This is probably just a coincidence, but

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SUMMARY

The discussion centers on the misunderstanding of kinetic energy equations, specifically the incorrect assertion that kinetic energy (EK) equals mv² without the 1/2 factor. Participants clarify that the correct formula is EK = 1/2 mv², applicable only at non-relativistic speeds. The conversation also addresses the concept of time dilation and the notion that all objects have a "sum velocity" of c, which is fundamentally flawed. The correct interpretation involves the four-velocity of particles and their behavior in spacetime, emphasizing the distinction between classical and relativistic physics.

PREREQUISITES
  • Understanding of Newtonian physics, specifically kinetic energy equations.
  • Familiarity with the theory of relativity and concepts of time dilation.
  • Knowledge of four-vectors and their significance in relativistic physics.
  • Basic grasp of energy-momentum relationships in physics.
NEXT STEPS
  • Study the derivation and implications of the kinetic energy formula EK = 1/2 mv².
  • Learn about the concept of four-velocity in special relativity.
  • Explore the relationship between energy and momentum in relativistic contexts, specifically E² = (mc²)² + (pc)².
  • Investigate popular science literature on relativity, such as works by Brian Greene, to understand common misconceptions.
USEFUL FOR

Students of physics, educators, and anyone interested in clarifying concepts of kinetic energy and relativity will benefit from this discussion.

Trepidation
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EK = mv^2
energy kinetic = mass times velocity squared

So... Isn't one way to interpret time dilation that everything moves through time and space with a sum velocity of c (somehow), and that as velocity in space increases velocity in time therefore decreases? I've read this in several places, anyway.

If this is the case, then the permanent space-time velocity of all objects is c... Which means that their kinetic energy would be:

EK = mv^2

EK = mc^2

So er... E=mc^2
 
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Trepidation said:
EK = mv^2
energy kinetic = mass times velocity squared
So... Isn't one way to interpret time dilation that everything moves through time and space with a sum velocity of c (somehow), and that as velocity in space increases velocity in time therefore decreases? I've read this in several places, anyway.
If this is the case, then the permanent space-time velocity of all objects is c... Which means that their kinetic energy would be:
EK = mv^2
EK = mc^2
So er... E=mc^2
The equation E=mc^2 only applies when v=0. The general equation for any v is
E=\sqrt{(mc^2)^2+c^2p^2}
where p is momentum.
So indeed just a coincidence.
 
Trepidation said:
EK = mv^2
energy kinetic = mass times velocity squared
So... Isn't one way to interpret time dilation that everything moves through time and space with a sum velocity of c (somehow), and that as velocity in space increases velocity in time therefore decreases? I've read this in several places, anyway.
If this is the case, then the permanent space-time velocity of all objects is c... Which means that their kinetic energy would be:
EK = mv^2
EK = mc^2
So er... E=mc^2

1. Since when is kinetic energy equal to mv^2? What happened to the 1/2?

2. What exactly are "velocity in space" and "velocity in time"?

3. What is "space-time velocity"?

4. This statement is puzzling: "... Isn't one way to interpret time dilation that everything moves through time and space with a sum velocity of c (somehow),... " Everything does NOT move with a "sum velocity of c (somehow)".

Zz.
 
ZapperZ said:
1. Since when is kinetic energy equal to mv^2? What happened to the 1/2?
Of course. I completeley overlooked this most obvious argument!
 
Trepidation said:
EK = mv^2
energy kinetic = mass times velocity squared

No, its E_K = \frac{1}{2} m v^2


If this is the case, then the permanent space-time velocity of all objects is c... Which means that their kinetic energy would be:
EK = mv^2
EK = mc^2
So er... E=mc^2

E=mc^2 only applies to a an object when v= 0 as another pointed out. It has nothing to do with kinetic energy.
 
ZapperZ said:
Everything does NOT move with a "sum velocity of c (somehow)".

The length of the velocity four-vector of any particle is 1 in geometric units, which corresponds to c in SI units. A particle in it's rest frame is moving through time at a second a second. To compare lengths with times, we multiply by c, to find that it is moving c metres a second.
 
masudr said:
The length of the velocity four-vector of any particle is 1 in geometric units, which corresponds to c in SI units. A particle in it's rest frame is moving through time at a second a second. To compare lengths with times, we multiply by c, to find that it is moving c metres a second.

But really, do you honestly think the OP knew about this and that this is what he/she is describing by making that erroneous statement? I highly doubt it.

Zz.
 
ZapperZ said:
But really, do you honestly think the OP knew about this and that this is what he/she is describing by making that erroneous statement? I highly doubt it.
Zz.

I believe one of Greene's recent popular science texts (with some grandiose title) expresses this idea (and I assume this is where the OP got the idea from); whether or not the OP fully understood it or not is, of course, another matter.
 
masudr said:
I believe one of Greene's recent popular science texts (with some grandiose title) expresses this idea (and I assume this is where the OP got the idea from); whether or not the OP fully understood it or not is, of course, another matter.

I'm not so sure... Greene cannot make a silly mistake of equating KE with mv^2 and missing out that 1/2, which is the OP starting premise.

Zz.
 
  • #10
I comprehend the idea, but the important thing is that I apparently don't comprehend simple Newtonian physics. Forgive me for the incorrect formula and the idiotic post... I just saw something that sort-of corresponded and then decided to post it, and consequently wound up with a bunch of gibberish.

Again, sorry. I don't know what I thought when I was writing EK=mv^2. Thanks for replying to this, anyway.
 
  • #11
ZapperZ said:
I'm not so sure... Greene cannot make a silly mistake of equating KE with mv^2 and missing out that 1/2, which is the OP starting premise.
Zz.

Sorry, I meant the notion of having a velocity of c at all times, as opposed to the definition of non-relativistic classical kinetic energy.
 

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