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View Full Version : Wrap your noggin around this one...

mugaliens
2008-Jul-18, 04:14 PM
Imagine a rocketship that leaves our solar system and accelerates, directly away from the sun, to 1/3 c relative to the sun.

It's second stage separates and accelerates directly away from the sun to 1/3 c relative to the first stage.

It's third stage separates and accelerates directly away from the sun to 1/3 c relative to the second stage.

How fast, relative to Sol, is the third stage travelling at the end of it's acceleration?

Neverfly
2008-Jul-18, 04:20 PM
Imagine a rocketship that leaves our solar system and accelerates, directly away from the sun, to 1/3 c relative to the sun.

It's second stage separates and accelerates directly away from the sun to 1/3 c relative to the first stage.

It's third stage separates and accelerates directly away from the sun to 1/3 c relative to the second stage.

How fast, relative to Sol, is the third stage travelling at the end of it's acceleration?

Depends on who made the Rocketship.

If it was Ford, the alternator would have blown out past the second stage and the tail lights wouldn't work.

If it was Chrysler, the transmission would have gone out when shifting from 1/3 to 2/3 gear.

if it was Honda, it would never pass the speed limit. But you can blame this one on the driver and the ship will never break down.

korjik
2008-Jul-18, 04:21 PM
:P

:D

By the way, just to WAG it, about .9c

Neverfly
2008-Jul-18, 04:22 PM

Yeah, these datgummed lazy whippersnappers always asking us their homework questions...

grant hutchison
2008-Jul-18, 04:26 PM
Seven-ninths of light-speed, according to the relativistic addition of velocities.

Grant Hutchison

Cougar
2008-Jul-18, 05:08 PM
Seven-ninths of light-speed, according to the relativistic addition of velocities.

Seven-ninths? That sounds like an improbable figure... :think: Well, I'll be darned. That's correct!

--formula (http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html)

OldGuySythe
2008-Jul-18, 05:15 PM
AAANNNTTT! Acceleration is absolute. You are describing a circumstance that has no meaning.

cjl
2008-Jul-18, 05:24 PM
AAANNNTTT! Acceleration is absolute. You are describing a circumstance that has no meaning.
The circumstance absolutely has meaning, and the answer is 7/9, as said before.

grant hutchison
2008-Jul-18, 05:27 PM
AAANNNTTT! Acceleration is absolute. You are describing a circumstance that has no meaning.The problem is stated in terms of relative velocities, and is quite meaningful. But observers scattered across the different inertial frames referenced in the problem will observe different accelerations, too.

Grant Hutchison

mugaliens
2008-Jul-19, 07:15 PM
Depends on who made the Rocketship.

In defense of the automakers...

If it was Ford, the alternator would have blown out past the second stage and the tail lights wouldn't work.

At least refurbished Ford alternators are a dime a dozen at any Pep Boys, NAPA, or other Auto Parts store, and if the light goes out, it's a \$12 bulb, rather than a \$130 tail light assembley (thanks, Honda...)

If it was Chrysler, the transmission would have gone out when shifting from 1/3 to 2/3 gear.

True, but the good thing about Chrysler transmissions is that the forward gearing is separated from reverse, so if that happened, you could simply flip 180 degrees and keep on going. You'll need a rearview mirror, though...

if it was Honda, it would never pass the speed limit. But you can blame this one on the driver and the ship will never break down.

I've owned two Hondas and am on my third Ford. To date, each of my Hondas has had more breakdowns than any one of my Fords, and aside from my current Ford, I owned each of my cars for roughly the same period of time and milage.

Come to think of it, total cost for parts and repairs on all my Fords comes to less than \$300, but for my two Hondas, it came to more than \$3,000.

Hmmm...

And I babied my Hondas, but drove my Fords into the ground.

mugaliens
2008-Jul-19, 07:21 PM
cjl, Neverfly, I confess - I gave myself a homework problem, then was too lazy to look it up myself.

Shame on me!

Actually, we've seen dozens of related questions over the last six months, and instead of tackling it each and every time, I hit upon this simple, straightforward scenario to highlight how 1/3c +1/3c + 1/3c <> c, from the relativistic point of view.

Thanks, Cougar, for sharing the link to that website (http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html)- good resource!