Does Mass Really Change with Velocity Near the Speed of Light?

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Discussion Overview

The discussion revolves around the concept of mass in the context of special relativity, particularly whether mass changes with velocity as one approaches the speed of light. Participants explore the distinction between relativistic mass and invariant mass, and how these concepts relate to energy and momentum.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the term "relativistic mass" is synonymous with energy, while others argue that invariant mass remains constant regardless of speed.
  • There is a discussion about the implications of defining mass as "confenced energy" and how this relates to momentum, with references to the formula p=ymu.
  • Some participants express uncertainty about the terminology used, particularly the distinction between "variant mass" and "relativistic mass," and how these terms affect the understanding of momentum in relativistic contexts.
  • Questions are raised about the meaning of "the classic idea of mass" and whether rest mass can be equated with "intrinsic energy of matter."

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether mass changes with velocity or if it is merely the energy capacity that changes. Multiple competing views remain regarding the definitions and implications of mass in relativistic physics.

Contextual Notes

There are limitations in the discussion regarding the clarity of terms used, such as "confenced energy," "variant mass," and "classic idea of mass." The discussion also highlights the need for context when interpreting the term "mass" in different scenarios.

mike232
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Hey guys,
I'm reading my modern physics book over break and I remember hearing that mass changes as you approach the speed of light. But is it really the mass that is changing or just the amount of momentum a certain mass can have. So is mass really varying or is it the energy capacity of mass that is changing with respect to velocity?
 
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mike232 said:
I remember hearing that mass changes as you approach the speed of light.

The "mass" that changes as your speed changes is more precisely termed "relativistic mass"; but that term is not used much in modern physics texts because it is just a synonym for "energy".

mike232 said:
is mass really varying or is it the energy capacity of mass that is changing with respect to velocity?

I'm not sure what you mean by "mass really varying" as opposed to just "energy capacity of mass" changing; if "mass" means "relativistic mass", then "mass" and "energy" are just different names for the same thing (see above).

OTOH, if "mass" means "rest mass", or better, "invariant mass", then it is an inherent property of the object and does not change as the object changes speed.
 
So by defining mass as simply confenced energy... I'm thinking of p=ymu. So because the variant mass is mass times 'demensionless velocity' the momentum increases differently than classic, the classic idea of mass changed but the rest mass, intrinsic energy of matter, doesn't change?
 
mike232 said:
by defining mass as simply confenced energy

Relativistic mass is the same as energy (not sure what you mean by "confenced"). Invariant mass is not. It's important to keep this in mind because the term "mass" and the symbol m won't always be explicitly defined as one or the other, you have to look at the context. See below.

mike232 said:
I'm thinking of p=ymu.

In this formula, m is invariant mass, not relativistic mass. So m in this formula is not the same as energy.

mike232 said:
because the variant mass is mass times 'demensionless velocity' the momentum increases differently than classic,

The momentum increases with velocity differently from the "classic" case (I assume by "classic" you mean "Newtonian") because of the factor ##\gamma##; you don't have to think of "variant mass" (by which I assume you mean "relativistic mass"--it really helps to keep your terminology consistent) at all.

mike232 said:
the classic idea of mass changed but the rest mass, intrinsic energy of matter, doesn't change?

What do you mean by "the classic idea of mass"?

As for rest mass, I'm not sure it is best thought of as "intrinsic energy of matter", because, as I noted above, invariant mass is not the same as energy.

It might help if you gave a specific quote from your book, or a specific example of a scenario you're not sure about.
 

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