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Homework Help: A questn

  1. Mar 18, 2005 #1
    hello i m new to this forum n dis is my first topic i was reading about relative motion yesterday that if a train is movin with 90 km to east the observer in the train will find dat train is stationary and the earth is moving backwards similarly for an observer on earth train is moving n earth is stationary and for the observer on the earth train has the energy and so for the train observer earth should also have energy with which it is moving backwards

    so from where the earth is getting energy??????/ :confused:

    or if it has no energy then v must agree that the energatics do not support the relative motion concept
  2. jcsd
  3. Mar 18, 2005 #2
    thats the grat thing about relativity. The oservers do not agree with eachother, but they are both right.


  4. Mar 18, 2005 #3
    buddy i m talking abt energy

    dat frm where earth is getting during its motion backwards
  5. Mar 18, 2005 #4
    read my post again. And think about what Im saying.


  6. Mar 18, 2005 #5
    Nenad is right, the observers do not agree, but they are both correct. When an observer is accelerating relative to the Earth, he sees the Earth gaining an incredible amount of kinetic energy. However, he is not in an intertial frame of reference during that time. He does not actually have to do an enormous amount of work to accelerate the earth, since it is he who's accelerating. No one said that you will see the energy being conserved when you switch from one intertial reference frame to another. So if you were standing on top of a board attached to a compressed spring which had 1000 joules of potential energy, and then the spring was released, an observer standing on the Earth (in this problem the Earth is essentially an interial frame) would see you gain a kinetic energy of 1000 joules. However, you would see the Earth gain a "gazillion" joules of kinetic energy, much more than was stored as potential energy in the spring. But then, you did not remain in the same inertial frame.
  7. Mar 18, 2005 #6
    The earth isnt moving backwards, it just looks like it.
    Think of passing cars on the free way, when you pass a car, is it moving backwards? no, your moving ahead of it. Thats whats happening here. Energy is always relating two objects, the kinetic energy of the train in this case is 1/2*mv^2, where v is the velocity measured from a spectator on earth. A person on the train measuring the kinetic energy on teh train would measure 0, since to him his velocity is 0. His energy measurement of the earth would be .5mv^2, and equivalent to what the earth spectator would measure.

    These two concepts tie into einstein's theory of relativity because when measuring things requiring a frame of reference, you will almost always get different answers for different frames of reference.
  8. Mar 18, 2005 #7
    The above explanation is dubious at best, and totally incorrect at worst.
  9. Mar 19, 2005 #8
    right whozum bt this .5mv*2 is coming frm where ?i mean dat 5 mv*2 is of the earth as measured by the train observer n this energy is coming frm where it must have some sort of energy

    similarly an observer on earth will measure train s energy as 5 mv * 2
    n this is frm fuel so earth should have energy as well relaive to the train,s obs. n what u r saying that earth is not moving i think it is wrong b coz there is no absolute rest nad there is no absolute direction
    of any motion
  10. Mar 19, 2005 #9
    The Earth's rotation? :confused:.
  11. Mar 19, 2005 #10

    Your on the right track, there is no absolute rest or motion, but the idea is relative rest or motion. The train doesnt need fuel to move, you sitting in your chair right now are zooming across the galaxy at a couple hundred thousand kmph, but you arent doing anything. You only need an energy source to accelerate.

    If you consider every object as its own system like you are thinking of the earth, then everything has an infinite amount of energy, however when you define your system, in this case, the earth and the train, it is irrelevant which one is moving, since there is no correct answer as to which one is aboslutely still and which one is absolutely in motion, but what matters is that there is motion between the two with magnitude of energy .5mv^2

    Maybe this will give you the right train of thought?
  12. Mar 19, 2005 #11
    Peacebird, now do you see what I meant by my post. It is all relative.
  13. Mar 20, 2005 #12
    yues whjozum i can get wht u r saying n thnx nenand fr ur comments
  14. Mar 20, 2005 #13
    Thre is sense to talk about kinetic energy
    - only when we look at the difference of energy between two instances;
    - and only when we are in an inertial base.

    Why? If you don't understand I can explain, but first try get it on your own.

    PS: it's a good idea to take the Physics textbook for half an hour before going to bed every day! :wink:
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