Special Relativity Question (1 Viewer)

[littlemic21]

There are 3 rocks. One is stationary. Another is travelling at a constant velocity of 0.6c and the last rock is travelling at a constant velocity of 0.6c in the opposite direction.

ok so if you're on the stationary rock, relative to you, both rocks are travelling at 0.6c in opposite directions

If you're on one of the moving rocks what is the other moving rocks speed relative to you?

i keep gettin 1.2c which is obviously wrong as the speed of light is the fastest you can get, or can you not do it this way for some reason?

dnt know wat the real answer is... ne1 got ne ideas/explanations

 

[deyveed]

oooo...thats a thinker...

 

[ND]

No you can't just add them together, there is some equation you use to find the relative velocities, don't remember what it is though...

 

[Inhuman]

My first instinct is to add them, but this obviously doesn't work. SO, would time dilation and suchlike be relevant?
...I have a headache...

 

[...]

nonono...

ND is right, i remember my teacher said something like that...u can't just add the speed vectors....i think its gotta somehow time i relative to each other and shit...

oh man, stop throwing special relativity q's in this forum..our brains will blow up!!!!

 

[Constip8edSkunk]

u cant add the velocity vectors if they are travelling close to c...

i think the formula is something like

V=(v_a - v_b) / sqrt [ 1 - (v_a*v_b)/c^2]

where V is the velocity of an object 'a' to 'b' and v_a,v_b are the corresponding velocities to an observer

EDIT: actually have a look at post by wogboy, who actually knows what he's talking about

 

[wogboy]

i think the formula is something like

V=(v_a - v_b) / sqrt [ 1 - (v_a*v_b)/c^2]

Close, the formula actually is:

V_rell = (v_a - v_b)/(1 + |v_a*v_b|/c^2) (plus instead of minus in the denominator, and no sqrt)

so if v_a = 0.6c and v_b = -0.6c (travelling in the opposite direction), then the relative velocity is:

v_rel = 0.6c - (-0.6c) / (1 + 0.6c*0.6c/c^2)
= 1.2c/(1 + 0.36)
= 1.2c/1.36
~ 0.88c

[Edit] Crap, forgot the absolute values in the denominator, for those who are mathematically fussy.

 

[...]

Originally posted by wogboy
Close, the formula actually is:

V_rell = (v_a - v_b)/(1 + v_a*v_b/c^2) (plus instead of minus in the denominator, and no sqrt)

so if v_a = 0.6c and v_b = -0.6c (travelling in the opposite direction), the the relative velocity is:

v_rel = 0.6c - (-0.6c) / (1 + 0.6c*0.6c/c^2)
= 1.2c/(1 + 0.36)
= 1.2c/1.36
~ 0.88c

you always amazes me with ur talent...

 

[wogboy]

Bah I'm just regurgitating Einstein, he's the mad physicist with the crazy ideas/talent, and even crazier hair

 

[...]

Originally posted by wogboy
Bah I'm just regurgitating Einstein, he's the mad physicist with the crazy ideas/talent, and even crazier hair

i'll pay u 20bucks to do my physics exam!

 

[wogboy]

lol ok and you can try doing my physics exam at uni.

 

[...]

Originally posted by wogboy
lol ok and you can try doing my physics exam at uni.

pfft...only general relativity...

 

[littlemic21]

kool thanx alot wogboy... so this type of question wouldnt be on the hsc exam?... my friend asked me it out of nowhere... ill try find out where he got it from