Understanding the Relativistic Velocity Equation

[Lynny]

A particle moves along the x' axis of frame S' with velocity .4c. Frame S' moves with velocity .6c with respect to frame S. What is the velocity of the particle with respect to frame S'?

I used the realistic velocity equation:
u=(u' + v)/(1+u'v/c^2)

u=(.4 + .6)/(1+.4*.6/(3*10^8)^2)

The answer I get is 1 but the real answer is .81c

What am I doing wrong?

Thanks

 

[The_Duck]

You are using u' = .4 and v = .6 instead of u' = .4c = .4*3*10^8 and v = .6c = .6*3*10^8.

 

[Lynny]

I see. I keep thinking c is a unit of measurement. It all works now. Thanks!