A block hangs from a vertical spring. Downward is chosen as the positive y-axis. At one instant the block is y=+0.020m below equilibrium while moving upward. What is the sign of the spring force on the block at that instant?
(a) Positive, because Fs=ky and the displacement y is positive.
(b) Negative, because the spring force acts opposite the displacement, upward toward equilibrium.
(c) Zero, because 0.020m is very close to equilibrium, so the spring force is essentially negligible.
(d) Positive, because the acceleration that slows the upward motion is downward, and downward is the positive y-direction, so Fs=ma is positive.