Problem 5. The Sun rotates about its own axis once every 25 days. Assume that th
ID: 1289199 • Letter: P
Question
Problem 5. The Sun rotates about its own axis once every 25 days. Assume that the Sun is a solid sphere with uniform density. After about 5 billion years, when its nuclear fuel is exhausted, the Sun will collapse and become a much denser body called a white dwarf of radius 5.80x l0^6m. (Assume for simplicity that the mass of the Sun is unchanged during this process, which is strictly not true.) (a) What is the Sun's angular momentum now? (Again Appendix F is your friend) (b) How will it take for the white dwarf to rotate once about its axis?Explanation / Answer
a) w1 = 2*pi/T = (2*3.14)/(25*24*60*60) = 2.91e-6 s^-1
I1 = (2/5)*M*R^2 = (2/5)*1.98e30*6.95e8*6.95e8 = 3.825558e+47 kg m^2
L1 = I1*w1 = 1.113237378e+42 kg m^2 /s
-----------------
b) I2 = (2/5)*M*r^2 = (2/5)*1.98e30*5.8e6*5.8e6 = 2.664288e+43
L2 = I2*w2
L2 = L1
I2*w2 = 1.113237378e+42
w2 = 4.178e-2
T2 = 2pi / w2 = 6.28/(4.178e-2) = 150.31 s