Free fall: measuring g
What students will learn
- Measure the acceleration of a falling object
- Compare a measured value against an accepted one and talk about error honestly
What you'll need
- Photogate
- A picket fence (a clear strip with evenly spaced dark bands) or a small ruler to drop
- Something soft to catch the drop
Setup
Clamp the photogate so a dropped picket fence falls straight through the beam. Put a cushion below. Each dark band interrupts the beam, giving a burst of precisely spaced times.
What students do
- Hold the picket fence just above the gate and drop it cleanly. No throwing.
- Read the velocity at each band crossing and plot velocity against time.
- Fit a line. The slope is your g.
- Repeat five times and average.
- Compare with 9.8 m/s² and list the reasons for the gap, out loud, as a class.
What to expect
Values typically land between 9.5 and 9.9. A slightly tilted drop reads low, which becomes a genuinely useful conversation about systematic error rather than "we did it wrong."
Teaching notes
This one is exam-relevant: measuring g is a standard AP and IB practical, and the error discussion is where the marks live. Keep every group's five values on the board; the spread across the class is the best error bar you can show.