Problem 5. The Sun rotates about its own axis once every 25 days. Assume that th

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

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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

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