- #1

Fendergutt

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## Homework Statement

The Sun rotates about its own axis once every 26.0 days. Its mass is m_sun = 2.0E30 kg and radius is r_sun = 7.0E8 m. Assume that the Sun is a solid sphere with uniform density.

Astrophysicists predict that the Sun will collapse into a white dwarf in some billions of years. Its density will be high, and its radius 5.8E6 m. Assume the mass of the Sun to be unaltered (a good approximation for our calculation).

(i) What is the Sun's angular momentum now?

(ii) What is the Sun's angular momentum as a white dwarf?

(iii) How long will it take for the white dwarf Sun to rotate about its own axis?

## Homework Equations

I = (2/5)*m*r^2

**L**= I

**ω**= Iω

**k**(k being the z-axis unit vector)

## The Attempt at a Solution

(i)

ω_sun = (2pi)/(26 days) = 2.8E-6 rad/s

**L**= I_sun*ω_sun*

**k**= (2/5)*m_sun*r_sun²*ω_sun*

**k**= (2/5)*2.0E30 kg * (7.0E8 m)² * ω_sun *

**k**= 1.1E42 kgm²/s

(ii)

I don't know the new spin angular velocity of the white dwarf, do I? If I leave ω_white = ω_sun the formula for

**L**above with inserted new value for radius r_white gives appr. 7.5E37 kgm²/s

**k**, ie. the angular momentum is lowered. Can this be correct?

(iii)

I don't know what to do here. I want to find the new period T, but I suspect there is something odd in (ii). Is there a relation I have overseen?

Thanks for any help.