# I Does mass increase as velocity increases?

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1. Jan 21, 2017

### oz93666

First let me see if I understand what mass is ...it's the measure of an objects ability to attract other masses , and also resist acceleration ... the two always come together and define the term "mass" ..... there are no subdivisions in the term 'mass' ... no different kinds of mass .

I was brought up to understand that the mass of an object increased with it's speed ... now it seems this was wrong ...

So does the mass of an object increase to infinity at light speed or not?

2. Jan 21, 2017

### PeroK

No. I know you've been pointed at the FAQ that explains why. Also, you may like to note that with relativistic mass defined as $m = \gamma m_0$, you do not get $\vec{F} = m \vec{a}$ except where $\vec{F}$ is perpendicular to $\vec{v}$. For 1D motion you get:

$F = \gamma^3 m_0a = \gamma^2 ma$

This implies also that you have a different relativistic "mass" depending on the direction of the force.

https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

Last edited: Jan 21, 2017
3. Jan 21, 2017

### Staff: Mentor

It would be nice if that were true. Unfortunately there are several kinds of mass. There is relativistic mass, invariant mass, inertial mass, and gravitational mass.

4. Jan 21, 2017

### DrStupid

No, it implies that relativistic mass and M in F=M·a are different things.

5. Jan 21, 2017

### PeroK

The OP implicitly defined relativistic mass so that $F = ma$, or a "measure of an object's ability to resist acceleration". The OP would need a directional relativistic mass by that definition.

6. Jan 21, 2017

### DrStupid

No, he don't. The force required for a specific acceleration at a given velocity is always proportional to mass and relativistic mass, Thus both are a measure of the object's ability to resist acceleration and non of them depend on direction.

He also assumed mass to be "the measure of an objects ability to attract other masses". Your "directional relativistic mass" has nothing to do with this property and is therefore off-topic.

The OP just mixed classical mechanics with relativity. In classical mechanics there is indeed only one kind of mass which is both a measure of gravity and inertia. But this is not the case in relativity.

7. Jan 21, 2017