How can i track different objects in consecutive images?

I have 2 consecutive images of some balls falling. The only thing that changes between one image and the other one is the position of the balls. What I want to do is to detect the balls in the images and messure their velocity. How can I write the code to tell which ball corresponds to the same ball in the next image so I could calculate their velocities? I just need a few tips because I'm new to image processing.