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# Scary Relativity- Mass?

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### #1 brentwood

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Posted 14 December 2013 - 06:28 PM

Now that we all have that pesky headlight thing sorted out! ( ), here's another. Hopefully this is easier to describe.
As a vehicle is pushed to close to light speed, its mass increases. This is explained, sort of, by the fact that all that energy has to go somewhere as it cannot be used to propel the vehicle any faster. The energy is converted to mass. The question is, what kind of mass? more atoms? what kind of atoms, where do they go?
I can imagine a craft zipping along at just below c, with a big lump on the tail, the extra mass!
Is this a question that can be answered without an equation?

### #2 PeterR280

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Posted 14 December 2013 - 07:18 PM

Every particle in the car, every electron proton neutron gains mass.

### #3 llanitedave

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Posted 14 December 2013 - 07:27 PM

And again, to the occupant of the car, nothing within it has changed.

### #4 PeterR280

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Posted 14 December 2013 - 07:33 PM

As long as you have non-zero rest mass, it will follow the transformation laws. You can actually calculate the kinetic energy of a moving body by calculating the difference between the rest mass and the mass at velocity and applying the famous equation E=mc^2. for low velocities the kinetic energy will be the same as .5mv^2. At higher velocities it will be the relativistic kinetic energy which will be a little different depending on speed.

### #5 brentwood

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Posted 14 December 2013 - 08:28 PM

I may not get this one at all due to my lack of particle physics knowledge. So the total number of particles doesn't increase, just the mass of each one of them? So is this the same kind of extra mass that any moving object seems to acquire as measured when it hits something, Or is this totally different, apart from the obvious difference in magnitude?

### #6 PeterR280

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Posted 14 December 2013 - 11:08 PM

It's really a very simple transformation. Anything with a non-zero rest mass, whether a subatomic particle or a baseball follows the same rules for calculating the relativistic mass.

### #7 GregLee1

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Posted 15 December 2013 - 06:13 AM

Every particle in the car, every electron proton neutron gains mass.

But if the people in the car had a particle accelerator and used it to shoot particles in the opposite direction to the car's motion, the particles would not be moving as fast as the car, so they would become lighter (i.e., fail to gain as much mass as the rest of the car). Since they would be moving fast with respect to the people in the car, however, to them, the particles would seem to become more massive.

### #8 brentwood

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Posted 15 December 2013 - 09:11 AM

Thanks for the explanations. Just a clarification though, above Peter said " for low velocities the kinetic energy will be the same as .5mv^2. " yet if I throw a weight at a scale, it will show quite an increase. So are we talking about two different processes? One the normal apparent increase in mass due to inertia, the other the relativistic real increase in mass at very high velocities.

### #9 PeterR280

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Posted 15 December 2013 - 10:17 AM

Kinetic energy is a different concept. It goes back to Newtonian mechanics.

### #10 FirstSight

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Posted 15 December 2013 - 10:29 AM

As a vehicle is pushed to close to light speed, its mass increases. This is explained, sort of, by the fact that all that energy has to go somewhere as it cannot be used to propel the vehicle any faster. The energy is converted to mass. The question is, what kind of mass? more atoms? what kind of atoms, where do they go?
...
Is this a question that can be answered without an equation?

Sort of, kind of, but physics for poets can only take you so far. I say this as someone who at one time took a lot of math courses (e.g. calculus through differential equations) but who has forgotten probably 95% of it all (and that's being generous to myself), and who realizes that like my potential to lower my golf handicap, there are simply limits to my math capabilities beyond which it's forbiddingly difficult for me to ever go.

The Feyneman Lectures on Physics (3 vol set available in paperback) do go into the math (including on relativity), but are surprisingly accessible on many subjects (including relativity) to folks with only a decent background in basic algebra, not the hard parts thereof. Richard Feynman was not only a Nobel Laureate, but a brilliant teacher who had the gift of being able to lucidly explain things to more ordinary mortals using apt examples rather than relying solely on math for explanations. It's really worth picking up the 3-volume set to have around as a basic reference.

### #11 brentwood

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Posted 15 December 2013 - 11:05 AM

"with only a decent background in basic algebra," Well that lets me out! I seem to be algebraic dyslexic, with geometry I'm fine and I can add large columns of numbers in my head but show me XY and it looks like Albanian to me!
One more question on the energy=mass aspect, am I correct in assuming that with a just sub c craft approaching a massive object, the gravitational pull of the object will also add mass to the craft as well?

### #12 PeterR280

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Posted 16 December 2013 - 10:08 AM

Brentwood, I am not sure what you meant by the question but you have to differentiate between kinetic energy and relativistic mass. The reason the scale momentarily shows more than the weight of an object when you throw it is not due to the increase in mass. It's due to the kinetic energy doing work. The increase in mass is negligible for low velocities. It is true that objects appear to have more mass as they accelerate with respect to you but the velocity has to be very high to be noticeable. It would be hard to learn basic mechanics in this forum.

Interestingly though, even for low velocities, E=mc^2 works, so you can use it to calculate kinetic energy.

### #13 brentwood

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Posted 16 December 2013 - 03:11 PM

Thanks for trying to explain it Peter, but I guess I'm just not up to it, I seem to be going in circles. Maybe I'll be back later after I've learnt some basic mechanics.

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