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

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# Observers and velocity

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