The Bumper and Launcher Kit, order code BLK, $89 includes two hoop springs, one stiffer than the other.
It is easy to measure the spring constants by placing a Dual-Range Force Sensor, order code DFS-BTA, $109 in the Bumper Launcher Kit bracket, and a spring on the sensor. Attach the BLK to a Vernier Dynamics Track: Combination 1.2 m Track/Optics Bench, order code TRACK, $129.
Attach either a Motion Detector, order code MD-BTD, $89 or the Vernier Motion Encoder System, order code VDS-EC to measure the position of a cart. While collecting data, press the cart against the spring, recording force and position data.
Graph Force vs Position, and select only the portion of the data corresponding to the cart in contact with the spring. This section will be nearly linear.
For the light spring, the slope is about 35 N/m. For the heavy spring, it is about 65 N/m. Spring constants are approximate, as the springs are not designed to be highly consistent.