The Russian Version is a translated one hence might contain Errors
The English one Shall be referred if Possible
Thank You .
I wanted to Ask a Doubt I am not A Russian So I have a hard time figuring out how to add the Image of the Question into this place like the Other themes have done.
(Honestly I don’t even know if themes is the Right place to ask Questions, but seemed to me something like that) hope I have not made some trouble yet .
Thank You Miz 
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So what is your question?
I Have Doubt in How to solve the R Part.
What the Authors have done is to use a “Result” which I want to Understand Better.
That “Work Done by Net Friction on the System is Independent of The Reference Frame from which It Is Measured”
And They Hence Write :
W on Belt wrt Ground + W on Block wrt Ground = W on Belt wrt Belt + W on Block wrt Belt .
But Where does this Result Come from ?
I kinda need a proof or something for It .
The friction work is equal to W_{fr.}=-F_{fr.}s_{rel.}, where the s_{rel.} is the distance that was covered by an object with respect/relative to the moving frame.