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Relative Motion


Klaynos

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I wasn't talking about the train. I was talking about the rocket ship. You have plenty of reference points to compare the train's velocity against.

 

Yes you do, and when you pick one you can't change in the middle of the thought experiment, because the reference point has already been established.

 

The train travels 20m/s from a specific reference point. If you want to talk about a different reference point you need to change the 20m/s to the appropriate velocity, as it is no longer valid from a different frame of reference.

 

20m/s has already been established.

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There are three balls(one red, one blue, and one yellow). Each ball has a mass of 1kg. The red and blue balls are at rest with respect to each other, but are moving with respect to the yellow ball. From a reference frame in which the yellow ball is at rest, the red ball has the energy from mass, but since the red ball is moving, it also has kinetic energy. Now, let's move our reference frame to one in which the blue ball is at rest. The red ball still has the same energy due to mass, but, since it is at rest relative to the blue ball, has no kinetic energy. The red ball has more energy in the reference frame of the yellow ball than it does in the reference frame of the blue ball. Thus energy is dependent on the reference frame and is not conserved from frame to frame. Mass, however is the same in every frame of reference. If we consider the three balls as a system and use the same reference frames, we get different values for the total energy of the system. In the reference frame in which the yellow ball is at rest, there is a certain amount of energy in the system from the mass of all three balls and the kinetic energy of the red ball and the blue ball. In our other reference frame, we have the energy from the mass of all three balls and the kinetic energy of just the yellow ball.

 

As stated, each ball has a mass of 1kg. In the reference frame where the red and blue balls are at rest, the yellow ball is observed to be moving at 100m/s with respect to the red and blue balls. In the reference frame where the yellow ball is at rest, the red and blue balls are each observed to be traveling at 100m/s with respect to the yellow ball.

 

How much [acr=Kinetic Energy]KE[/acr] does the system have?

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All that is true. I did point out that from different frames of reference you get different velocities. From the point of view of someone sitting in the station the velocity of the train is 20m/s. Someone hurtling through space sees it as 7600m/s. They're both right. That's relativity.

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All that is true. I did point out that from different frames of reference you get different velocities.

 

Yes, but we already discussed the train was traveling at a velocity of 20m/s, and that the ball was initially not accelerating compared to the train (on the table, or in the hand of the guy before he throws it), so the ball is also traveling 20 m/s in the same reference frame. For all practical purposes, the ball is the train when it is not accelerating compared to the train.

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...How does that change what I said? You can still measure different velocities of it from different reference frames.

 

But you gave the ball's initial velocity of 20m/s. In order for the ball to have a velocity of 10 m/s when the guy throws it, he must have thrown the ball in the opposite direction of the train's travel. In other words, the guy would have had to "decelerate" the ball from 20m/s to 10m/s in order for the ball to have a velocity of 10 m/s. The only way the guy can do that is to throw the ball in the opposite direction of travel of the train's 20 m/s. Basically, the guy is reducing the ball's velocity from 20 m/s to 10 m/s when he "accelerates" the ball in the opposite direction of travel of the ball's initial velocity.

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This thread isn't talking about trains.

 

 

 

No, he didn't. Read the original problem statement in the correct thread.

 

 

Whether he realizes it or not, if the ball was traveling along with the train that is traveling at 20m/s, and the ball was say, resting on the table on the train, the ball is part of the train. It is not accelerating compared to the train. The ball's velocity is 20m/s while traveling with the train. The train's engine had to accelerate the ball, along with the train. The additional mass of the ball caused the train to accelerate at a reduced rate compared to the train accelerating without the ball, just like your car's acceleration is reduced at wide open throttle when there is extra people (mass) in the car.

 

Does adding a grain of sand to the trunk of your car slow the car's maximum acceleration rate? You better believe it!

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I'm done, anyway. I've made my point and backed it up with sound scientific and mathematical facts. The threads will be closed if I continue, I am told.
At least take a stab at my word problem in the other thread.
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But you gave the ball's initial velocity of 20m/s. In order for the ball to have a velocity of 10 m/s when the guy throws it, he must have thrown the ball in the opposite direction of the train's travel. In other words, the guy would have had to "decelerate" the ball from 20m/s to 10m/s in order for the ball to have a velocity of 10 m/s. The only way the guy can do that is to throw the ball in the opposite direction of travel of the train's 20 m/s. Basically, the guy is reducing the ball's velocity from 20 m/s to 10 m/s when he "accelerates" the ball in the opposite direction of travel of the ball's initial velocity.

 

No. If he's standing in the train and lobs the ball at 10m/s in the direction of the train's travel, then measures the ball's speed with a radar gun, he'll get an answer of 10m/s. A guy standing on the platform will get an answer of 30m/s, because the train was initially moving at 20m/s. Both of them are correct.

 

It is apparent to me that you have not expended any effort at all in trying to teach yourself more about relativity. You do not seem to be interested in learning.

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It is apparent to me that you have not expended any effort at all in trying to teach yourself more about relativity. You do not seem to be interested in learning.
This happens so often with Relativity. It runs counter to what seems true because, as humans, we often fail to consider any perspective but our own.

 

Then you find out others don't get it either and suddenly it's a big conspiracy. :rolleyes:

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As YDOAPS has pointed out (I just love reading my name in the paper, Butch) all speeds/velocities HAVE to be in reference to something. There is no such thing as "it moves at 20 m/s" The train moves at 20 m/s with respect to the platform. But to a guy on the train, the train is not moving — with respect to him. To him, the train is at rest.

 

The people at the train station standing on the Earth measure the train at 20m/s. If you want to include the Earth's velocity than you need to tell me what initial velocity the people at the train station have.

 

This tells me you have a grasp of reference frames. Stop being obtuse.

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Relative reference does not always reflect reality. Relative reference can create illusions. For example, you are in the train, and have the ball sitting on your lunch tray. The train begins to take a turn. Relative to your reference there does not appear to be any force acting, yet the ball mysteriously moves due to telekinesis. That is the theory for that reference. The person sitting on a hill watching the train also sees the ball start to have an additional motion. His theory based on his reference is centrifugal force. If they were both the same we would not have this problem.

 

Nobody in the train will believe telekinesis. Instead they will shift reference in their minds to a better zero reference and conclude the train is turning, i.e., use the hill reference, because their reference is not adequate to give a good explanation. When velocity approaches relativistic one need to include the third or mass parameter because some of the references will create illusions that violate the conservation of energy.

Edited by pioneer
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Relative reference does not always reflect reality. Relative reference can create illusions. For example, you are in the train, and have the ball sitting on your lunch tray. The train begins to take a turn. Relative to your reference there does not appear to be any force acting, yet the ball mysteriously moves due to telekinesis. That is the theory for that reference. The person sitting on a hill watching the train also sees the ball start to have an additional motion. His theory based on his reference is centrifugal force. If they were both the same we would not have this problem.

 

Nobody in the train will believe telekinesis. Instead they will shift reference in their minds to a better zero reference and conclude the train is turning, i.e., use the hill reference, because their reference is not adequate to give a good explanation. When velocity approaches relativistic one need to include the third or mass parameter because some of the references will create illusions that violate the conservation of energy.

 

Not all reference frames are created equal. An accelerated frame is not an inertial one.

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I'm done, anyway. I've made my point and backed it up with sound scientific and mathematical facts.

Ok, it is known that a bullet from a gun travels at lest 330m/s. This is enough to kill someone.

 

So if I have a bullet on train going at 330m/s down the tracks, then according to you, that bullet is travelling at 330 m/s. If I then get a friend to slowly (1m/s) throws the bullet towards me and hits me with it, then I should be killed as we know that a bullet travelling at 330 m/s can kill you.

 

However, I understand relativity and know that motion is relative. I know that although the Bullet is travelling at around 330 m/s (+/- 1 m/s), that I too am travelling at that 330/ms and our relative speed is only around 1m/s. A bullet travelling at 1m/s is not enough to kill me and so I know that I won't be killed by it.

 

This is what is meant by relative motion. In the case of the bullet, it doesn't matter if both it and me are travelling at 300m/s or 100,000km/s so long as the difference between our speeds is not that great, I'll survive.

 

Now, lets consider a poor bystander on the platform. My friend seeing that a slowly moving bullet doesn't kill me, decides to chuck that bullet out the window of the train. He doesn't throw it very fast. Just the same 1m/s that he hit me with. He saw that 1m/s can't kill someone, so he concludes that the bullet chucked out of the train at 1m/s won't hurt any one.

 

However, as the train is moving at 330m/s relative to the person on the platform, the bullet hits them at a high enough speed to kill them. Because although the bullet was only moving 1m/s relative to me and my friend at all times, it was moving at a different speed relative to someone standing on the platform.

 

Of course we could try this experiment again but this time the train is moving slowly, only 1m/s. This time the bullet hits the person on the platform at only 2m/s and they survive.

 

But what if instead my friend chucked it out the other window as another train was passing at 330m/s? The bullet is only travelling at 1m/s according to my friend and me and 2 m/s according to the person on the station. So at most, according to the thinking you are doing, that bullet can not kill someone on that other train.

 

However, according to relativity, that other person is moving at around 330m/s relative to us and so that bullet will hit them at around 330m/s and kill them.

 

Can you now see your mistake?

 

According to you, the bullet could never kill any one in the last examples because it is only travelling at most 2m/s. But under relative motion, you have to compare the speeds of the observer to work out the situation relative to them.

 

In the last example, the relative motion of the bullet gave 1m/s, 2m/s and 330m/s. All observers were observing the same bullet, there was no illusion as this is the difference between someone getting killed or not getting killed which can be directly measured. It can't be a case of the delay of the light takes to reach me that I see them live and another person sees them killed.

 

It is why there are laws that make it illegal to throw things out of moving vehicles. People can be and are hurt be these kinds of events.

 

If you gently chuck an apple core in a car travelling at 100km/s to the person sitting next to you, you are not going to bruise them, but if you such that same apple core with the same force out of that moving car moving at 100km/s at a person sitting on the side of the road you will bruise them.

 

The reason is that motion is relative. Your motion relative to the person sitting next to you is 0. But the motion relative to the person on the side of the road is 100km/h.

 

So, your science is wrong. All motion is relative. There is no absolute motion. I just proved it (several times).

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You have mistaken my understanding of relative motion. I fully understand what you are saying about the bullet, and never claimed otherwise. Why do people insist on telling me I don't understand relative motion??? Please provide a quote of mine that disagrees with the notion of relative motion.

 

The example in a previous thread was of a ball on a train. The train's velocity was given, 20 m/s. What is that 20 m/s relative to? Let's go really slow here answering one question at a time, so that we can see where the problem is, OK? Not big long posts, just direct answers to one question at a time, and stick to the direct question.

 

What is the 20m/s relative to?

Edited by Motor Daddy
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Presumably, it is given relative to the rails.

 

OK, I agree with that.

 

The track is marked like a ruler, every meter. Like a 1/4 mile drag strip, there is a timer. That timer is activated when the train passes the starting line on the track. When the train reaches the 20 meter mark on the track, relative to the starting line where the timer was activated, the timer is deactivated, and a time is shown on a display board, just like a drag strip.

 

Q: How long does it take the train to travel from the start line to the finish line that is exactly 20 meters away from the start line?

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