# Conservation of Angluar Momentun question

## Homework Statement

When a star like our Sun no longer has any hydrogen or helium "fuel" for thermonuclear reactions in its core, it can collapse and become a white dwarf star. Often the star will "blow off" its outer layers and lose some mass before it collapses into the rapidly spinning, dense white dwarf. Suppose a star with mass 1.0 Msun , with a radius of 6.96×108 and rotating once every 25 days, becomes a white dwarf with a mass of 0.60 Msun and a rotation period of 131s.

What is the radius of this white dwarf? (You may assume the progenitor star and the white dwarf star are both spherical.)

## Homework Equations

I'm assuming that there are no external torques so it's angular momentum is conserved.

So,
I1$$\omega$$1=I2$$\omega$$2

rotational inertia of a solid sphere I=(2/5)mr2

$$\omega$$1=2.91*10-6 rad/s (from 1 rotation every 25 days)
$$\omega$$2=0.04796 rad/s (from rotation period of 131s)

## The Attempt at a Solution

After rearranging the angular momentum equation from above:

r22=(m1r12$$\omega$$1)/(m2$$\omega$$2)

r22=[(1.0 Msun)(6.96*108)2(2.91*10-6)]/[(0.6 Msun)(0.04796)]

so r2=6.99*106m

The problem I'm having is that when I try to enter in this answer into masteringphysics it tells me I'm wrong. I've checked over my work a few times but I can't see my error anywhere.

EDIT:
Looks like the same problem was posted here: https://www.physicsforums.com/showthread.php?t=255031

I guess I'll just have to confirm with the professor tomorrow.

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