If you were to make a deep well in the earth to the other side of our planet, and then crawl into it with a ladder, you would have to be ‘turned around’ at some point. And what happens if you throw a ball into it?
Answer
Earth’s gravity is greatest at the Earth’s surface. As you descend, the gravity gradually diminishes, because part of the earth is above you and not below you.
As you correctly suspect, the gravity in the center is effectively zero, because the mass is symmetrical around you. That’s the point where you “turn around” so to speak. This reversal is not sudden.
If you throw a ball in a tunnel through the earth, which is also empty at the same time (so as not to have friction), what happens then?
-the ball falls faster and faster. The ball goes at its fastest in the center of the earth. But it no longer accelerates because the gravitational force F is effectively zero (F=m*a, so a=0).
-beyond the center the acceleration will reverse, ie the ball will slow down. When the tunnel is perfectly void – no friction – the ball will come to a stop just on the surface on the other side.
-for information: the ball reaches the center after 21 minutes and then “falls” at a maximum speed of approximately 20000 km/h. It continues flying and reaches the other side after 42 minutes.
Answered by
Engineer Bart Dierickx
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