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：

左邊15個球起始右行，編號L1,L2,L3, ...L15
右邊16個球起始左行，編號 R16,R15,R14, ...R1

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