The figure shows a conical compression helical coil spring where R1 and R2 are the initial and

Question:

The figure shows a conical compression helical coil spring where R1 and R2 are the initial and final coil radii, respectively, d is the diameter of the wire, and Na is the total number of active coils. The wire cross section primarily transmits a torsional moment, which changes with the coil radius. Let the coil radius be given by