Firing a gun on a train doing 2000 mph

[jajrussel]

It would seem to me that going in the same direction as the train the bullets velocity would already be 2000. From that view if you do not add force to the bullet to accelerate it 2000 in the direction of the train, you haven't accelerated the bullet. So, when you aim in the direction the train is going and fire/accelerate the bullet the bullets velocity increases 2000. I don't want to be on the train in front of the bullet, because the bullet would be moving toward me at 2000. The ground/platform observer measures the trains speed at 2000 and the bullets speed at 4000.

 

Now I turn around and fire/accelerate the bullet in the opposite direction at 2000. Again, I don't want to be on the train in front of the bullet, because I would be moving toward the bullet at 2000. From my point of view the bullet is not sitting there suspended in air waiting for me to catch up, it is moving toward me at 2000.The ground/platform observer sees the gun moving away from the bullet at 2000, sees the bullet fall straight down at roughly 9.8. The bullet still has kinetic energy. If it lands on the train one thing happens. If it lands on the ground another thing happens. When it comes to bullets I am a coward. To the ground/platfom observer it may appear that the bullet is simply falling straight down with 9.8 worth of kinetic energy, but the coward in me says observe from a distance. That 2000 worth of kinetic energy is still there.

 

The only way the bullet simply drops at 9.8 is if you simply drop it. The ground/platform observer is not going to see it as falling straight down.

 

I could be wrong, but this is how it seems to me.

 

Foreign the gun opposite to the trains direction does make my head hurt when I think about it, but if I were in space and came in contact with something of similar mass. I should be able to push off the other mass even if we are both moving in the same direction. I am thinking that there should be plenty of mass for the bullet to accelerate from zero to 2000 when it per say pushes off.

Edited January 31, 2017 by jajrussel

[Janus]

It would seem to me that going in the same direction as the train the bullets velocity would already be 2000. From that view if you do not add force to the bullet to accelerate it 2000 in the direction of the train, you haven't accelerated the bullet. So, when you aim in the direction the train is going and fire/accelerate the bullet the bullets velocity increases 2000. I don't want to be on the train in front of the bullet, because the bullet would be moving toward me at 2000. The ground/platform observer measures the trains speed at 2000 and the bullets speed at 4000.

 

Now I turn around and fire/accelerate the bullet in the opposite direction at 2000. Again, I don't want to be on the train in front of the bullet, because I would be moving toward the bullet at 2000. From my point of view the bullet is not sitting there suspended in air waiting for me to catch up, it is moving toward me at 2000.The ground/platform observer sees the gun moving away from the bullet at 2000, sees the bullet fall straight down at roughly 9.8. The bullet still has kinetic energy. If it lands on the train one thing happens. If it lands on the ground another thing happens. When it comes to bullets I am a coward. To the ground/platfom observer it may appear that the bullet is simply falling straight down with 9.8 worth of kinetic energy, but the coward in me says observe from a distance. That 2000 worth of kinetic energy is still there.

 

The only way the bullet simply drops at 9.8 is if you simply drop it. The ground/platform observer is not going to see it as falling straight down.

 

I could be wrong, but this is how it seems to me.

Yes, you are wrong. KE is not absolute, it is relative. If the bullet has 0 velocity with respect to you, then it has 0 KE with respect to you. It doesn't matter that it was given some value of KE relative to the train. If the bullet is fired backward, it will just fall to the ground and hit it with the same force as if someone standing on the ground had just dropped it.

[jajrussel]

Yes, you are wrong. KE is not absolute, it is relative. If the bullet has 0 velocity with respect to you, then it has 0 KE with respect to you. It doesn't matter that it was given some value of KE relative to the train. If the bullet is fired backward, it will just fall to the ground and hit it with the same force as if someone standing on the ground had just dropped it.

Maybe I am, but I need to think about it. It seems to me that if the bullet and I are both moving in the same direction, and then it suddenly stops, I would then slam into it at 2000, or pass it with that velocity. Seems to me that an observer on the platform should observe exactly that. The result should imply a lot of kinetic energy.

[Janus]

Maybe I am, but I need to think about it. It seems to me that if the bullet and I are both moving in the same direction, and then it suddenly stops, I would then slam into it at 2000, or pass it with that velocity. Seems to me that an observer on the platform should observe exactly that. The result should imply a lot of kinetic energy.

In this scenario, the bullet would go from having 0 velocity and 0 KE with respect to you to having a non-zero velocity and a non-zero KE with respect to you. In your frame of reference the bullet didn't "stop", but accelerated up to 2000 mph.

The change in energy had to come from whatever changed the bullet's velocity.

 

There is no such thing as an absolute velocity. When you say that that you and the bullet were moving in the same direction, you mean in respect to some frame of reference. There is nothing more real or preferred to that frame of reference than your own.

 

Thus in one frame, you can say that you and the bullet were moving at 2000 mph, the bullet stopped and you ran into the bullet, But in another you can say that you and the bullet were at rest and the bullet sped up to 2000 mph and struck you. The result of these two frames will be exactly the same. You will suffer exactly the same amount of damage from the bullet according to both frames. It doesn't matter how you considered the final difference in speed between you and bullet came about, only what it is at impact.

 

With the bullet fired backwards from the train, the same holds true, As far as the person on the ground is concerned, it doesn't matter how the bullet came to have a zero velocity with respect to him, being always at rest and dropped or fired backwards from the train, the KE of the bullet relative to him is the same.