Equation for velocities at relativistic speeds

[Uno Lee]

If a baseball player driving a convertible in a straight line at 75 kmph and throws a ball the same direction at 50 kmph, the ball would be going about 125 kmph. but if one was traveling in a spaceship going half the speed of light and fired a particle beam in the same direction going three-quaters the speed of light the particle beam be going 99.9999...% of the speed of light.
What is the equation that would make both these
statements true?

 

[jtbell]

Your question has to do with relativity, not quantum physics, so I expect one of the moderators will pounce on this thread and move it over to the relativity forum.

Nevertheless, to get an answer to your question, look up the relativistic "velocity addition" formula and apply it to both of your situations. That is, actually calculate the results. You'll see that for the baseballs, the exact relativistic result is sooooo close to the non-relativistic simple addition of velocities, that it's impossible to tell the difference, in practice.

 

[Uno Lee]

Thanks, I think I found what I was looking for in relative velocity. Half the time seems to be spent figuring out the right word, term, phrase or category.