want to know this mystery objects mass as it relates to Earth... just take some basic measurements and know the objects composition.
Not mass... weight changes... mass is constant.

#### YULitle

##### Hall of Fame
When I was a teen learning physics, the whole idea of mass drove me insane. They said weight is the force exerted on an object by the earth's gravity, and that mass was "the amount of matter in an object". Then why the hell did they use a scale that relies on earth's gravity to measure both???

Back to the topic, the only answer I can give the OP is:

Density = mass / volume

So if you know the density and volume of an object, then you can calculate the mass.
There are two different types of measuring devices. A scale and a balance. Both can measure mass. A balance operates like it sounds, it balances two things. On one side you have the object you wish to measure and on the other you have objects of known mass. A scale operates in a number of different ways but is typically spring loaded. Objects placed on the scale depress the spring and how much it is depressed gives you the mass (to varying degrees of accuracy.)

A balance works on the moon (and any other planet where there is enough gravity to keep the objects on the trays.)
A scale does not work on the moon (unless you have it calibrated to work on the moon.)

Also, weight is a measure of force and is dependent on two measures: the mass and the acceleration (or, gravity.) So, since gravity is nearly constant everywhere on earth, weight has been frequently confused with mass.

#### Bud

##### Bionic Poster
Not mass... weight changes... mass is constant.
Yes, I'd already edited it... I assumed the OP was referring to the "Earthly" weight of an object.

In other words... if scientists discovered a 300' x 300' heavenly object... and determined its composition to be graphite :twisted: ... how much would it then weigh here on Earth.

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#### TourTenor

##### Professional
... Watch for the next launch of the Space Shuttle Endeavour that has a priest, a choir boy and the congregation on board???

#### Bud

##### Bionic Poster
... Watch for the next launch of the Space Shuttle Endeavour that has a priest, a choir boy and the congregation on board???
Doh! :shock:

#### flyinghippos101

##### Legend
Yes, I'd already edited it... I assumed the OP was referring to the "Earthly" weight of an object.

In other words... if scientists discovered a 300' x 300' heavenly object... and determined its composition to be graphite :twisted: ... how much would it then weigh here on Earth.
I'm assuming the OP is referring to a unknown mass being taken to space and how it would be determined in a zero-environment of space. Don't think he';s referring to objects found form space and brought to earth for measurement.

#### 35ft6

##### Legend
Congratulations. I think Odds & Ends on the TW message boards is the perfect place to pose this question.

#### Bud

##### Bionic Poster
I'm assuming the OP is referring to a unknown mass being taken to space and how it would be determined in a zero-environment of space. Don't think he';s referring to objects found form space and brought to earth for measurement.
That wouldn't make sense since mass isn't determined by gravity... weight is. He specifically mentions a zero-gravity environment... which tells me he's wondering how the "Earthly" weight of an unknown space object would be determined.

Some people use mass and weight interchangeably when referring to Earth-bound objects.

Here's the OP, BTW:

OK, if an object of unknown weight (and mass) is taken to space, in zero-gravity environment, how could it's MASS be determined?

I don't think there'd any be any "correct" answer, but let's see the possibilities.

Cheers!

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#### YULitle

##### Hall of Fame
That wouldn't make sense since mass isn't determined by gravity... weight is. He specifically mentions a zero-gravity environment... which tells me he's wondering how weight (on Earth) is determined.

Some people use mass and weight interchangeably when referring to Earthly-bound objects.
That's not how I took it. You can easily measure mass on earth, because there is gravity. You need gravity in some measure to work a balance or a scale. In a near-zero gravity environment, those tools would be difficult to use in the same way that we use them on earth.

#### Bud

##### Bionic Poster
That's not how I took it. You can easily measure mass on earth, because there is gravity. You need gravity in some measure to work a balance or a scale. In a near-zero gravity environment, those tools would be difficult to use in the same way that we use them on earth.
Mass isn't dependent on gravity... weight is. On Earth, mass and weight are interchangeable. Off of Earth, they are not.

How do you think scientists come up with the mass of other planets like Jupiter? Do they bring them back to Earth and weigh them?

It appears we need the OP to further clarify what he was referring to.

Perhaps... the OP could have just stated... how do scientists determine the mass of other heavenly bodies.

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#### Bud

##### Bionic Poster

This seems to answer the OP's question:

"The only way we can measure a planet's mass is through its gravity. This has been the way Earth's mass was measured, too (we can't directly probe what's in Earth's interior, but we can measure the gravity on the surface). Since nobody ever visited other planets and was able to measure gravity on the spot, we usually have to resort to other methods. The most commonly used technique is to observe a body orbiting or passing close to the planet and see how its path is affected by the planet's gravity.

For example, if we see a moon orbiting a planet at certain distance from it, the orbital period of the moon at that particular distance will depend on the planet's mass only. Heavier the planet, stronger it attracts the moon and faster the moon moves. It is straightforward for astronomers to calculate the planet's mass after they have observed the motion of one of its moons for a while.

Mercury and Venus have no moons, so their exact mass was not known until a few decades ago. Before spaceflight was developed, the only way to measure their gravity was to see how they affect other planets' orbits. Astronomers would measure very small changes in, say, Earth's orbit, that were caused by the attraction of Venus. These changes are small and it was hard to get the exact mass of Venus by this technique. But once the spacecraft were launched to Venus and they flew close to it, scientists could easily measure its mass by tracking how were these probes attracted while passing by Venus. The same technique was used for Mercury when the Mariner X spacecraft flew by it."

- - - - - - - - - -

So, it appears that the OP's unknown object's mass could be determined by observing it orbiting another body of known mass... then noting the 'attraction' of the smaller (unknown) body to the larger (known) body.

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#### Camilio Pascual

##### Hall of Fame
OK, if an object of unknown weight (and mass) is taken to space, in zero-gravity environment, how could it's MASS be determined? Let's see your creative answers ... all for fun. I don't think there'd any be any "correct" answer, but let's see the possibilities.
1. not creative
2. correct
Take an object of known mass and put it into orbit about the object. Measure its velocities and distances from the object. It will take little time if the object of unknown mass is spherical and uniform in density. It will take more time if the object is irregularly shaped and not uniform in density, but the answer is attainable.

#### Bud

##### Bionic Poster
1. not creative
2. correct
Take an object of known mass and put it into orbit about the object. Measure its velocities and distances from the object. It will take little time if the object of unknown mass is spherical and uniform in density. It will take more time if the object is irregularly shaped and not uniform in density, but the answer is attainable.
I beat you to the punch... see post #21 :twisted:

I think many of us in the thread weren't clear what the OP was seeking.

#### YULitle

##### Hall of Fame
Mass isn't dependent on gravity... weight is. On Earth, mass and weight are interchangeable. Off of Earth, they are not.

How do you think scientists come up with the mass of other planets like Jupiter? Do they bring them back to Earth and weigh them?

It appears we need the OP to further clarify what he was referring to.

Perhaps... the OP could have just stated... how do scientists determine the mass of other heavenly bodies.
You missed my point entirely. The way in which we measure mass on earth IS DEPENDENT on gravity. You need gravity to act on both sides of a balance and you need gravity to pull the object onto the tray of a scale. Without gravity (or some other acceleration,) a balance and a scale are useless. That's why the OP asked how we'd do it without gravity.

#### Camilio Pascual

##### Hall of Fame
I beat you to the punch... see post #21 :twisted:
Eh, the only reason you did is because that was what I was drinking.
I'm pretty sure there is another way that I'm trying to remember that was used to make the first credible estimate of Earth's mass. What I'm thinking about was in a book I read within the last year.
I'm thinking it was a measurement of the movement of an object of known mass by magnetic force against Earth's gravity, but I'm not sure.

#### albino smurf

##### Professional
Where is there zero gravity?

#### LuckyR

##### Legend
Put it next to an object of known mass and measure the gravitational attraction between the two objects.

#### Bud

##### Bionic Poster
Put it next to an object of known mass and measure the gravitational attraction between the two objects.

#### YULitle

##### Hall of Fame
This still requires gravity...
Gravity exists between all objects. So... check!

#### Camilio Pascual

##### Hall of Fame
OK, if an object of unknown weight (and mass) is taken to space, in zero-gravity environment, how could it's MASS be determined?
Okay, so you are ruling out ALL gravity in your zero-gravity environment, which is what you are LITERALLY saying, then ALL solutions involving gravity are out.
Which also means there is NO gravitational attraction between any atoms, correct? So, elements and compounds could exist in your environment, but not mixtures, correct?
I'm asking these questions because I now doubt you understand the implications of your model. But, maybe you do. Let's see.

#### Camilio Pascual

##### Hall of Fame
I think many of us in the thread weren't clear what the OP was seeking.
I think the OP is not clear, either.

#### raiden031

##### Legend
I think the OP is not clear, either.
OP seems clear to me and acknowledges there is no one correct (if any exist at all) answer. The OP is basically looking for the least gravity-reliant way to measure.

#### Bud

##### Bionic Poster
OP seems clear to me and acknowledges there is no one correct (if any exist at all) answer. The OP is basically looking for the least gravity-reliant way to measure.
Perhaps... the OP can give us an example of what he's asking.

#### iamke55

##### Professional
Count the number of protons and multiply by 1.67 * 10^-27 kg, then count the number of neutrons...

#### anirut

##### Legend
WOW .... lots of answers ... and questions ... thanks for joining the fun

OK, I'll make my question "more clear":

There's an object of unknown on weight (while on earth). An astronaut took it into space with him (riding in a space shuttle, I suppose). Now, while in the space shuttle, he wants to determine its MASS.

What are the possible methods he can use to determine its mass?

Assume: gravity in the shuttle and in space is zero.

Alright, let's take another go ...

#### YULitle

##### Hall of Fame
I stand by all of my statements as that is exactly what I thought you meant from the get-go.

#### Bud

##### Bionic Poster
WOW .... lots of answers ... and questions ... thanks for joining the fun

OK, I'll make my question "more clear":

There's an object of unknown on weight (while on earth). An astronaut took it into space with him (riding in a space shuttle, I suppose). Now, while in the space shuttle, he wants to determine its MASS.

What are the possible methods he can use to determine its mass?

Assume: gravity in the shuttle and in space is zero.

Alright, let's take another go ...
Assuming you know what the item is or it's composition (you stated an item of unknown weight only)... simply take the physical measurements and research the density.

Density = M/V

M = D * V

#### YULitle

##### Hall of Fame
Assuming you know what the item is or it's composition (you stated an item of unknown weight only)... simply take the physical measurements and research the density.

Density = M/V

M = D * V
You'd have to know the volume too. Also, what if weren't a homogeneous density. Let's say it was a watch. That's got glass, metal and plastic... and certainly not all the same kind of each.

#### Court_Jester

##### Hall of Fame
Hope I'm not too late for this party...

I got a method that will not rely on knowing the composition of the unknown object. It uses the concept of a simple harmonic oscillator. You attach one of a spring of known spring constant k to a wall and on the other end the object whose mass is to be determined. If you pull on the mass and stretch the spring by a length x and release, it will start to oscillate with a certain frequency f, which can be measured. This oscillation can be described by the differential equation below:

md^2x/d^2t = -kx​

The solution of the above equation yields a frequency given by

f = [(k/m)^0.5]/2pi​

Pretty elegant, if I may say so.

#### chess9

##### Hall of Fame
The accepted manner is by the inferred gravity based upon perturbations in the orbits of nearby masses, as I recall. But, I last had a physics class about 1968.

Our next question is: "How long is a piece of string?"

-Robert

#### ramseszerg

##### Professional
Too easy.. have the astronaut push against the object. Measure the astronaut's acceleration, the object's acceleration, ask the astronaut how much he weighs, and solve for the object's mass from Newton's third law of motion. Next?

Ok solve for the object's weight from Newton's third law of motion. The object's mass is simply the astronaut's weight times his acceleration. Or is that right? But anyways that's the idea. I'm not doing my bachelors in physics, although sometimes I wish I were.

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#### J011yroger

##### Talk Tennis Guru
This is all very elementary. I am surprised that you thought it would be difficult to answer for a small object.

There are about a billion ways to do it. The easiest are to accelerate it to a known velocity, and collide it with an object of known mass, and measure the velocity of the 2nd object, or accelerate it to a know velocity, and put it up against an opposing force of known quantity, and measure how long it takes to come to a stop.

J

#### flyinghippos101

##### Legend
Too easy.. have the astronaut push against the object. Measure the astronaut's acceleration, the object's acceleration, ask the astronaut how much he weighs, and solve for the object's mass from Newton's third law of motion. Next?
Ok so wait, how would you apply the third law to this? So I assume you're using f=ma and just equaling both situations to each other to solve for m. I would think that you would use momentum if you got someone to push the mass.

#### damasta55

##### Rookie
Newton's law of gravitation works pretty well in space until you have to explain voids...where we don't know what dark matter is

#### ramseszerg

##### Professional
momentum works too.. actually momentum would be easier to measure than force cause for force you would have to measure acceleration while the pushing is occurring.

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#### Camilio Pascual

##### Hall of Fame
Now, while in the space shuttle, he wants to determine its MASS.
Assume: gravity in the shuttle and in space is zero.[/B]
You should define mass, according to some definitions the answer to your question could be zero.
Since I think the definition below is the one you are probably most closely thinking about, Ramseszerg looks to me that he was on the right track.
"Mass is the quantity of inertia possessed by an object or the proportion between force and acceleration referred to in Newton's 2nd Law of Motion."
BTW, your questions are very sloppy. Good physics questions are economical with NO irrelevancies or ambiguities.

#### anirut

##### Legend
BTW, your questions are very sloppy. Good physics questions are economical with NO irrelevancies or ambiguities.
May be. I'm no native speaker any way. And in no way I'm a physiscist. Thanks for joining in anyway.

#### Claudius

##### Professional
Centripetal force = (m * v^2)/ r

Attach a light string to the object and swing it in a circular path above your head.

If you know the speed of the object as it's swinging, the length of the rope, and the time it takes for the object to undergo one revolution, you can determine the mass.

An alternative method is Newton's universal law of gravitation. For this you would need to know the mass of the nearest celestial body, and the object's distance to it (the center).

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#### J011yroger

##### Talk Tennis Guru
^^^ You would need to know the value of the centripetal force. The time it takes to make one revolution is determined by velocity and length of string, Cent force and mass don't enter into that.

J