# physic

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A rotating star expands to become a red giant, increasing its average radius from one million km to 100 million km without changing its mass. What happens to the velocity of a piece of this star located at the average radius?

any help would be great THANKS

• physic -

Angular momentum has to be conserved.

angular momentum sphere= 2/5 mr^2 * w

so if you are increasing r by a factor of 100, then w must decrease by a factor of 10000 to keep angular momentum constant.
So, velocity at the new average radius is wnew*rnew= 1/10000 * 100= 1/100 of before.

check my thinking.

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