| Difficulty | Secondary School | High school/other |
| ★ | Basic | Easy |
| ★★ | Demanding | Basic |
| ★★★ | Demanding | |
| ★★★★ | Difficult | |
| ★★★★★ | Very difficult |
Determine the acceleration in free fall.
Tip: Measure the time spent by the bubble at certain distances and move the values to the $(t^2,s)$ coordinate system. Fit a straight line to the points. What can you expect from the slope of the line?
Answer: $10\ m/s^2$