(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

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 M_{sun}, with a radius of 6.96×10^8 m and rotating once every 25 days, becomes a white dwarf with a mass of 0.62 M_{sun}and a rotation period of 131 s.

2. Relevant equations

M_{sun}= 1.99 × 10^{30}kilograms

M_{1}= 1.0 M_{sun}

R_{1}= 6.96 * 10^{8}

T_{1}= 25 days = 2 160 000 seconds

M_{2}= 0.62 M_{sun}

R_{2}= ?

T_{2}= 131 seconds

T = 2 pi r / v

= 2 pi r / w r

= 2 pi / w

.:. w = 2 pi / T

I = (2/5)MR^{2}

3. The attempt at a solution

I_{1}x w_{1}= I_{2}x w_{2}

(2/5)(M_{1}x R_{1}^{2})(2 pi / T_{1}) = (2/5)(M_{2}x R_{2}^{2})(2 pi / T_{2})

(M_{1}x R_{1}^{2})(1 / T_{1}) = (M_{2}x R_{2}^{2})(1 / T_{2})

(M_{1}/ M_{2})(T_{2}/ T_{1})(R_{1}^{2}) = R_{2}^{2}

sqrt((M_{1}/ M_{2})(T_{2}/ T_{1})(R_{1}^{2})) = R_{2}^{2}

R_{2}= sqrt((1.99*10^{30})/[(0.62)(1.99*10^{30})](131/2160000)(6.96*10^{8})^{2}

= 6.88*10^{6}m

Checking the result by plugging into equations shows that I am roughly correct. But Mastering Psychics says I am wrong. Where is the mistake?

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Angular Momentum Conservation Problem

**Physics Forums | Science Articles, Homework Help, Discussion**