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Homework Help: Energy and momentum.The misunderstanding have prevailed

  1. Apr 14, 2005 #1
    Energy and momentum.The misunderstanding has prevailed

    Two train wagons of the same mass will impact.
    Wagon A has velocity 10 and wagon B 0.When they impact a lock will make them conjoint.
    Which will be the velocity of two wagons and which the energy after the collision;

    We use the conservation of momentum (....not the energy)and we will find
    that the velocity will be 5.We note that we have a loss of KE. AND we say with endless silliness and easiness that kinetic energy transformed to heat (and sound) :smile: the simple logic obey me to ask: <<<<<How the particles of the wagons have heat without momentum? Since all the momentum is visible to us, as velocity of the wagons.>>>>>>

    All we know that must eventuate to the conservation of two quantums K.E. and P (for the most is only equations because of misunderstanding of the real meaning)

    Type at google:{ James a Putnam Momentum and Energy} you will find a very helpfull article.
    I think that few know their nescience , fewer know it, but can not do anything to learn ( ineffectual searching on books ,web).The people who know is the rarest.
    I want to know the fundamental of physics like a scientist and no like accountant who has memorized the methods of solving problems and finish them automatically and ignores what the represent
    Despite the previous i have a thought of the "two wagons" in the case of pendulum and bullet...

    -We have answer of relativity that energy represents the matter.
    -In 19th century scientists had disagreement of which of two quantums represent the element of difficulty to change velocity(see halliday and resnick book).Note that force is not the ordinary word ,people use ,it is a mathematical variable.Also they had engrossment how the same force to same object need different energy supplys depend of its different previous velocity.

    If you have understand the collision of different mass object "in action-reaction level" the meaning of energy in classical mechanics
    I please you to reply me or send me the adresses of the answers.Thanks of reading.Sorry if i have done language mistakes.
    Last edited: Apr 15, 2005
  2. jcsd
  3. Apr 14, 2005 #2
    The 'vibration' components of the particles' velocities, that result in heat energy actually cancel out, resulting in zero momentum due to heat. There are billions (much greater order actually) of particles where the probability of vibrating at any direction is the same.
  4. Apr 14, 2005 #3
    c c
    c b-> c
    We have a container and a bullet (of the same mass for easiness)
    bullet is inside and has a speed from left to right.when it impacts to the right wall of the container ,it stops and the container now move.because practically we can not see inside the container or because the impact are continual and fastly we observe that the system moves constant with the half of course, speed.
  5. Apr 14, 2005 #4
    So what's the problem? Momentum is conseved m1v1 + m2v2 = (m1 + m2)v
    Overall energy is also conserved (Kinetic + heat + sound). Of course the kinetic energy of the system is decreased.
  6. Apr 14, 2005 #5
    but we cannot seperate the system.the system consists from the bullet and container .they have the quantums.And we want to examine exactly how the energy and momentum exists on it.Of course it is easily for us when the object is like a wooden pendulum which harmed by a bullet(complicated situation) to simplify the problem .ans say that energy went at heat and sound and ignoring how the particles have thermal-move without momentum.
    i think your first reply is near the example of the container and bullet.
    The essenetial is that if we don't want to conflict we must think of intermitent move of the system
  7. Apr 14, 2005 #6
    We cannot take every single particle and compute its movement! Yet if we could, we would still get consistent results.
    But it is more useful to abstract and view the system as two bodies where momentum is conserved and just say that the energy is lost as heat and sound, i.e. we take a more global perspective.
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