On Wed, 30 Apr 2008 16:43:25 +0200, YBM
wrote:

The following describes a very elegant and simple derivation of the relativistic
formula for the addition of velocities, w = (u+v)/(1 + uv/c^2).

It is due to David Mermin.

See:
http://dorigo.wordpress.com/2008/04/...ivistic-train/



[Hammond]
Are you kidding.... nobody but a rank amateur would screw
around with algebraic formulas when adding velocities.
Any expert will tell you all you do to add two velocities
in SR is to

ADD THE RAPIDITIES STUPID

The rapidity is defined as:

rapidity = r =acrtanh (v/c)


If you want to add two velocities (2/3)c and (3/4)c, all
you do is add r1 and r2 and then take the tanh of the
result:

r1 = arctanh(2/3) = .80472

r2 = arctanh(3/4) = .97296

total = 1.7777

Vtotal = tanh(1.7777) = .94444 c

So, (2/3)c + (3/4)c = .94444 c