发表机构
University of Amsterdam（阿姆斯特丹大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
研究发现，钢球在纸堆上的反弹并非随厚度单调递减，而是因层间空气的声学效应出现恢复峰值，使纸堆成为“蹦床”。
AI 中文摘要
一个球在更厚的垫子上弹跳时，反弹应更少。我们表明情况并非总是如此。测量钢球落在由N张标准A4打印纸组成的纸堆上的恢复系数e，我们发现e先下降，然后上升至一个显著的最大值，之后才下降至最小值。最大值出现在声波穿过纸堆的往返时间与冲击接触时间相匹配之处。对于由纸堆本身决定的接触时间，可恢复经典的棒冲击条件，即纸堆与球的质量比约为1。然而，棒冲击仅预测出较弱的恢复，且对周围气体不敏感。对于纸堆，恢复较大，且当纸堆被抽真空时，无法分辨出最大值：是纸张之间滞留的空气，而非纸张本身，使纸堆成为蹦床。
英文摘要
A ball bouncing on a thicker cushion should bounce less. We show that this is not always so . Measuring the coefficient of restitution $e$ of a steel ball dropped on a stack of $N$ sheets of standard A4 printer paper, we find that $e$ first drops, then rises to a pronounced maximum, and only then decreases towards a minimum. The maximum occurs where the acoustic round trip through the stack matches the contact time of the impact. For a contact set by the stack itself, one recovers the classical bar-impact condition of a stack-to-ball mass ratio of order one. Bar impact, however, predicts only a weak recovery, insensitive to the surrounding gas. For paper stacks, the recovery is large and no maximum is resolved when the stack is evacuated: the air trapped between the sheets, not the paper alone, makes the stack a trampoline.
Comments12 pages, 12 figures
