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Mass change

  1. Mar 27, 2004 #1
    what happens when there is a mass change in an object?
    the mass change is due to its change in its velocity, like when it accelerates from rest to near speed of light, there would be an increase in mass, but how does this happen?
    would there be a change in the atoms that makes up the object itself???
  2. jcsd
  3. Mar 27, 2004 #2


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    The term "mass," as used in modern physics, means "rest mass." This mass does not ever change, no matter what velocity the object has.

    - Warren
  4. Mar 27, 2004 #3
    Prepare to be told that you're speaking of relativistic mass and not proper mass (aka rest mass). Some people believe that when the term "mass" is used without qualification that it refers to rest mass. This is clearly not the case as is evidenced in the CERN URL


    I use the term to refer to relativistic mass. So that is what I mean below.

    Mass increase is a direct result of time dilation and Lorentz contraction.

    I disagree with chroot's claims that the term "mass" always mean "rest mass" in modern physics. The contrary is an emperical fact. E.g. the texts

    Relativity: Special, General and Cosmological, Wolfgang Rindler, Oxford Univ., Press, (2001)
    Cosmological Principles, John A. Peacock, Cambridge University Press, (1999)
    Understanding Relativity: A Simplified Approach to Einstein's Theories, Leo Sartori, University of California Press, (1996)
    Basic Relativity, Richard A. Mould, Springer Verlag, (1994)
    Introducing Einstein's Relativity, Ray D'Inverno, Oxford Univ. Press, (1992)

    Are perfect counter examples

    One simply has to look in the relativity literature to see this is not the case. At best there is a majority of particle physicists who use the term that way. In cosmology that does not appear to be the case. In almost all cases authors define the term once and use it as such.

    For a list of examples from places such as Fermi-Lab, Cern and universities around the globe who use the term "mass" to mean "inertial mass" aka "relativistic mass" please see

    Last edited: Mar 27, 2004
  5. Apr 1, 2004 #4
    but i still ain't quite sure about the explanation..
    my level of studies doesn't offer me the knowledge of time dilation and lorentz contraction, could u explain it to me pls??
  6. Apr 1, 2004 #5
    I think you are asking about the mechnism that changes the mass of particles as they are accelerated towards light speed. As I understand it, a particle must absorb a photon, or some other particle, in order to increase its kenitic energy and thus speed. The extra energy comes from the absorbed photons.
  7. Apr 4, 2004 #6
    That is incorrect. That is not the mechanism. In fact what you suggested is an actual increase in the intrinsic mass of a particle. That is not directly related to a velocity dependant mass.

    alchemist - The concpet of "relativistic mass" is based on things like time and length. Momentum is defined as mass times velocity where velocity is defined as change in distance traveled divided by time required to travel that distance. Since those two things are velocity dependant then this leads to a velocity dependance of mass.
  8. Apr 4, 2004 #7
    so what exactly is the mechanism based on? and how does it work?
    and if an object reaches the speed of light, it will have an infinite mass??
  9. Apr 5, 2004 #8
  10. Apr 5, 2004 #9


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  11. Apr 5, 2004 #10
    In agreeance with DW, I wanted to quote a source by David W. Hogg:

    I agree with pete that often it's dependent on the source in WHAT way they use the term mass (above he defines it as rest mass), but Hogg also mentions that "relativistic mass" is an older way of referring to the change in an object's 4-momentum.
  12. Apr 5, 2004 #11

    It looks as though you're getting a good tutorial on relativity terminology, but unless I'm mistaken, you're not really getting the answer you're looking for!

    Try this. Think about a clock radio, and imagine looking at it face on. You hold up a yardstick to measure how long it is. Let's say the 7" mark lines up with the left end of the radio and the 19" mark lines up with the right end of the radio. So you say your clock radio is 12" long. Then you put the clock on a scale. The scale says it weighs about 2.2 lbs. So you say its mass is 1 kilogram. Then you look at what time it is. The clock says 6:00. A little while later, you look at the clock again and it says 6:15. So you say that 15 minutes have past since the last time you checked the time.

    Then you get another clock radio that's identical to this one. That is, it's 12" long, its mass is 1 kg, and if you synchronize it with the first one at, say, 6:30, they'll still be synchronized at 6:45.

    Now imagine one clock sliding sideways (parallel to the direction of the length we measured) at some speed v. And imagine that just as the moving clock passes by, both clocks read 7:00. We're going to sit with the stationary clock and measure (somehow) the length and mass of the moving clock and measure what time the moving clock reads when the stationary clock reads 7:15.

    Here's what Einstein's theory of relativity says we'll get. Our measurement of the length of the moving clock will be less than 1 ft. Our measurement of the mass of the moving clock will be greater than 1 kg. And when the stationary clock reads 7:15, the moving clock won't have gotten to 7:15 yet.

    It also says this (which seems even more strange). If somebody rides along with the moving clock and makes the same kind of measurements on our stationary clock, they'll measure that IT'S the shorter, heavier and (this one's really strange) slower running one!

    So where does the increase in mass (or the decrease in length, or the decrease in clock rate) come from? Physics doesn't really answer that kind of question. It only answers questions like this: If I measure something in nature, what result will I get? For example: If I measure the speed of light in empty space, will the result depend on whether the source and/or my measurement system are/is moving. The answer is no. The strange results about mass, length and time that relativity says we'll get are all consequences of this strange answer.
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