The potential energy of a spring is: U = ½kx2
A block connected to a horizontal spring sits on a frictionless table. The
block is moved to compress the spring and is released from rest. What happens
to the energy initially stored in the spring? What is the maximum speed of
To solve, use D0EL (energy conservation).
Step 1: Define/draw system and coordinates. (see transparency)
Initial position i and final position f (equilibrium position).
Step 2: Define potential energy zero.
Define the zero of the potential energy at the equilibrium point.
Step 3: Energy conservation statement.
Ui + Ki = Uf + Kf
Initial kinetic energy Ki = 0
because the particle is released from rest.
Final potential energy Uf = 0 by definition.
Conclusion: Ui = Kf or kxi2 = ½ mvf2 →   vf = 2 xi (k/m)½
The total energy remains constant and merely transforms back and forth between potential and kinetic.