Hi everyone, i dint post this i homework section because i know how to solve the problem.(adsbygoogle = window.adsbygoogle || []).push({});

So here is a problem

So i am standing and in my own reference system S i observe two objects moving toward each other at speed 2*10^5 km/s each. Object in it's own reference system S' is observing object moving toward him at speed 2*10^5 km/s and it's own S' is moving with speed u(relative to S) is moving object 2, and object 2 in it's own reference system S'" is also observing object moving toward him at speed 2*10^5 km/s and it's own S'' is moving with speed u(relative to S). Two objects are moving at the same line in different directions i.e. toward each other, with speeds 2*10^5 km/s(relative to the standing observer). What is the speed of one body relative to the other, calculate by classical laws and by relativistic laws of velocity-addition.

So to think of this problem in another way lets say i am in a car with speed u1=2*10^5 km/s moving toward another car with speed u2= 2*10^5 km/s. Speed of that second car relative to me is should be q=u1+u2= 4*10^5km/s. That is by classical laws but by relativistic laws it should be V'=(u1+u2)/(1+ (u^2)/(c^2)). Where u1 is speed of my car relative to standing observer and u2 speed of second car relative to my reference frame. Also i think that it is + between u1 and u2 since we are moving in opposite directions. Then the difference is V-V' and that's that. Problem arises when i asked my teacher a question "In whose reference frame are we calculating these speeds" and she said "In reference frame of standing observer."

This is weird to me because if i am in a car 1 i should detect car 2 traveling at speed u1+u2=

4*10^5km/s and how can that observe standing observer? Should he be able to detect only speeds of car 1 and 2 and only that or also relative speed of car 1 and car 2. My "logic" says to me that it can only detect speeds of car 1 and car 2. Also in my text book they wrote solution with V' and i guess that V' is reserved for S' system that is moving and S that is standing and only observing.

I hope you understand my question i tried to write this as clearly as possible.

Thank you

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Observers and velocity

**Physics Forums | Science Articles, Homework Help, Discussion**