How Fast is Spaceship A Traveling Relative to Spaceship B?

[Gott_ist_tot]

I am doing an extra problem from my textbook as review and I am stumped.

An Earth based observer sees to spaceships approaching Earth in the same direction. Spaceship A is traveling at 0.500c and Spaceship B is traveling at 0.800c. How fast is spaceship A traveleing as viewed by an observer on spaceship B?

I used a velocity transformation like:

(0.500c - 0.800c)/sqrt(1-[(0.5c)(0.8c)/(c^2)])

The book gets -0.500c.

I am finding it difficult to see that the Earth based observer and the spaceship moving at 0.800c observe that spaceship going the same speed.

Thanks for your help.

 

[Doc Al]

I used a velocity transformation like:

(0.500c - 0.800c)/sqrt(1-[(0.5c)(0.8c)/(c^2)])

That square root shouldn't be there.

See https://www.physicsforums.com/showpost.php?p=905669&postcount=3"

 

[Gott_ist_tot]

Wow! It's always the simplest of mistakes. Thanks a lot.