When a ship enters into a sea from a river?

SGPGI Nursing Officer-2024

General Knowledge

Hard

Appeared in: SGPGI Nursing Officer-2024

Explanation

The core concept is Archimedes' principle, which states that the buoyant force on a floating object equals the weight of the fluid it displaces.
For a ship to float, this buoyant force must be equal to the ship's own weight, which remains constant.
Seawater is denser than river water because it contains dissolved salts.
To generate the same buoyant force in a denser fluid, a smaller volume of that fluid needs to be displaced.
Displacing a smaller volume of water means less of the ship's hull is submerged, causing the ship to rise slightly.

Option B: The ship's material determines its overall weight and whether it can float at all, but the change in its level when moving between two fluids is determined by the difference in the fluids' densities, not the ship's material.
Option C: The ship would sink a little only if it were moving from a denser fluid to a less dense fluid, as it would need to displace a larger volume of the less dense fluid to support its weight.
Option D: The level remains the same only if the density of the fluid does not change. Since seawater and river water have different densities, the buoyant force dynamics change, causing a change in the ship's level.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Archimedes' principle and the effect of fluid density on buoyancy as background academic context rather than a clinical decision trigger.
Understanding fluid density and buoyancy is fundamental in nursing for interpreting specific gravity tests of urine using a urinometer. A higher specific gravity can indicate dehydration, while a lower one can suggest overhydration or kidney issues.
The principle is applied in hydrotherapy, where the buoyancy of water supports a patient's body weight, reducing stress on joints and allowing individuals with arthritis or injuries to perform therapeutic exercises with less pain.
What if? If the ship were moving from the Dead Sea (which has extremely high salt concentration and density) to the ocean, it would sink a little, as it is moving to a relatively less dense fluid.

How to Approach the Question

First, identify the key change in the scenario: the ship is moving from a river (freshwater) to the sea (saltwater).
Recall the primary physical difference between these two bodies of water: their density. Seawater is denser than freshwater due to its salt content.
Apply Archimedes' principle, which states that a floating object displaces a weight of fluid equal to its own weight.
Recognize that the ship's weight is constant. Therefore, it must displace the same weight of water in both the river and the sea.
Deduce that to equal a constant weight, a smaller volume of the denser seawater is needed compared to the larger volume of less dense river water.
Conclude that displacing a smaller volume means the ship will not sink as deeply, and therefore, it will rise slightly.

Concept Tested & Keywords

Concept Tested: Archimedes' principle and the effect of fluid density on buoyancy.
Stem keywords: ship, sea, river
Lead-in keywords: When

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