this is not homework help.I want to know.

can any one suggest why for a dipole m placed in a magnetic field B
F=grad(m.B)
and N=mxB?

Force is always the gradient of a potential energy, provided the force is conservative.

The torque can be derived from first principles.

Both of these things are probably easier to see in the case of electric dipoles rather than magnetic dipoles.

OK,what you told is right,but the actual rigorous derivation appears in Jackson.