# Can mass be measured directly in the absence of gravity?

1. Aug 24, 2008

### phyzmatix

I was wondering about this and can't seem to think of a way this could be done so I have to ask. Is it possible to measure an object's mass directly in the absence of gravity? If so, how would you do it?

2. Aug 24, 2008

### Defennder

Think in terms of oscillations.

3. Aug 24, 2008

### clem

The mass of elementary particles is usually found by measuring their energy and momentum and then using the equation m^2=E^2-p^2.

4. Aug 24, 2008

### nicksauce

How about subjecting a particle/object of known charge to an electric field and measuring its acceleration.

5. Aug 24, 2008

### ZapperZ

Staff Emeritus
You may want to look into the standard mass spectrometer, where gravity is irrelevant in determining the mass of various particles.

Zz.

6. Aug 24, 2008

### Staff: Mentor

An ordinary balance works the same way in an accelerating rocket ship, or in a large rotating space station, as it does in the Earth's gravitational field.

7. Aug 27, 2008

### phyzmatix

Thanks for the input.

Going to Google it right away!

8. Aug 27, 2008

### PhilDSP

You can also use the Newton force equation and bounce the particle off another particle of known mass. That assumes that gravitational mass and inertial mass are equivalent (or that you want to measure inertial mass).

9. Aug 28, 2008

### Troponin

Set up an apparatus that measures the resistance to change of an object when a force is applied. Base it off of a measurable constant such as some oscillatory constant.

10. Aug 28, 2008

### f95toli

That's easy, as Defennder has already pointed out you can just use a spring.
BTW, if I am not misstaken this is how keep track of how much the astronauts on the International Space Station weigh.

11. Aug 28, 2008

### phyzmatix

Now that I finally have some time, I can ask some more questions! :tongue:

As in spring?

That would mean you don't really measure the mass directly, but calculate it from measurements of other properties of the object. Thanks though

...on the side, how would you measure momentum?

Thanks, but I had larger things in mind

And that would still mean that you calculate the mass based on the measured acceleration.

This is one of those things that makes you slap your forehead and go "of course" as soon as someone tells you

One question though, you'd still have to know the mass of the balancing pieces before you could make any meaningful measurements wouldn't you? And that would've had to be done say, on earth or somewhere else where a gravitational field is present (?)

Fair play, but you'd still be stuck if you didn't know the mass of the other particle.

Would you mind giving me a more detailed breakdown of your idea?

How would you use a spring to measure the mass of an object directly?

12. Aug 28, 2008

### Defennder

Yes the period of oscillation of a mass on spring is related to the mass. There isn't any way to measure mass "directly", or perhaps any other physical quantity. All these are measured by noting their effect on some quantifiable and observable physical phenomena. For example, a watch doesn't measure time; all it does it trace out a circular path, though it does so in an ordered and controlled means which allows us to interpret the result as time measurement.

13. Aug 29, 2008

### Dschumanji

A gravitational field isn't needed to define a standard for mass. If you had no kilogram around but instead had a small space rock, you could define that as one unit of mass. You could then carry out measuring masses of other object by applying newton's second law and measuring masses in multiples of space rock.

It wouldn't be measured in standard SI units, but it will still convey meaningful information.

Last edited: Aug 30, 2008
14. Aug 30, 2008

### phyzmatix

Thank you Defennder, that's very well said actually.

An excellent point. Didn't think of that.

15. Aug 30, 2008

### Staff: Mentor

Note that the official definition of the kilogram is "the mass of a specific object held in custody by the International Bureau of Weights and Measures in Paris." All other mass measurements ultimately reduce to comparision with this object.

You don't need a gravitational field to compare the masses of two objects. You can put them on a balance scale in an accelerating spaceship or rotating centrifuge, or you can attach them to a spring and measure the periods of oscillation, etc. (If two objects oscillate with the same frequency when attached to the same spring, they must have equal mass.)

Last edited: Aug 30, 2008
16. Aug 30, 2008

### phyzmatix

And so we learn!

I must say that I have a much better understanding of mass now than I had before asking this question (which is a bit ridiculous when you think about it, seeing that I've been using the term and doing various calculations with it for quite some time now), just goes to show, the things we take for granted...

17. Aug 30, 2008

### InertialViper

The most simple why i can think of is to simply determine the exact number of atom's from that object you want to weigh and then simply using mendeleev table you can determine the exact mass in the absence of gravity. Probably with a high performace spectrometer or an improved one you could determine the exact number and type of atoms from an object. Is this helpfull?

18. Aug 30, 2008

### HallsofIvy

THAT'S "simple"??!!

19. Aug 30, 2008

### Defennder

No apparently that's not enough. The weight of a composite material is different from a collection of homogenous ones, so you can't average the material composition for their respective individual elemental weights to get the mass.

The whole is more than the sum of its parts.