A submarine adjusts its depth by controlling the amount of w

A submarine adjusts its depth by controlling the amount of w - HiSET Prep

Question

A submarine adjusts its depth by controlling the amount of water in its ballast tanks. Which best explains how increasing water in the ballast tanks causes the submarine to descend?

Answers

It increases the submarine’s total volume, lowering its density.
It decreases the weight of the displaced fluid, reducing buoyancy.
correct

It increases the submarine’s overall mass without changing its volume, raising its density.
It decreases the upward pressure acting on the hull.

Explanation

Correct Answer: C. It increases the submarine’s overall mass without changing its volume, raising its density. A submarine changes buoyancy by adjusting mass within essentially the same hull volume: taking water into ballast tanks increases the vessel’s total mass while the external volume remains essentially constant, so the submarine’s average density (mass/volume) rises. If density becomes greater than that of surrounding seawater, the submarine sinks; if density is reduced (by expelling water and filling tanks with air), it rises. This principle is a practical application of Archimedes’ principle: descending occurs because the increased mass requires a larger displaced fluid mass to provide equal buoyant force — the submarine achieves this by submerging more deeply.

Why the Other Options Are Incorrect:

A. It increases the submarine’s total volume, lowering its density.
Adding water to ballast tanks does not significantly change the external hull volume — the hull encloses the same space. The mass increases, not the external displacement volume. Lowering density would require increasing volume or reducing mass, neither of which occurs by filling ballast with water.
B. It decreases the weight of the displaced fluid, reducing buoyancy.
Taking on water does not reduce the weight of the displaced fluid; rather, to balance the increased weight the submarine displaces more fluid by submerging deeper. The buoyant force depends on displaced fluid weight; it adjusts with immersion depth but the fundamental reason for descent is increased vessel density from added water.
D. It decreases the upward pressure acting on the hull.
Hydrostatic pressure at a given depth depends on ambient fluid; filling ballast does not reduce the external upward pressure. The buoyant response comes from displaced fluid mass, not a change in upward pressure due to internal water addition.