When a star collapses it significantly shrinks in size and spins up (similar to a figure skater but at a much greater scale).
Consider a star with a mass of M = 4.15 x 10" kg and an initial radius of R; = 6.5 x 10" km. Find the new rotational
period of this star after it collapses to a final radius of R = 6.5 x 10 km and had an initial period of rotation of T; =
36.7 days. Treat the star before and after the collapse as a solid sphere with uniform mass distribution (which is not true,
of course, but good enough for an estimation).
The new rotational period of the star, If =

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Find the ratio between the final and initial rotational kinetic energies of the star.
The factor by which the kinetic energy of the star increases, KE KE; =
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The increase in the rotational kinetic energy of the star comes from gravity. How much work is done by the gravity force
to collapse the star? Note: to enter a very big or small number you can use 'E' as follows: 1.23 x 10* = 1.234E+45 or
1.23 x 104 = 1.23E4.
The work done by gravity, W =
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