英语时差：硬币可以叠多高？（上）

Here’s a question of burning scientific importance. How high can you stack pennies before they fall over? The answer? It depends. In a perfect universe, with perfectly flat pennies that align exactly on top of each other, the stack could go as high as you wish. In the real universe, however, you’ll be lucky if you can get them above about one foot. Why Do They Always Fall Over? It’s because of the center of gravity. The center of gravity is the point where all the individual weights average out. The entire stack acts as if its weight were located only at the center of gravity! Now, since the center of gravity is the average of all those pennies’ weights, it will always be in the middle of the stack. That means the higher you make the stack, the higher the center of gravity goes.