The RCX can record harmonic motion of the spring.
Harmonic motion means that the spring is bouncing up and down.
Place a plate with holes, firmly, to the back of the RCX and hook the spring through one of the holes. Hang the spring with the RCX attached, very carefully, and place a light directly below but shining upward toward the light sensor. Reflected light percentage is being data logged on the RCX and will be graphed after being uploaded to the laptop computer. Now take the RCX over to the computer and place it in front of the IR Tower. Click on upload and the data will be transferred from the RCX to the computer where the program will graph the resulting data.