12. assignment (simple harmonic motion)

A 500 g block on a frictionless surface is attached to a rather limp spring of constant k = 8.7 N/m. A second block rests on the first, and the whole system executes simple harmonic motion with a period of 3.5 s. When the amplitude of the motion is increased to 68 cm, the upper block just begins to slip. What is the coefficient of static friction between the blocks?
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