19. In what sense is the Space Shuttle falling toward the earth?
When the space shuttle circles the earth, it's experiencing only one force: the force of gravity. As a result, it's perpetually accelerating toward the earth's center. If it weren't moving initially, it would begin to descend faster and faster until...splat. But it is moving sideways initially at an enormous speed. While it accelerates downward, that acceleration merely deflects its sideways velocity slightly downward. Instead of heading off into space, it heads a little downward. But it never hits the earth's surface. Instead, it arcs past the horizon and keeps accelerating toward the center of the earth. In short, it orbits the earth—constantly accelerating toward the earth but never getting there.