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by Carl Sagan

Joss was watching her closely. He didn’t ask her whether she was sure, he neglected to warn her about falling forward, he offered no cautions about giving the bob a horizontal component of velocity as she let go. Behind her was a good meter or meter and a half of level floor, before it started sloping upward to become a circumferential wall. If she kept her wits about her, she said to herself, this was a lead-pipe cinch. She let go. The bob fell away from her. The period of a simple pendulum, she thought a little giddily, is 2 π, square root L over g, where L is the length of the pendulum and g is the acceleration due to gravity. Because of friction in the bearing, the pendulum can never swing back farther than its original position. All I have to do is not sway forward, she reminded herself. Near the opposite railing, the bob slowed and came to a dead stop. Reversing its trajectory, it was suddenly moving much faster than she had expected. As it careened toward her, it seemed to grow alarmingly in size. It was enormous and almost upon her. She gasped. “I flinched,” Ellie said in disappointment as the bob fell away from her.