What is this happening ?

 

U first told that total m is same for both cases , now u are including mass of the wheels . And deriving answers on this basis !!

 

 It would be fair to stick to ur qn .

 

Again anchitsaini  drop the 1/2 I w^2 term in the energy expression . According to the original qn it is not allowed .

 

Instead apply no slipping condition and see the result .