# What are the different types of Mass?

1. Jul 26, 2014

### Natsirt

I've heard terms like solid mass,or maybe still mass and I'm sure there are others so can I get a little education?

2. Jul 26, 2014

### Staff: Mentor

As far as physics and the equations like $E_k=\frac{mv^2}{2}$ or $F=ma$ are concerned, there's only mass, and adjectives like "solid" are irrelevant. In colloquial English, the word "mass" is also used to mean some lump of some sustance and then people say things like "solid mass" or "huge globular mass" and the like, but that has nothing to do with the ter as used in physics.

3. Jul 26, 2014

### Natsirt

OK I'm glad to hear its not as complicated as I thought it would be. So why are those Mass terms used in the first place?

4. Jul 26, 2014

### SteamKing

Staff Emeritus
To be descriptive?

5. Jul 26, 2014

### Natsirt

Thanks Nugatory you have been very helpful.

6. Jul 26, 2014

### Staff: Mentor

Can you give us quotations that show how these terms were used, in context? In particular, I've never heard of "still mass."

I wonder if these might be bad translations into English. If that is the case, seeing how they are used would help us tell you what the proper English terms are.

7. Jul 26, 2014

### Natsirt

I'm sorry my knowledge on the subject is very little. I actually meant to say rest mass.

8. Jul 27, 2014

### Staff: Mentor

"rest mass" is the mass of an object as measured by an observer who is at rest relative to the object.

9. Jul 27, 2014

### Natsirt

What is the difference between measuring a mass when your at rest relative to said mass and not being at rest relative to said mass?

10. Jul 27, 2014

### Staff: Mentor

At sufficiently high speeds (we're talking an appreciable fraction of the speed of light, so many thousands of kilometers per second, the kinetic energy of the object becomes so great that we have to include it in the mass, using Einstein's famous $E=mc^2$. This usually only matters for subatomic particles, as they're the only things you'll encounter that move at anywhere near those speeds.
(Be aware that this explanation is somewhat simplified. If you want to get the next level of detail down, you'll have to learn some relativity first. For now, suffice to say that when physicists speak of the "mass" of an object, they mean the mass as measured by an observer at rest relative to the object, and sometimes they'll say "rest mass" just to reinforce that point)

11. Jul 27, 2014

### Bandersnatch

When not at rest, you measure the mass to be higher. The greater the relative speed, the greater the mass.
More to be found here:
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html#c3

The two "kinds" of mass not mentioned so far are the inertial and gravitational mass.
The first one is the mass in Newton's 2nd law of motion $F=ma$, and is the measure of resistance to having the state of motion changed. The more of it you have, the harder it is to accelerate you.
The second kind is both of the masses in Newton's law of gravitation $F_g=GMm/R^2$. It's the measure of the "charge" inherent to an object, that produces gravitational field.
Turns out they are equal as far as we can tell(their equality has been measured to some ridiculous degree by now).
For the above reason, they're treated as one thing, but their equality is not something obvious or necessary.

12. Jul 27, 2014

### Natsirt

So when subatomic particles are going near the speed of light the amount of energy creates a measurable amount of more mass?

13. Jul 27, 2014

### DrGreg

14. Jul 27, 2014

### Staff: Mentor

You might want to think about what it would mean to measure the mass of a subatomic particle moving close to the speed of light - your question is more subtle than it looks at first glance.

But yes, it is true that a particle moving rapidly relative to me will accelerate differently when I apply a force to it than would the same particle at rest relative to me. From there, a naive application of Newton's $F=ma$ law would leave me with the conclusion that the mass is different. You will find this treatment in some obsolete textbooks and (sadly) in too much of the popular literature.

However, the modern understanding is that what you measure when the particle is at rest relative to you is the mass, we say "rest mass" occasionally to remind ourselves, and the funniness we get when we apply $F=ma$ to a particle moving at relativistic speeds should be treated as a relativistic correction to the momentum instead.

I have to say again: what I'm saying here is oversimplified to the point of being almost just plain wrong. Until you're ready to seriously study relativity, you can just say that there's just one mass, it's always the same whether the object is moving or not, and leave it at that.

15. Jul 27, 2014

### Natsirt

I found this helpfu . Very simplified, it sounds like energy warps spacetime because E=mc2 but a mass does not have to be solid to do so. In other words it can still pass through things. Is that right?