Relative movement and agreement about an object's speed

[Quarlep]

I am confused about Relative Motion.Lets suppose we have one train and two observer.One of them inside the train and the other one is outside.Lets think the outside observer see the train moves a constant velocity v.The observer in the train will be think it is not moving cause of Galileo Principle(It means you cannot prove your are moving or not moving).The outside observer will think kinetic energy will be 1/2mv^2 but the inside observer can't say anything(or will say zero cause he will think its not moving).Now let's think a special situation which two observer can agree the speed.(I know its not possible but let's think that's possible).In this case Can two observers agree on kinetic energy of train(In this case again its violets galileo principle cause the observer in the train know its moving but he can't proof)here I am started to confused.What can be solution Can they agree on kinetic energy cause of the observer inside the train no proof of movement ?

 

[A.T.]

Can they agree on kinetic energy cause of the observer inside the train no proof of movement ?

http://en.wikipedia.org/wiki/Kinetic_energy#Frame_of_reference

 

[Quarlep]

Thank you that's the thing what I want

 

[Quarlep]

I didnt understand something.Total kinetic energy will be A momentum energy and normal relative kinetic energy ? How can we calculate momentum energy ? (My main language is not english so I am getting trouble to understand logic)

 

[russ_watters]

There is no such thing as "momentum energy". I think you misread/mistranslated.

 

[Quarlep]

Whats Ei snd MV^2/2 can someone explain me this simpler just what these things mean and what's the COM frame please
Thanks

 

[Dale]

The COM frame is the frame where the total momentum is 0. The mv^2/2 is just the non relativistic formula for kinetic energy.

I have no idea what Ei might be referring to. Perhaps you could provide a citation or some context.

 

[Ibix]

Whats Ei snd MV^2/2 can someone explain me this simpler just what these things mean and what's the COM frame please
Thanks

Assuming you are talking about the formula in the "Frame of reference" section of the Wikipedia article linked above, E_i is the kinetic energy measured by observers at rest in frame i, which is the Center Of Mass (COM) frame. E_k is the kinetic energy measured by observers at rest in some other frame which is moving with speed V with respect to frame i.

Note that the formulae in that web page are non-relatisistic ones. There may be a relativistic section - I haven't read the whole thing.

 

[Quarlep]

So in my sitatuion The observer in the train will measure a kinetic energy E_i plus the outside observer kinetic energy (which observer i see that objects moves a velocity v) ,that will be his (the observer in the train) total kinetic energy.If that's true how can we calculate E_i.

 

[Ibix]

In your case, since there's only one thing (the train) it is stationary in its Center Of Mass frame, so E_i=0. Its kinetic energy in a frame moving at speed v with respect to the train is just $\frac{1}{2}mv^2$, where the mass of the train is m. Or $(\gamma-1)mc^2$, where $\gamma=1/\sqrt{1-v^2/c^2}$ if we're going for the full relativistic treatment.

 

[Quarlep]

Inside the observer will detect train kinetic energy 1/2mv² isn't it. You said something different I guess.Just I want to be sure I have bad english.If you write me again I will be very happy.
Thanks

 

[russ_watters]

So in my sitatuion The observer in the train will measure a kinetic energy E_i plus the outside observer kinetic energy (which observer i see that objects moves a velocity v) ,that will be his (the observer in the train) total kinetic energy.If that's true how can we calculate E_i.

That's not how it works. Since KE is a square function of speed, you have to do your speed transformation first to calculate how fast the observer on the train is moving with respect to the chosen reference. Then you plug that speed into the KE equation.

 

[Quarlep]

In my question, I asked what we get ,If the observer in the train calculates train kinetic energy.Now the observer inside the train things train speed is zero so train kinetic energy will be zero.But it can't be.The observer which its outside the train call A and the observer which inside the train call B.Train total energy for B will be E_T=E_i+1/2Mv² now our referance frame will be train or B (they are in the same referance frame) russ said I need to calculate speed of observer (B) to referance frame that's zero so we get zero energy (again its not possible) here what I am trying to say .
B will thing" I see A observer moves velocity v " so train kinetic energy will be 1/2mv² or train is not moving so v is zero . Then E_i is not zero so total energy will be E_i.
Thanks

 

[PeterDonis]

I asked what we get ,If the observer in the train calculates train kinetic energy

Zero. If an object is not moving, it has no kinetic energy. Kinetic energy is frame-dependent.

Train total energy for B will be ET=Ei+1/2Mv2

If you're just looking at a single object, or a system that has all of its parts at rest relative to each other, then $E_i$ is zero, and since $v$ is also zero in the train frame, the train's total kinetic energy is zero. That does not mean its total energy is zero; it still has its rest energy, $Mc^2$.

Then Ei is not zero

No. $E_i$ is only nonzero if the system consists of internal parts that are moving relative to its center of mass. For example, suppose there is a merry-go-round inside the train, whose center is anchored to the train, and it is rotating. Then the merry-go-round will have some rotational kinetic energy relative to the train's center of mass; this energy will appear as a nonzero $E_i$ in the equation you see on the Wikipedia page. But this energy has nothing to do with the velocity $v$ of the train as a whole relative to something else.

 

[russ_watters]

In my question, I asked what we get ,If the observer in the train calculates train kinetic energy.

Kinetic energy requires that you define a frame of reference against which to measure it. What is that frame?

Now the observer inside the train things train speed is zero so train kinetic energy will be zero.But it can't be.

Well, it can be, but only if he defines a frame of reference in which the train is stationary. He can do that, but why would he if what he is really looking for is the train's KE with respect to the ground?

The observer which its outside the train call A and the observer which inside the train call B.Train total energy for B will be E_T=E_i+1/2Mv² now our referance frame will be train or B (they are in the same referance frame) russ said I need to calculate speed of observer (B) to referance frame that's zero so we get zero energy (again its not possible) here what I am trying to say .
B will thing" I see A observer moves velocity v " so train kinetic energy will be 1/2mv² or train is not moving so v is zero . Then E_i is not zero so total energy will be E_i.
Thanks

So again: you can't use three different reference frames in different parts of the problem. Pick a rest frame, pick an observer and do the whole problem using only those two things. The consistency will eliminate the logical contradictions you are creating.

 

[Quarlep]

Kinetic energy requires that you define a frame of reference against which to measure it. What is that frame?

The observer inside the train

Well, it can be, but only if he defines a frame of reference in which the train is stationary.

I am defining that stationary referance frame observer in the train.

So again: you can't use three different reference frames in different parts of the problem. Pick a rest frame, pick an observer and do the whole problem using only those two things. The consistency will eliminate the logical contradictions you are creating.

Referance frame is B observer is also B.

 

[russ_watters]

The observer inside the train

I am defining that stationary referance frame observer in the train.

Referance frame is B observer is also B.

OK, so since the train is stationary with respect to that frame, the kinetic energy with respect to that frame is zero.

 

[Quarlep]

So If I change referance frame to A, B will se 1/2Mv^2 and If I pick referance frame to ground also its 1/2Mv^2 .If these things are rhen true thanks for help I understand the idea.

 

[russ_watters]

So If I change referance frame to A, B will se 1/2Mv^2 and If I pick referance frame to ground also its 1/2Mv^2 .If these things are rhen true thanks for help I understand the idea.

Yes, it is true that kinetic energy is frame dependent, just like (because) velocity is frame dependent. As long as you recognize that asking about the kinetic energy with respect to an additional frame results in a different answer and the two answers don't have to match, then you should be fine.

 

[Quarlep]

I ll will check that If I found correct answer and I ll post it.For now thanks

 

[Dale]

train speed is zero so train kinetic energy will be zero.But it can't be.

It is zero in that frame. Why do you think it couldn't be. The correct answer is 0 in that frame.

It will be non zero in other frames. Energy is frame variant.

 

[Quarlep]

http://en.wikipedia.org/wiki/Kinetic_energy#Frame_of_reference
There says this " By contrast, the total kinetic energy of a system of objects cannot be reduced to zero by a suitable choice of the inertial reference frame, unless all the objects have the same velocity"

 

[Ibix]

http://en.wikipedia.org/wiki/Kinetic_energy#Frame_of_reference
There says this " By contrast, the total kinetic energy of a system of objects cannot be reduced to zero by a suitable choice of the inertial reference frame, unless all the objects have the same velocity"

Yes. You've only got one object, the train. So all your objects have the same velocity. So the KE can be reduced to zero by a suitable choice of frame in this case.

 

[Quarlep]

So the KE can be reduced to zero by a suitable choice of frame in this case.

Is this is a suitible choice of frame ?

 

[Ibix]

Is this is a suitible choice of frame ?

Is what a suitable choice of frame?

 

[Quarlep]

Our situation

 

[russ_watters]

Is this is a suitible choice of frame ?

We don't know: the chocie of frame is yours to make, depending on what it is you want to know. It doesn't seem suitable to me, though, since it appears that you actually want to know the train's KE with respect to the ground, but instead keep trying to calculate the train's KE with respect to the passenger:

Previously, I asked you what frame you wanted to know about and you said the observer on the train. But then after you calculated the KE to be zero, you immediately discarded the answer and asked about the observer on the ground. So it appears to me you really didn't want to know about the observer on the train even though you said you did. This doesn't make sense to me.

 

[Dale]

@Quarlep this is not acceptable. When asked for clarification you should provide it. For this discussion the following must be clarified:

1) what reference frames are of interest
2) what system is of interest
3) what is the scenario of interest (completely specified)
4) what assumptions/simplifications are used

I have assumed that you are interested in the system of an inertially moving train in the train and embankment frames using the simplification that the train is a single rigid object moving at v relative to the embankment.

 

[Janus]

http://en.wikipedia.org/wiki/Kinetic_energy#Frame_of_reference
There says this " By contrast, the total kinetic energy of a system of objects cannot be reduced to zero by a suitable choice of the inertial reference frame, unless all the objects have the same velocity"

The "total kinetic energy of a system". In this case, the system of objects can be the train and the embankment observer. In the embankment frame, the train has a non-zero kinetic energy and the embankment observer has zero kinetic energy, But in the train frame, the train has zero kinetic energy and it is the embankment observer that has a non-zero kinetic energy.

In other words, you cannot find an inertial frame where the sum of the kinetic energy of the train and embankment observer is zero. But this does not mean that you cannot choose a frame in which one or the other has zero kinetic energy. Also note that while it says that the total kinetic energy of such a system can not be reduced to zero, it does not say that it must remain constant.

On the other hand, if you reduce your system to just the train, then it can have its total kinetic energy reduced to zero, because this fulfills the requirement of the phrase "unless all the objects have the same velocity".

 

[Quarlep]

Here I made a pic.I wrote everything down there.