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Does stopping a very fast object makes it lighter?

  1. Sep 24, 2012 #1
    Hey guys,
    If we stop a very fast moving object will it lose weight ?..i need a hand on this matter. Thanks
     
  2. jcsd
  3. Sep 24, 2012 #2

    mfb

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    No, unless something additional happens which you did not specify here.
     
  4. Sep 24, 2012 #3
    No ideal what to add..

    maybe: speed of moving object = half c , stop point in reference to the point of initial contact..

    in a way , since you mention "unless" i assumed it is positive
     
  5. Sep 25, 2012 #4
    Well ... hot brakes surely have more mass than cool brakes.

    Now, if the mass of a car can not change, then other parts of a car must lose mass, when the brakes are gaining mass.
     
  6. Sep 25, 2012 #5
    time [itex]\downarrow[/itex]

    m=1
    ........:cool:............... fast moving
    v=c/2-->

    m=1
    ........:grumpy:<--F=1...... x=0 start stopping
    v=c/2-->

    m=?
    .......:mad:<--F=0....... x=1 End here
    v=0
     
    Last edited: Sep 25, 2012
  7. Sep 25, 2012 #6
    Let us postulate that:
    1: mass is conserved in a closed system
    2: radiation has mass

    Now let us consider a fast moving electron that enters a magnetic field. In a magnetic field electrons:
    1: radiate
    2: decelerate

    Mass is conserved in the closed system consisting of the electron and a magnet, so the mass increace caused by the creation of radiation is cancelled by the mass decreace caused by the deceleration of the electron.
     
  8. Sep 25, 2012 #7
    Awesome! mass was decreased then we have "light electron"
    but will the -e absorb radiation & increase mass if magnet field is reverse to accelerate it?
     
  9. Sep 25, 2012 #8
    I'm not sure about this - surely in some reference frame any moving object will be stationary, or in another reference frame have a much greater velocity. Wouldn't it only be apparent that it had 'higher energy' if it was collided with something?
     
  10. Sep 25, 2012 #9
    A discharging battery loses mass, this can be verified experimentally.
    A charging battery gains mass, this can be verified experimentally.

    An electric car, with two batteries, accelerates using battery 1, and then decelerates charging battery 2 with the energy generated during the deceleration, has the same mass at the end as at the beginning, except that battery 1 lost some mass, and battery 2 gained some mass.

    Now let us ask: How did mass travel from battery 1 to battery 2 ?

    Well my answer is that when the car was moving at high speed, the mass had left battery 1, but the mass had not entered battery 2, so the mass was in the material of the car.
     
  11. Sep 25, 2012 #10
    No magnetic field is needed in this case.

    Let's weigh a cold piece of metal.

    Then we warm the piece of metal using a micro wave owen.

    Then we weigh again.

    We notice that the weight incraced. The electrons in the metal absorbed radiation, and now the collection of electrons weighs more.
     
  12. Sep 25, 2012 #11
    correction.."light electron" is not especial. same as any electron at rest..

    #10 Interesting , thanks

    a reliable source informed me that minimum mass of an object is when it is at rest..
     
    Last edited: Sep 25, 2012
  13. Sep 25, 2012 #12

    mfb

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    Both postulates are wrong. Energy is conserved, and radiation has energy (but not mass). Mass is not conserved.
    You can assign an effective mass to a system, based on its total energy content in its center of mass. That is conserved, but it is not the sum of masses of its components.

    Do you know of any publication showing this?
    The effect is there (with the effective mass as above), but it is so tiny that I did not know that it can be measured yet.
     
  14. Sep 25, 2012 #13
    What if the object is a meteor that just missed the moon and is suddenly prevented by a superman from colliding with our planet successfully halted before entering the atmosphere?

    Maybe far out example of no heat exchange involved, the doubt is: when we push on object it would gain mass to not reaching c, and halting the meteor is like pushing it too.

    s.o.s.
     
  15. Sep 25, 2012 #14

    mfb

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    Why do you expect any changes?
    If you do not expect changes, why did you invent that scenario?

    The concept of relativistic mass leads to all sorts of misconceptions, and you won't find it in modern physics any more. Just in some old textbooks and at bad science websites.
     
  16. Sep 25, 2012 #15
    sorry..lets omit the "to not reaching c"
    i expect answer.
    scenario is part of the query.

    oh.. is relativity passe
     
  17. Sep 25, 2012 #16

    mfb

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    That is wrong.
    Unless you use a definition of "mass" which was abandoned some decades ago.
     
  18. Sep 25, 2012 #17
    confusing one mfb
    you mean That is right only if I use abandoned definition of "mass".
    so where can i get this decades old "mass'' definition?


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    Last edited: Sep 25, 2012
  19. Sep 26, 2012 #18
    I see.

    So: When two fast objects are stopped, the effective mass of the object pair decreases.

    For example, when an electron hits the magnetic field of the earth, the effective mass of the electron-earth system decreases, and radiation is produced with effective mass equal to the lost effective mass.

    I meant that with good enough instruments the mass change of charging battery could be measured. I think there is no need to call this mass change "change of effective mass".
     
    Last edited: Sep 26, 2012
  20. Sep 26, 2012 #19

    mfb

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    Right. See relativistic mass for details.

    I would not talk about a single mass for "electron+earth" together... but if you do that, and the radiation is lost, the effective mass of "electron+earth" is reduced and the leaving photons carry some energy. However, that change is completely negligible.
     
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