# What physically happens to the object as it increases in mass

1. Aug 28, 2008

### mr-tom

What physically happens to the object as it increases in mass (when its velocity increases). Where does the matter come from that allows it's mass to increase?

2. Aug 28, 2008

### Staff: Mentor

Re: Mass

An object doesn't "add matter" (by the usual definition of "matter") when its velocity increases.

Consider that an object's speed relative to you can increase because you accelerate. Why should the amount of matter in an object (which from its own point of view is just sitting there minding its own business ) increase when you start moving?

Furthermore, you and I might well be moving past the object with different speeds. To the two of us, the object has different amounts of relativistic mass.

Relativistic mass is not a property of the object itself, but rather, of the relationship between an object and an observer (more precisely, an inertial reference frame). The "amount of stuff" in an object corresponds to what physicists call the "invariant mass," which is often called the "rest mass" and is a property of the object itself.

3. Aug 28, 2008

### mr-tom

Re: Mass

So it's mass is always the same from the objects 'point of view' and only changes to an observer? Could somebody explain why this is (in non layman terms)

4. Aug 28, 2008

### mgb_phys

Re: Mass

The point of relativity is that everything moving with you (in your frame of reference) is 'normal' from your point of view and your observations of anything else depends on the relative speed.
So even if you are moving your clock, rulers etc measuring anything moving with you will work normally. But if you sue them to measure somethign moving relative to you it will show them differently. Similairly anybody measuring you will think you have changed.

5. Aug 28, 2008

### Staff: Mentor

Re: Mass

I would recommend you forget relativistic mass, it is not a useful concept as far as I can tell. It is much better to just realize that momentum is a non-linear function of velocity and leave it at that.

6. Aug 29, 2008

### DrGreg

Re: Mass

Einstein's famous equation $E = mc^2$ tells us that mass is a form of energy. A moving body has energy due to its rest mass, and more kinetic energy due to its motion relative to the observer. There is a group of people who like to take this total energy (rest-mass-energy + kinetic-energy) and convert it back to mass and call it "relativistic mass". In fact this group will usually say just "mass" when they mean "relativistic mass". There is another group of people, which seems to include the vast majority of modern professional physicists, who don't like to do that; they prefer to leave the energy as energy, and whenever they say "mass" they mean "rest mass" (also known as "invariant mass").

When you read books about relativity, you need to check whether "mass" means "rest mass" (excluding kinetic energy) or "relativistic mass" (including kinetic energy). Different books use different conventions.

The "increase in mass" is better thought of as "kinetic energy".