15 balls with the same spacing and speed move along a straight line, another 16 balls with the same spacing and speed move along the same straight line but at opposite direction. The balls are all the same size and mass and the spacing between the balls at both directions is the same. When two balls collide, they will reverse their moving directions, but maintain the same speed (frictionless moving and collision). How many collisions will occur? 2th question: how many collisions for each ball？

解：

左边15个球起始右行，编号L1,L2,L3, ...L15
右边16个球起始左行，编号 R16,R15,R14, ...R1

Lk 撞击2(k-1)+1次
Rk 撞击2(k-1)+1次， k=1,2,...15
R16撞击30次。
共480次