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Mass expansion

  1. Nov 7, 2009 #1
    relativity states that as v approaces c mass increases
    but my question is that if we are going fast and suddenly mass increases then simultaineously we must slow down and thus we will be back to original state
    by this hypothesis i believe that mass expansion is only for minute seconds
    AM i right ?
  2. jcsd
  3. Nov 7, 2009 #2
    Increasing mass of a body would not slow it down. The mass increase as one approaches the speed of light is in the form of photons. The energy that is applied to accelerate a body as it approaches the speed of light turns into photons rather than pushing the body faster.

  4. Nov 7, 2009 #3
    cant understand could u PLZ explain in detail
    and also
    if possible can u explain how mass is converted to energy and energy is converted to mass
    i cant understand how this happens. I want the physical meaning behind this
    i console myself by thinking of this example: the food we eat is mass . It is getting digested to give energy so mass is converted to energy. But i am sure that this is wrong
  5. Nov 7, 2009 #4
    Mass and energy are equivalent by E = mc[tex]^{2}[/tex]. So a body's mass can be increased by adding energy in the form of photons. The conversion of mass to energy and vicesa versa just happens. Its a process of matter that is simply accepted.
  6. Nov 7, 2009 #5
    but photon has zero mass how can addition of photon increase mass also what is the source of photons
  7. Nov 7, 2009 #6


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    Staff: Mentor

    You can increase the speed of an object only by pushing on it with something. The object that is doing the pushing does work on the pushed object, which increases the pushed object's kinetic energy according to the work-energy theorem:

    [itex]K_{final} = K_{intitial} + W[/itex]

    This in turn increases the pushed object's total energy

    [itex]E = m_0 c^2 + K[/itex]

    where [itex]m_0[/itex] is the pushed object's "rest mass," and the "relativistic mass" increases along with the energy according to [itex]E = mc^2[/itex].

    Both the total energy and momentum of the object vary with speed differently in relativistic mechanics than in classical mechamics:

    [tex]E = \frac {m_0 c^2} {\sqrt {1 - v^2 / c^2}}[/tex]

    [tex]p = \frac {m_0 v} {\sqrt {1 - v^2 / c^2}}[/tex]

    instead of

    [tex]E = \frac{1}{2}m_0 v^2[/tex]

    [tex]p = m_0 v[/tex]

    No. Photons are not involved in this, unless of course you are accelerating the object by using photons, as in a solar sail. But here the photons are what is doing the "pushing," and they either disappear or are reflected backwards, depending on whether the sail absorbs or reflects light.
    Last edited: Nov 7, 2009
  8. Nov 7, 2009 #7
    i cant understand einsteins mass energy equivalence that how mass can be converted to energy also the concept of rest mass and rest energy
    what i thought about mass converted to energy is that

    the food we eat is mass . It is getting digested to give energy so mass is converted to energy. But i am sure that this is wrong.
    if possible can u also derieve E=mc2
  9. Nov 7, 2009 #8


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    Staff: Mentor

    Digestion is a series of chemical reactions. In any spontaneous chemical reaction, the sum of the masses of the reactants is greater than the sum of the masses of the products. The difference in mass (which is very tiny) corresponds to the energy released in the reaction.

    Why? (It's dangerous to be sure something is wrong before you fully understand it. :wink:)
  10. Nov 7, 2009 #9
    You're right it means that kinetic energy is not realize as linear higher velocity as the object is pushed towards the speed of light.
  11. Nov 7, 2009 #10
    Most of our food energy comes from the oxidation of carbohydrates, such as the monosaccharide C6H12O6 (dextrose). The available energy is about 29 eV per molecule. The gram molecular weight is about 180 grams, so very roughly Mc2 is 180 AMU or 931 MeV x 180 = 167,000 MeV. So the available chemical energy is about 0.2 parts per billion.
    Bob S
  12. Nov 8, 2009 #11
    You are probably thinking that momentum conservation implies if the mass is higher the velocity must be slower. But remember, to increase the velocity in the first place you are adding momentum to the system. So the velocity and mass can both increase simultaneously. Some of the momentum goes to increasing the velocity, and some to increase the mass. The amount of this which is divided between the two depends on the object's speed. If v is close to c, the object will hardly accelerate, but gain a significant amount of mass. If v is near zero, the additional mass will be tiny.
  13. Nov 8, 2009 #12
    ok done but what is the source of that momentum which increases both mass and velocity
  14. Nov 8, 2009 #13
    Fermilab routinely puts about 1012 protons, and 1012 anti-protons (in opposite directions) in the Tevatron (a 6280-m circumference synchroton), accelerates them to over 980 GeV total energy using RF cavities, where they can coast all day (synchroton radiation is not a significant problem). The RF cavities are the source of the added momentum. The mass is not increased, but the total energy is.
    Bob S
    Last edited: Nov 8, 2009
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