Something about the second ball feels a bit off.. Maybe the spacing, as in the gaps between bounces is a bit small. Is it a rigged ball or no? That might also be it. For such a bouncy ball, the squash and stretch would be really exaggerated. I love the balls with the coon tails though, and that sneaky dude looks nice and smooth. We haven't gotten to human-like rigs yet, that's the next project I believe.
It's probably the absence of any squash and stretch the assignment was to do a realistic bouncing ball of some sort; specifically without cartoony and exaggerated squash and stretch. No, it's not a rigged ball, just a generic sphere being moved around and graphs tweaked. Thanks! They were so much fun! When you do get to it, use Richard William's "Animator's Survival Kit" as a crutch. It's extremely helpful!