Question

as work=f.d=joule & torqe=r*f=N m. why it is not joule?

Answer 1

Joule and Newton.meter are indeed dimensionally equivalent, but torque is not the same concept as work - so the practice is to define them differently in terms of SI units to avoid mistakes.

This might help you: http://en.wikipedia.org/wiki/Torque#Units

Answer 2

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Answer 3

On top of that, Energy is a dot product \[W=\bar F·\bar d\]while Torque is a cross product:\[\bar M=\bar F \times \bar d\] they represent different physical concepts then cannot use same units though dimensions are the same

Answer 4

***I meant work is a dot product