Theory of relativity

[wuliheron]

...Wow. Ok, So we're done here, then? Great.

Hey, don't ask me. You're the one who started this crap, who keeps bragging about being a physicist somehow making you an expert on metaphysics, and now getting paranoid about kooks. Perhaps you'd like the number of a good psychiatrist? They're doing miracles these days with psychopharmacology.

 

[QuantumPion]

Hey, don't ask me. You're the one who started this crap, who keeps bragging about being a physicist somehow making you an expert on metaphysics, and now getting paranoid about kooks. Perhaps you'd like the number of a good psychiatrist? They're doing miracles these days with psychopharmacology.

Ahh, liberals. Always standing on your head and telling the world they're upside down. The expression never fails.

Just to clarify, you are in fact the on that started the demonstrably false philosophical conjecture. When I corrected you, you ignored my links to factual information. You didn't even bother to investigate them. You didn't argue with my statements. Instead, like a typical progressive, you simply resort to accusing me, of making "metaphysics arguments", which is precisely what you have been doing this whole time.

When you debate against reality, you lose. End of discussion.


I'm done here.

 

[wuliheron]

I'm done here.

You keep saying that. I'll wait for the movie.

 

[KZ0]

*Hoping this thread hasn't been completely killed*

Got a question related to relative speed and energy, not necessarily the theory of relativity, though.

Person A stands still, person B moves west with a speed of 10 m/s relative to A. A throws a ball of mass 1 kg with a speed of 20 m/s eastwards. From A's reference frame, the ball's kinetic energy difference of the ball is 0.5*1*20^2 - 0 = 200 J. From B's reference frame, the ball's kinetic energy difference is 0.5*1*30^2 - 0.5*1*10^2 = 400 J.

If A throws the ball to B, who decelerates the ball to 0 m/s relative to A.

Visually (speeds relative to A)
A         *                B
.     20 m/s -->     <-- 10 m/s

after sphere deceleration:

A          B         *
.     <-- 10 m/s     .

(. is no movement, * is sphere)

Seems like energy is created. Is the solution applying conservation laws for energy and moment, meaning B will be slowed down, and "loose energy"?

 

[Puppies04]

Kinetic energy doesn't magically change depending on your "reference frame", yes the ball may appear to have different totals of kinetic energy from each perspective but if either party is trying to work out scientific equations without first checking the figures actually represent reality then they might aswell go home. The only reason "reference points" were even mentioned in this thread was to try to explian how light has a uniform speed.

*Disclaimer, into my second bottle of red at the moment so if i missed your point completly i apologize.

 

[Gibsons]

If A throws the ball to B, who decelerates the ball to 0 m/s relative to A.

I thought B was moving? How does he get the ball to 0 m/s to A if he's moving relative to A?

 

[Paul98]

Kinetic energy doesn't magically change depending on your "reference frame", yes the ball may appear to have different totals of kinetic energy from each perspective but if either party is trying to work out scientific equations without first checking the figures actually represent reality then they might aswell go home. The only reason "reference points" were even mentioned in this thread was to try to explian how light has a uniform speed.

*Disclaimer, into my second bottle of red at the moment so if i missed your point completly i apologize.

Sure kinetic energy changes depending on the frame of reference. How are you going to have velocity with out a frame of reference?

 

[Ghiedo27]

The total energy is the same regardless of your frame of reference.

Both people will recognize the energy added from the throw. Person B will add in the 10m/s that person A and the ball were going before the throw and person A will add the force of person B reversing the direction of motion by 10m/s.

 

[KZ0]

I thought B was moving? How does he get the ball to 0 m/s to A if he's moving relative to A?

Simplifying my statement: B makes the speed of the ball the same as A.

The total energy is the same regardless of your frame of reference.

Both people will recognize the energy added from the throw. Person B will add in the 10m/s that person A and the ball were going before the throw and person A will add the force of person B reversing the direction of motion by 10m/s.

So by conserving total momentum as well, the energy will level out?
If I understand you correctly, from A's reference:
Throwing the ball makes him move a little westwards, meaning throw requires more work, and B receiving the ball makes him loose speed westwards, removing a little energy, that could even out the apparent difference?

 

[Ghiedo27]

Whether throwing the ball makes A move a little westward or not doesn't really mater. The energy of the collision between the ball and person B is consistent because they're closing in on each other at 20 m/s. Whether you think that person A was moving towards person B before the throw or the reverse doesn't make any difference.

 

[iCyborg]

Whether throwing the ball makes A move a little westward or not doesn't really mater. The energy of the collision between the ball and person B is consistent because they're closing in on each other at 20 m/s. Whether you think that person A was moving towards person B before the throw or the reverse doesn't make any difference.

This is not so simple, yes, the speed delta is the same, but the kinetic energy is not, because it is a square dependence on speed. I had a discussion about this same problem about a year ago, albeit slightly differently phrased: say an observer B on a train is traveling at 10 m/s relative to some observer A. How much energy does he need to give to a 1kg ball to give it a speed of 10 m/s relative to himself B in the same direction the train is traveling?
The answer for B is 1*10^2 - 0 = 100J.
Now consider A, he also measures speed delta of 10m/s, but it is from 10 to 20, and he notices that B has given the ball 1*20^2 - 1*10^2 = 300J, 3x more (this is what I meant that speed delta is the same for both observers, but kinetic energy delta is not).
So how much extra energy did the ball get, 100J or 300J?

So by conserving total momentum as well, the energy will level out?
If I understand you correctly, from A's reference:
Throwing the ball makes him move a little westwards, meaning throw requires more work, and B receiving the ball makes him loose speed westwards, removing a little energy, that could even out the apparent difference?

That's exactly why, to conserve momentum, B (or in the above case something on the train or the train itself) had to lose a bit of speed and this will account for that extra 200J that the ball got from seemingly nowhere. I worked it out with momentum conservation taken into account, and everything is ok.

This is actually a known effect, and has a name: Oberth Effect.
http://en.wikipedia.org/wiki/Oberth_effect

In the page they discuss rockets and why they get more boost from the fuel at certain velocities than others. Quote from there:
"It may seem that the rocket is getting energy for free, which would violate conservation of energy. However, any gain to the rocket's energy is balanced by an equal decrease in the energy the exhaust is left with."

 

[KZ0]

@iCyborg: Excellent explanation. Thank you.

 

[Sunny129]

might i suggest a moderator move this section of the thread (from post #29 onward) to a new and appropriately titled thread, as it has absolutely nothing to do with Special or General Relativity, or the specific topic of this thread for that matter...

...not that KZO doesn't pose a good question - its just that it deserves its own thread and title so that 1) people can reference it if they choose to search this topic, and 2) folks searching for "relativity" don't get result threads whose titles imply that they're more about relativity than they truly are...