consider a newtons cradle consisting of decreasing masses from the left to the right. if the left most ball is displaced and collides with the others, we end up with a fan like picture whereby the lightest ball, furthest to the right, has the greatest velocity, and the largest ball has the least velocity. The balls go up, reverse their motion, and then all meet in the middle again. the collisions are reversed, and all balls except the largest stop dead. What i am wondering is how come the frequency of oscillation for all the balls is the same, i.e. they meet again in the middle, when their masses are different. Thanks.