Why will an unanchored boat in the ocean move up and down in place but not move towards shore?

Answer 1

To be clear, motion to the left and right also exists in the ocean, and the unanchored boat stays in place because there is no net movement in the direction of the shore. Of course, we'll assume that there is no net wind movement in a set direction to discuss your question.

The movement of the unanchored boat is produced by the transfer of energy through seawater via wave motion. To understand how waves propagate through seawater, think of the ocean as a flexible bulk material.

To understand how the waves propagate through the ocean, think of seawater as a flexible, continuous bulk medium held together in its form by the extensive network of hydrogen bonds between individual water molecules.

Consider a wave of air molecules moving toward the surface of the ocean. The energy of the wave transfers through a cascade of collisions between neighboring air molecules and propagates into the ocean. As the wave propagates through the ocean, oscillations and vibrations begin to deform the seawater in a given direction (say, to the left). The force of the leftward moving wave progressively weakens as the energy of the wave disperses outward until it equals the intermolecular forces between water molecules that pull the wave in the opposite direction (to the right).

The continuously increasing leftward energy transfer begins to form a rightward restoring force that reverses the seawater deformation to the left. Seawater restoration proceeds until the intermolecular forces of water molecules weakens to the point where rightward wave motion becomes dominant again, completing a full cycle of left to right wave motion.

This cycle repeats over and over again for each of the waves propagating in all directions, producing the up-down, left-right periodic oscillations of the unanchored boat. Since the seawater deformation caused by each wave is fully reversed by a restoration force equal in magnitude and opposite in direction, there is no net movement toward the shore and the boat just continues oscillates in place.