But...you wouldn't feel it then?
http://www.xkcd.com/1366/
I don't understand momentum anymore.
Let's say this is actually how it works - the train is stationary, and the earth is rotating. Why would I be thrown forward on the train as the earth stops spinning?
I don't understand momentum anymore.
Let's say this is actually how it works - the train is stationary, and the earth is rotating. Why would I be thrown forward on the train as the earth stops spinning?
| Why would I be thrown forward on the train as the earth stops spinning? |
The earth is spinning really fast
This is why I like this comic, it spawns fun thought experiments like these.
@LB: There's way too little information for a proper thought experiment here.
SCENARIOS:
- If the train is imparting a force on the planet then the planet is imparting force on the train. So even if from your frame of reference you appear to not be moving, you actually are. When you stop the friction from the breaks will transfer the kinetic energy from the planet to the train until relative to one and other, neither body appears to be moving. This is one scenario where you would experience a change in velocity.
- If the planet is moving independently of the "train", say the train launches itself into space and then comes down when your destination is below you, then you will have to match the planets rotation as you come down, or your would experience and instantaneous change in velocity as your train comes into contact with the surface. That is another way you would observe a change in velocity.
- If the rotation of the planet is controlled by a fourth body and the planet completely stops moving when your destination is below you then you would not experience a change in velocity other then the movement up and down. This is not the scenario that the comic described though.
@LB: There's way too little information for a proper thought experiment here.
SCENARIOS:
- If the train is imparting a force on the planet then the planet is imparting force on the train. So even if from your frame of reference you appear to not be moving, you actually are. When you stop the friction from the breaks will transfer the kinetic energy from the planet to the train until relative to one and other, neither body appears to be moving. This is one scenario where you would experience a change in velocity.
- If the planet is moving independently of the "train", say the train launches itself into space and then comes down when your destination is below you, then you will have to match the planets rotation as you come down, or your would experience and instantaneous change in velocity as your train comes into contact with the surface. That is another way you would observe a change in velocity.
- If the rotation of the planet is controlled by a fourth body and the planet completely stops moving when your destination is below you then you would not experience a change in velocity other then the movement up and down. This is not the scenario that the comic described though.
Let's say relative to the sun the train is not moving - is that your third scenario? Why is it different if the train is moving relative to the sun?
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The way I interpreted the comic was the track system moves the planet and the train just sits on the tracks, never actually moving.
That's the same thing basically, since I'm only concerned about what passengers in the train feel.
It's not so much whether the train is moving or not. That lurch forward you are describing is caused by a change in velocity (propagation of information and all of that stuff).
But the train is not changing velocity, only the planet's rotational velocity is changing.
The point is that the train is tied to the earth (via gravity) hence it's dragged with movement of the earth. When the earth stop the momentum conservation sets in.
Due to the difference of the masses you will be thrown forward.
Due to the difference of the masses you will be thrown forward.
Right, in that third scenario you would not observe that 'lurch forward'.
But that is not what was described in the comic. In the comic the train was controlling the planets movement and in order to stop the planet the train would have to exert force on it. For every action there is an opposite and equal reaction i.e. the planet moves the train as the trains breaks bleed away the planets momentum. That change in momentum that you then experience is the train moving you.
But that is not what was described in the comic. In the comic the train was controlling the planets movement and in order to stop the planet the train would have to exert force on it. For every action there is an opposite and equal reaction i.e. the planet moves the train as the trains breaks bleed away the planets momentum. That change in momentum that you then experience is the train moving you.
I'm still having a hard time grasping why the third scenario is different from the first scenario.
| SCENARIOS: - If the train is imparting a force on the planet then the planet is imparting force on the train. So even if from your frame of reference you appear to not be moving, you actually are. When you stop the friction from the breaks will transfer the kinetic energy from the planet to the train until relative to one and other, neither body appears to be moving. This is one scenario where you would experience a change in velocity. - If the planet is moving independently of the "train", say the train launches itself into space and then comes down when your destination is below you, then you will have to match the planets rotation as you come down, or your would experience and instantaneous change in velocity as your train comes into contact with the surface. That is another way you would observe a change in velocity. - If the rotation of the planet is controlled by a fourth body and the planet completely stops moving when your destination is below you then you would not experience a change in velocity other then the movement up and down. This is not the scenario that the comic described though. |
The first two are the same scenario, and are exactly what is going on now in reality, except that the train is so small compared to the Earth, that the train changes the rotation of the earth so little as to be immeasurable. The comic suggests the opposite happens, that the trains inertia is so great that when it rotates the Earth, it's change in velocity is so miniscule it's immeasurable.
Under the scenario the comic suggests, you would not feel any lunge when you stop rotating the Earth. There would be a miniscule lunge, but it wouldn't be detectable.
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