Hook's law I

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Horizontal restoring force

The force produced by a spring is said to be a "restoring" force. The force is always trying to restore the spring to its equilibrium state. For the horizontal configuration shown in the figure below, the equilibrium position is the position in the center of the block when the spring is relaxed (not compressed or elongated).

When the spring is stretched by a distance x to the right, the spring pulls to the left. This restoring force is given by Hooke's Law as F=-kx. The movie shows what happens if the spring is stretched and released. The verticle arrow in the movie points to the equilibrium position.

1. Is there a difference in the spring force when the spring is compressed instead of stretched?
2. Why does the cart oscillate about the equilibrium position in the movie?
3. How would the motion of the cart differ if there was extra mass on top of the cart?