# Density Formula for Moving Particles

• FeDeX_LaTeX
In summary, the conversation discussed the concept of density for a moving object and the different kinds of mass in relativity. It was mentioned that the density formula for rest mass may not apply for a moving object, and the use of Einstein's General Theory of Relativity was suggested. Additionally, it was pointed out that the density of the object may appear to increase from the viewpoint of an external observer, but it ultimately depends on the purpose for which the density is being used.

#### FeDeX_LaTeX

Gold Member
Hello;

I know that the formula for density is mass/volume. However, this is only true for rest mass. What about when the particle is moving?

Thanks.

This opens up the whole can of worms about the different kinds of mass in relativity. I suppose the answer would depend on what you want to use the density of the moving object for.

Mass increases with velocity, right?

Do I have to use Einstein's General Theory of Relativity and somehow use that with my original density formula? I'm confused

Note that the particle itself would not know if it is moving or not, unless it was being accelerated. So according to the particle, it's density would not increase.
However, from the viewpoint of an external observer, it could be said that the density of the object had, in a way, increased.

In the end, whether this density is what would fit in with your usual concept of density or not, depends on what you want to use it for, as jtbell said.

However, from the viewpoint of an external observer, it could be said that the density of the object had, in a way, increased.

I don't get it... what do you mean by this?