A star with the same mass and diameter as the sun rotates about a central axis w

Question

A star with the same mass and diameter as the sun rotates about a central axis with aperiod of about 26.2days. Suppose that the sun runs out of nuclear fuel andcollapses to form a white dwarf star with a diameter equal to that of the earth. Assumethe star acts like a solid sphere and that there is no loss of mass in the process.

Radius of the Sun = 9.96x10^8

Radius of the Earth = 6.37x10^6

(a) What would be the new rotation period (s) of the star?

(b) What is the ratio of nal to initial kinetic energies (Kf=Ki)?

I know how to calculate (a). The answer for (a) is 189.6. I need help figuring out (b)

Explanation / Answer

Moment of Inertia of A Sphere = 2mr2/5A.) Initial Moment Of Inertia - Ii = 2m(6.96*108)2/5 Final Moment of Inertia = If = 2m(6.37*108)2/5 Initial Angular Velocity = 2?/(28.8*24*3600) We know Momentum will be conserved 2m(6.37*106)2/5 * 2?/Tf = 2m(6.96*108)2/5 * 2?/(28.8**24*3600) => Tf/28.8*24*3600 = (6.37*106)2/(6.96*108)2 => Tf = 208.433 s = 3 min 28.43 sB.) Final Angular Velocity = 2?/208.433 Initial K.E. = (1/2)Ii*?i2 Final K.E. =

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