Your art style is nice, particularly the landscape in the aeroplane one. One thing I recommend you brush up on are your basic principles of animation, for example, your ball seems to magically stop in mid-air as it breaks the glass and then continue downwards at a constant speed. Work on getting those frames closer together as the ball reaches the peak of its trajectory. Same issue with the car as it hits the box - it looks to decelerate, stop, then decelerate again.
Remember to maintain the volumes of your drawings! I noticed that as the front of the car stopped the rear kept on going for a couple of frames. If you perhaps bent the car a little as it hit the box and made the rear bounce back out again the force of the impact would have read much more nicely.
However, for a beginner's piece, this stuff is pretty cool. I suggest you research the terms mentioned in the following link:
http://www.animationtoolworks.com/library/article9.html ;)
