Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Taking and a uniform Earth:
At a planet's surface . At a height : . At a depth in a uniform planet: . A spinning planet's equator needs of it for the circular motion.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) , so .
Step 2. (b) .
Step 3. (c) , so .
Answer: (a) . (b) . (c) .
Check. For (c), the shortcut gives . Nearly the same: is small next to .
Watch out. In (a), the height is , not .
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