Instead of a physics test at our school in 5th grade, we do this: Battlebots.
How to win:
1. Knock a piece off the enemy battlebot
2. Push the battlebot out of the arena
3. Flip the battlebot over
Building materials: 1-3 5th graders in a group, along with one lego dacta kit (Dacta = father of mindstorms).
Duration : 0:8:27