Falling Drops


The falling drops demonstration is a way for students to observe the motion of an object under a constant acceleration (in this case, from gravity). By timing the flashing of a strobe light to sync with a water pump, fluorescing falling water drops appear to freeze midair. Students can measure the distances between drops, and confirm the predictions of the basic kinematic equations.


To observe the position of objects accelerating due to gravity, as is predicted by the kinematics equation:

x = at2


  • Glass bowl
  • Two plastic hoses
  • Pump
  • Veriac (transformer)
  • Tower, with funnel attachment
  • Strobe light
  • Scope (sets frequency of pump)
  • Fluorescent Dye


All parts are located in room 202, the lecture prep demonstration room. The glass bowl, containing one hose and the pump, are on shelf R1. The tower, with the second hose and a funnel attachment, is on the floor in front of shelf J1, just next to where the bowl is stored. The strobe light and the pump are on E1, extra strobe light bulbs can be found on K4. The fluorescent dye, labeled “fluorescein disodium salt” is on shelf F3.

Take the pump out of the glass bowl and plug its cord into the back of the Veriac. Using an extension cord, plug the Veriac into a wall outlet. One end of the hose from the glass bowl will stay in the glass bowl. The other end attaches onto the “input” end of the pump. The other hose (setup on the tower) should connect to the “output” end of the hose. Place the glass bowl beneath the funnel. Using the arm attached to the strobe light, set the strobe light up on the top of the tower, so that the light faces downward into the bowl.


Fill the bowl with water until the hose is submerged by at least an inch of water. When you turn on the pump, water is pulled up the hoses and drips down through the funnel into the bowl. Make sure to get all air out of the lower hose, usually lifting the hose and moving it around will work. For the upper hose, pinching where there are air bubbles usually does the trick. Plug the scope into a power outlet, and then plug the strobe lamp into the scope.

How it works

Turn on the pump. The pump runs at 60 Hz, which means approximately 60 drops fall out of the nozzle a second. Adjust the veriac until a clear drop pattern is established – this usually occurs around 120 volts. Turn on the strobe light and adjust the frequency until the drops appear to freeze in midair. This should occur when the strobe light frequency and the pump frequency are synchronized – so when the strobe light is flashing 3600 times a minute. If you can’t perfectly adjust the strobe light frequency on the scope, turn the scope’s “line sync” switch on – now the scope will adjust its frequency in increments of 100. As you adjust the strobe frequency with the scope, the drops will appear to rise or fall if the strobe light is not synchronized with the pump. They’ll appear to travel up if the strobe frequency is too low, and down if the strobe frequency is too high.

Now add a small amount of the dye into the water (about what you can scoop out on the tip of a knife blade) and turn off the lights. This dye fluoresces under UV light, and creates the effect of the drops freezing in midair under the strobe light. Students can get a meter stick to measure the distances between drops.

Falling drops setup
Falling drops setup, just before strobe light and pump are synchronized

Clean up

The water can be dumped down the drain. Be careful unhooking hoses, it’s easy to accidentally spill water. It’s helpful to let the upper hose drain for an afternoon before putting it away.