# Measure mass

1. Apr 17, 2014

### Infinite/Zero

We all can measure the mass of a body by any common instrument like Spring Balance.
But if we go from one place to another place the gravitational force will change,
like if we go from 1st floor to 10th floor or from coastal region to plateau region.
The gravitational force depend on the height.
Is there any way to calculate mass of a body without gravitational force ?
Please tell me if i am wrong, i will be thankful to you.

2. Apr 17, 2014

### Chronos

The only 'invariant' way to define mass is using the stress energy tensor.

3. Apr 17, 2014

### 256bits

Using gravitation, m=F/g.

Use m = F/a, whereby if one can set up a system to determine the force F and the acceleration a, then surely the mass m can be calculated.

4. Apr 17, 2014

### UltrafastPED

You can measure mass by means of its inertia, or with a force balance.

5. Apr 17, 2014

### Infinite/Zero

Thank you all

6. Apr 17, 2014

### Infinite/Zero

Can you please explain me "stress energy tensor" .

7. Apr 17, 2014

### ZapperZ

Staff Emeritus
Actually, you may still use the mass-spring system to measure the mass if you simply measure the period of oscillation, which depends only on the ratio of k/m. Moving to a place with different g only changes the equilibrium position, not the period.

Zz.

8. Apr 17, 2014

### Infinite/Zero

What do you mean by k/m ?

9. Apr 17, 2014

### Staff: Mentor

The strength of a spring is described by a quantity called its "spring constant", traditionally written as $k$. If an object is hanging from a spring and you start it oscillating, the time for each oscillation will be $\sqrt{k/m}$ where $k$ is the spring constant and $m$ is the mass (not the weight!) of the object.

Thus, we can find the mass by hanging the object from a spring of known strength and measuring the time for one oscillation.

10. Apr 17, 2014

### Infinite/Zero

Thanks Nugatory for responding my question.
Can your method of measuring mass of a body will give a constant mass at different hight from earth ?
Is it related to gravity ?

11. Apr 17, 2014

### Staff: Mentor

It will give the same mass reading at all heights, and even if you are weightless in outer space.

It's actually a specific example of the general approach that 256bits suggested back in #2 of this thread: "Use m = F/a, whereby if one can set up a system to determine the force F and the acceleration a, then surely the mass m can be calculated."

12. Apr 17, 2014

### Infinite/Zero

Which force will oscillate the body hanging on a spring in outer space ?

The second method m=F/a is good. Thanks

13. Apr 17, 2014

### Staff: Mentor

Another way to do it is to use a two-pan balance scale that doesn't involve a spring. Here you use calibrated masses on one side of the scale to balance your unknown mass on the other side of the scale. This works on the top of a mountain, or at sea level. This is how we used to do it in chemistry lab back in the day.

Chet

14. Apr 17, 2014

### ZapperZ

Staff Emeritus
15. Apr 17, 2014

### Infinite/Zero

Thanks Zapperz:zzz:

16. Apr 17, 2014

### sophiecentaur

At Christmas time in the space station, when the Astronauts (Cosmonauts?) give each other presents wrapped up in boxes, they can get a clue about what's inside. They can shake the presents to get an idea of their mass (is it a cuddly toy or a bottle of Vodka?), even though the presents will both float in front of them. They are unconsciously using the equation F = ma to estimate the value of mass: more mass is harder to shake.