This was a fun little project for my son's physics class that we have designed together. You drop a 15 mm ball from variable height and measure it's nozzle velocity with a pretty good precision using two optical gates and Arduino nano. You may measure how far it lands (don't forget that the nozzle is not exactly at ground level! :) ), compare experimental results to theory, compare different materials and try to find the air drag effect etc (styrofoam ball and steel ball will definitely give different results).

I used PLA but material is not critical. The model was designed to need no supports. Begin by printing two pieces of the tubing and try to stick them together. Adjust hole compensation in your slicer so that they fit together nicely, with just the right amount of friction. It is possible that you will have to throw some away before you find the perfect setup and only then print the rest of the apparatus. 34 pcs of the tubing fit in the box but you can make more of course, to achieve even higher velocity.

There is always an IR diode and phototransistor facing each other across the nozzle width in their respective shaft. I have soldered their resistors (220 ohms to the diode, 10k in the collector of the phototransistor) directly to their pins. More should be clear from the attached pdf. All is secured with hot glue.

Arduino Nano is held in an expansion board that can be easily found in robotics e-shops or on Aliexpress etc. The display is a 1602 2-line display with I2C connection, also still cheaply available. 

The display always shows the last measurement.

What you need: