Students applied the energy transfer model to devise an experiment to determine the spring constant of the spring in the projectile launchers. Every group eventually ended up launching the projectile straight up and equating the spring potential energy to the maximum gravitational potential energy. Groups used different measurement techniques, but capturing the maximum height with cell phones was popular. Some groups used both the yellow projectile and the steel projectile. Surprisingly, they calculated significantly different spring constants for the different projectiles. Clearly, some of our assumptions are not accurate. Fodder for a future discussion!