Two large ships travel on close, parallel courses in the same direction and tend to move towards each other. Which physical principle best explains this effect?

Introduction / Context:
When two large ships move in the same direction on close, parallel paths, observers sometimes notice that the ships seem to be drawn toward each other. This can be dangerous if they get too close. Understanding the physics behind this phenomenon helps in safe navigation and illustrates an important principle of fluid dynamics. This question asks which physical principle gives the correct qualitative explanation.

Given Data / Assumptions:

• Two large ships are moving in the same direction along nearly parallel paths.
• The distance between their hulls is relatively small compared to open water around them.
• Water flows between and around the ships as they move.
• We neglect small effects such as magnetism and treat the main fluid as water.

Concept / Approach:
Bernoulli principle states that in an ideal flowing fluid, an increase in the speed of the fluid accompanies a decrease in the fluid pressure, provided elevation changes are small. Between two closely spaced moving ships, water must rush through a narrower gap. Because the same volume of water has to pass through a smaller cross section, its speed in the gap increases. According to Bernoulli principle, this higher speed corresponds to lower pressure between the ships compared to the outer sides. The resulting pressure difference produces a net force that pushes the ships toward each other.

Step-by-Step Solution:
Step 1: Picture two large hulls moving side by side with a narrow channel of water between them. Step 2: As the ships move forward, water is forced to flow through this narrow gap. Step 3: The restriction in cross sectional area between the hulls causes water speed there to increase. Step 4: Apply Bernoulli principle: higher fluid speed implies lower fluid pressure in the region of faster flow. Step 5: On the outer sides of the ships, water speed is lower and pressure is relatively higher. Step 6: The higher external pressure and lower internal pressure create a net sideways force that pushes the ships toward each other.

Verification / Alternative check:
This effect is similar to the attraction felt when two sheets of paper hang loosely and air is blown between them. The faster air between the sheets lowers the pressure there, and the sheets move closer together. Many fluid dynamics demonstrations show the same principle with suspended balls or streamlines around obstacles. Navigation guidelines warn ship captains about the risk of bank suction and ship interaction in narrow channels, which are explained using Bernoulli principle and related fluid dynamic concepts.

Why Other Options Are Wrong:
Magnetic properties of the ship material: Most ships are made of steel, which can be magnetic, but the distances and weak fields involved cannot produce significant mutual attraction compared with hydrodynamic forces.

Newton third law of motion directly: While all forces obey Newton third law, this law explains action reaction pairs, not the specific origin of this sideways attraction; it does not directly account for the pressure differences in the water.

Earth gravitational attraction: Although there is a tiny gravitational attraction between any two masses, the effect is negligible compared with hydrodynamic forces from the moving water.

Common Pitfalls:
A common misunderstanding is to attribute any attraction between masses solely to gravity or magnetism, ignoring fluid effects. Others may think of Newton third law as a catch all explanation. To avoid this, always ask what medium is involved and how it behaves. Here, the key is the water flow pattern and the pressure changes it causes. Bernoulli principle gives the simplest and most direct explanation of the ships moving toward each other.

Final Answer:
The tendency of the two ships to move towards each other is best explained by Bernoulli principle on fluid pressure and speed.