Experiment
1) Set the frequency of the produced sound at 50Hz.
2) Turn on the water supply, while the flash frequency of the LEDs is being changed. When the frequency of the LEDs is equal to the frequency sound (fLED = fsound = 50Hz), then every time a drop comes out of the tube, the LEDs light up. So, every time the LEDs light up, take the same snapshot and the drops will appear motionless. When the flash frequency of the LEDs is slightly lower than the frequency of the sound, then the drops appear to be moving downwards, while when the flash frequency of the LEDs is slightly higher than the frequency of the sound, then the drops seem to be moving upwards.
3) Study the free fall of the water droplet, by choosing fLED = fsound = 50Hz, so as the droplets to appear stationary.
By measuring the distances between the droplets and by also knowing the frequency of the outflow of the droplets, we can calculate the gravitational acceleration.
Starting from the equation of the position of the droplet in the y-axis (y=1/2gt2), we end up to the following equation: Δy΄-Δy=gT2,
where Δy΄ and Δy are the displacements of the droplets as shown in the adjacent figure, g is the gravitational acceleration and T is the period of the outflow of the droplets.
The students can measure the positions of the “stationary” droplets by just using a ruler, as shown in the adjacent photo. They actually did it and they calculated the gravitational acceleration with very good accuracy.