The roller coaster chugs to the top of the hill. Then you speed downward. As your speed increases, everyone screams! As you reach the bottom and the track levels off, the coaster slows. Then it starts climbing the next hill. You grip the safety bar, knowing it's about to start all over again.

When a roller coaster goes uphill, its speed slows down. The coaster almost comes to a stop at the top of the hill. Then the roller coaster starts downhill and accelerates rapidly.