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    If there are two particles A and B connected by an inextensible string. A being on a rough inclined slope and B suspended from the top end of the slope by means of a pulley so that it hangs freely. now if i am going to consider the kinetic energy gained by the system, then should the work done by friction be calulated using only the mass of A (which is in contact with the rough slope) or the sum of the masses of A and B?

    All the work is done by the falling mass and is used up in a combination of Force X distance for the friction on A and the change in GPE by A. The total work done is the GPE lost by B.

    This is assuming that B is heavy enough. If A is heavy then the work is all done by A moving down the slope and the work is done on B by raising it's (i.e. Bs) GPE and Force X distance for the Friction on A.

    Friction does not do work, it has work done on it. That may be a small point but I think you need to think of work being done by gravity in any case and comes from changing the GPE of the falling body.
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