# Relativity question

Discussion in 'Astronomy, Exobiology, & Cosmology' started by thplinth, Mar 25, 2011.

1. ### KennycRegistered Senior Member

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110% * 1 year eh?

3. ### DeeCeeValued Senior Member

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"110% * 1 year eh? "

Yeh that seems the obvious answer. So time dilation is not apparent in my frame of reference?

Dee Cee

5. ### KennycRegistered Senior Member

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I never get the time dilation questions right though...

7. ### OnlyMeValued Senior Member

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The perfectly ridged material or body are hypothetical. There is nothing we know of that is perfectly ridged. Matter as we know is connected electromagnetically atom to atom and does not represent a perfectly ridged interaction. Math would come into it were you to try and actually work out how long a transfer of motion would take, end to end through a pole. And that would require a great deal of additional information of both atomic and relativistic conditions.

The time dilation calculations were covered very well in another thread, "Very simple math problem", I think.

The math gets complicated but a link to an Internet calculator, using the Lorentz Transformation formula for both length contraction and time dilation was given, (I hope I can insert the link properly)

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html

If you plug .90 into the calculator, it gives a time dilation of 2.294157338705618, which would then be divided into a distance in usually in light years to give the actual time of travel a 90% c for that distance.

From that if I have it right 60 minutes of time on earth would be 26.1533936613 minutes on your clock at 90% of c.

8. ### orcotValued Senior Member

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Nice calculator fun to know you would have to travel at yust under 71% of the speed of light, to experience a lightyear in less then one year (seen from earth it would still look like 1.4 years)

9. ### wellwisherBannedBanned

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The force impulse within the rigid rod can only move at C. As such, the end of the rod will take a light year to move.

As a different analogy, consider sending electricity down a wire so we can get an ipulse moving at C. If I move a meter stick at the beginning, this sends an electical impulse down the wire. When the electricity reaches the end, it triggers another machine that moves another meter stick. It still takes a year.

10. ### tashjaRegistered Senior Member

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Wow! Does that mean that an observer on a very distant planet measure my mass diferently than how I measure it in my frame of reference? Does that apply only when going close to the speed of light?

11. ### OnlyMeValued Senior Member

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The concept of mass can be confusing. There are essentially two definitions.

The first is "rest mass" and is observed in all frames of reference as being the same.

The second is "relativistic mass" and is a combination of rest mass and velocity.

Mass being in effect a measure of on object's inertia. It resistance to a change in motion.

Velocity is relative to a point of observation, a frame of referance. Since velocity is relative to the observer, an objects "relativistic mass" also depends on the observer's frame of reference.

Think of it like this. A bullet has a specific rest mass that does not change. Though it is not really the same thing you can think of it as weighing the same all of the time, where its weight represents a measurement of its rest mass at a given location, as the force it exerts on the scales.

Now think about a bullet fired from a gun. The bullet now has not only its rest mass, it also has a velocity, relative to both the gun and whatever it hits. The faster it is going the more force it hits the target with. The combination of its rest mass and velocity, become a representation of its relativistic mass.

If yo are at rest compared to the motion of the bullet it hits you hard. If on the other hand you are running away from the gun fast enough that the bullet is moving relative to you at only 32 feet per second, the bullet would hit only as hardnas if it had been dropped on you from a distance of 32 feet. (gravity on earth accelerates an object at 32 feetnpersecond for the first second of travel.)

It still hurts when it hits you in the head but it does not carry the same force as if you were not moving.

Relativistic mass changes with relative velocity. It looks different to people moving at different speeds compared to the object.

Rest mass is always the same for all observers regardless of their individual velocities.

12. ### tashjaRegistered Senior Member

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Yeah, but where does the 'relativistic mass' reside, OnlyMe? Is it around the body or inside the atoms of the body?

13. ### OnlyMeValued Senior Member

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Think of relativistic mass as if it were momentum or the force of the object. Where iESA sense you can think of rest mass as substance, where substance is a measure of an objects resistance of a change in motion, in this case from a state of rest, crudely expressed as an objects weight. Relativistic mass is the force generated by a combination of rest mass and velocity. Relativistic mass is force.

A small object moving at a specific speed has a certain force. If another object with twice the rest mass is moving at the same speed it has more force.

It is better to think of relativistic mass as force rather than as mass or as weight, (weightnis a dude analogy but convent because it is part of everyday experience.)

14. ### tashjaRegistered Senior Member

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OnlyMe,

Can I say, then, that we don't really gain mass as we speed up, but just add relativistic momentum?

15. ### OnlyMeValued Senior Member

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We don't gain rest mass. The inherent aspect of matter to resist a change in motion.

In some over simplified way it could be looked at as momentum.

The idea that mass gets bigger comes in part from thinking of energy and mass as equivalent. When in motion an objects rest mass and velocity together make up its relativistic mass. Its total energy.

Most of us think of mass as substance and there was a time in the past when that is how it was treated. You had mass or substance and energy. Today how it is treated depends in part upon what your physics background is. In quantum theory, mass is treated as a special form of energy. Personally though I understand it I don't like that.

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@tashja,