Because you have to have energy to have work.
First of all we need to convert everything into SI units.
Let's start with the initial angular speed,

. Keeping in mind that


we have

And we should also convert the angle covered by the centrifuge:

This is the angle covered by the centrifuge before it stops, so its final angular speed is

.
To solve the problem we can use the equivalent of

of an uniformly accelerated motion but for a rotational motion. It will be

And by substituting the numbers, we can find the value of

, the angular acceleration:
Answer:
Although the core and mantle are about equal in thickness, the core actually forms only 15 percent of the Earth's volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent.
Answer: greater but no freaking clue How to figure it out
Explanation: