What happens to relativistic mass as objects move faster?

Question

From the perspective of a non-moving observer, link one claims that things get smaller as they go faster. Link two says things get bigger, and link three says the mass is invariable at any speed.

Can someone harmonize the three in layman terms for me?

1. http://en.wikipedia.org/wiki/Length_contraction

2. http://answers.yahoo.com/question/index?qid=20080305204320AAP8stK

3. http://www.geocities.com/zcphysicsms/chap3.htm

Answer 1

I have no interest in harmony.

In laymans terms smaller most often refers to size, not mass.

Same with bigger.

The rest mass of an object is the mass when the object is not moving relative to the observer.  Your question about a "non-moving" observer indicates you just don't get it.  All observers are moving.  I hope you meant not moving relative to x-y-z, since there is no such thing as "non-moving" (except with respect to some other object).

Anywhoo, M = Mo / √(1-v²/c²)

so if v, velocity is 0.5c  (half the speed of light, c)

then M = Mo/√(1-.25) = Mo/√0.75 ~ Mo * 1.15

So the Mass, M is ~ 15% heavier than it would be at rest, Mo.

At v = .99c; M ~ 7Mo.  As v gets closer to c, M increases without limit and the energy needed to speed it up increases without limit also.  can never get to c.

Size is another question.

Answer 2

The first site deals with relativistic length not mass.

the second site states correctly that relativistic mass increases but without explanation.

the third site appears to deal with theoretically mass less particles at the quantum mechanics level. Do not go there without many aspirin.  

short form. Ie.without phd.

mr = m0 /sqrt(1 - v2/c2)

the relativistic mass = rest mass divided by the square root of  1- (velocity squared divided by the speed of light squared.)

As the speed of an object approches the speed of light the fraction V^2/C^2 approches 1 and the value of 1- V^2/C^2

approches 0. M0/0 is infinity.

Answer 3

1) From the perspective of a non-moving observer, lengths contract.  This is correct and straightforward.

2) This is less straightforward, because a lot of high school teachers teach archaic conventions.  The idea that mass increases is not incorrect, but it uses an outdated definition of mass that you ought to avoid.  A particle's ENERGY increases.  Now energy and relativistic mass (gravitational and inertial mass) are the exact same thing (up to a conversion factor that may be required depending on your system of units).  Consequently, there's no need to use the idea of a relativistic mass that increases with speed.  Any time a person uses the phrase "relativistic mass", they should be saying "energy".  So for convenience, we redefine mass as is done in....

3) This article gives the "correct" (ie, actually used by real scientists) definition of mass--the rest mass--the particle's energy in its own rest frame.  You don't have to wade through all the 4-vectors to understand this basic point.  Needless to say, this approach is MUCH more practical.  You do not want to be dealing with a bunch of electrons, each of which has its own mass.  Better to use a system in which they all have the same mass, but they have different energies.  Energy is what has the inertia and couples to gravity.  So while it is tempting to call that mass, you shouldn't.  It leads to confusion and people like you having to ask for clarification on what should be a simple question and getting a lot of wrong answers from folks who were similarly ill-served by their teachers.

Answer 4

Other people have explained 1 and 2, so I'll focus on 3.  I'll confess that I'm not a relativity specialist, so I may be wrong here.  But I did notice that the equation showing mass invariance was using four-vectors, which are generalizations of traditionally three-dimensional vectors like force and velocity to all four dimensions of spacetime.  It could be that mass is independent of four-velocity, or that relativistic mass drops out in some other way when you generalize to four-space.  In any event, as long as you're dealing with regular 3-D space, mass DOES increase at very high speeds.

Answer 5

The first two are correct. They are part of the Special Relativity theory. Mass increase if not "bigger"!

The object does get smaller (contracts) along the line of flight.

The object does have a mass increase. This is the main reason why it is impossible to get to the speed of light. As you approach it the amount of energy needed to go the next speed increment increases. The amount of energy needed approaches infinity as you approach C.

The third reference is too complicated for me to read in a short time.

.

Answer 6

The first link is only talking about the apparent length of an object in the direction of travel to an outside, stationary, observer. This doesn't mean that the object shrinks and loses mass, just that it looks shorter.

The second is talking about mass yes, and it does increase with velocity.

I'm not sure where you read that in the last link, but it was very long, so I skimmed it. Therefore I will say that for a person on a fast moving ship, they would not notice any change in their mass. And also invariable is the rest mass of the object.

The change in mass is due to the increase in kinetic energy. Since energy and mass are equivalent, gaining kinetic energy also results in a gain in mass.

The mass (and also therefore kinetic energy) of an object becomes infinite at a velocity equal to the speed of light, which is obviously impossible since there is only a finite amount of energy. Hence c is the universal speed limit.

Answer 7

Link one is correct, an object shortens in the direction of motion.

Link two is wrong.

Link three is wrong.

The theory of relativity states, at relativistic speed, an object gains mass, shortens in the direction of motion and time slows.