The Science Guys
Science Guys > May 2003
If a hole were bored through the Earth and you fell into the hole, at what point would "down" become "up"?
It is believed that an incredibly hot molten material that is in constant motion makes up much of the interior of the Earth. For the sake of our discussion, assume that we were technologically capable of drilling a hole through the Earth. We often say that "we’ll end up in China" if such a hole were bored, but in fact a hole drilled from Jackson, through the Earth’s center, and out on the other side would put you in the southern part of the Indian Ocean, off the west coast of Australia!
Such a hole would be an exciting way to travel. If you stepped into such a hole and allowed yourself to fall, you would accelerate toward the center of the Earth. At the Earth’s center, you would experience a gravitational pull of zero (you would be momentarily weightless), but you would be moving at a high rate of speed. In fact, you would be moving about 17,700 miles per hour!
Neglecting friction the fall from Jackson to the center would have taken only about 21 minutes. During this first 21 minutes, "down" would have been toward your feet, as usual. However, after passing through the Earth’s center and heading toward the Indian Ocean, your body would begin to slow down. Now "down" would be from your feet toward your head! This is because the Earth’s gravity determines "down" for us and gravity is always directed toward the Earth’s center.
After passing through the Earth’s center you would slow for the next 21 minutes since the Earth’s gravity would act opposite your motion and decelerate your body. Finally, as you reach the opening on the other side of the world, your body would come to rest at the surface. You would need to grab something or someone; otherwise you will start falling back toward Jackson. The entire trip would have taken only 42 minutes during which you would have traveled a distance of almost 8000 miles along the Earth’s diameter.
If you did not grab anything on the other side of the world, you would fall back into the hole, and the motion would be repeated. Your body would execute a cyclic or oscillating motion. Each time you passed through the Earth's center the direction of "down" would flip from your point of view.
It’s very interesting that if the hole were filled with air then air drag would slow your motion. With air present, you would lose speed with each cycle back and forth through the Earth. Eventually you would come to a stop, floating at rest at the Earth’s center, trapped! Even Disney World has nothing to compare with such a ride!