My copy of "Resnick and Halliday" states that
"$Cv$=$(f/2)R$ which agrees with the experimental evidence for diatomic gases($f$=5), but it is too low for polyatomic gases($f=6$).
Exactly what do they mean by the statement that I have highlighted in bold? What's too low? $Cv$? How come? Doesn't $Cv$ vary directly with $f$?
Please share your insights and help. MUCH thanks in advance :) Regards.