There are 7 equally spaced spaced balls on the left side of a
horizontal line, each moving to the right at constant speed 'S'. There
are 9 equally spaced balls on the right side of the same line,
each moving to the left at the same constant speed of 'S'. The balls are all of the same size and mass and the spacing between the balls on the left is
the same as the spacing between the balls on the right. When two balls
collide they will reverse directions, but each will maintain the same
How many collisions will occur ?