Mail Time: What's your demo using kinematics with a spring?!?

Logan writes in:

Hi Greg,

What's the "energy of a spring demo with kinematics/energy" [on your class-by-class planner]?

Logan, 

I think you're talking about the demo where I hang a 1 kg object from a vertical spring of known spring constant, displace the object 5 cm, and predict  how fast the object is going at equilibrium.  I do this using Newton's second law and kinematics.  Yes, really.  

Then I do the experiment (a motion detector placed underneath makes a velocity-time graphs, and we look at the maximum vertical axis value).  Off by 40%.  What?  Physics didn't work?

But energy gives an accurate prediction.

Point is for students to recognize when kinematics are applicable, and not applicable.  And to practice energy bar charts, o'course.