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View Full Version : Quick relativity question (NOT a debate!)

Wally
2004-Apr-28, 12:31 PM
I was talking SR with a friend yesterday. He asked a "what-if", and I couldn't answer (even though it was pretty basic). I'm having a brain-fart, I guess. . . Anyways, the senario is, you have 3 inertial reference frames; A, B and C. We are in frame C. From our viewpoint, A is moving at .6c to our left, and B is moving .6c to our right. Question is, what speed does frame A see frame B moving at? It can't be 1.2c, obviously. I couldn't answer. . . Help please?

Celestial Mechanic
2004-Apr-28, 12:36 PM
Velocities add according to the following formula:

V = (v1 + v2) / (1 + v1*v2/c^2) .

With v1 and v2 both equal to 0.6c, that means (0.6+0.6)/(1+0.6*0.6) = 1.2/1.36 = 15/17 c.

Wally
2004-Apr-28, 12:40 PM
thanks CM! Is that one of lorentz's (sp) formulas?

Normandy6644
2004-Apr-28, 02:26 PM
thanks CM! Is that one of lorentz's (sp) formulas?

It's a formula derived by Einstein known as "the relative addition of velocities." It's fairly straightforward, and easy to apply.

Note: I like how we all have to put disclaimers on our relativity threads now. It's funny. :lol:

Glom
2004-Apr-28, 02:34 PM
Of course the whole point about Newtonian mechanics is that for v &lt;&lt; c, the denominator is pretty much 1.

Kaptain K
2004-Apr-28, 03:44 PM
thanks CM! Is that one of lorentz's (sp) formulas?

It's a formula derived by Einstein known as "the relative addition of velocities." It's fairly straightforward, and easy to apply.

Note: I like how we all have to put disclaimers on our relativity threads now. It's funny. :lol: