Grades 3–5 · Machines
How can a submarine sink and rise again?
A submarine can travel below the sea instead of staying at the surface. Water pushes upward on it, just as water pushes upward on a floating boat. That upward push is called buoyancy. The submarine's weight pulls downward. Changing the balance between those pushes helps it sink or rise.
In the diagramBallast water changes a submarine’s total weightPressure hullMain ballast tank shown belowUpward push: 10Weight: 10Equal pushes: neutral buoyancyFully submerged; force numbers use invented units.
A submarine has ballast tanks that can contain air or seawater. Letting water into these tanks makes the submarine and the water it carries heavier. In our fully underwater example, its outside size stays the same. It still takes the place of the same amount of surrounding water, so the upward push stays the same while the downward weight changes.
When the upward and downward pushes are equal, the submarine has neutral buoyancy: neither push is larger. Adding more ballast water gives it a tendency to sink. For rising, compressed air can push water out of the main ballast tanks. Losing that water reduces the weight. Separate adjustments help balance the front and back, and moving control surfaces help a traveling submarine change direction and depth. Ballast gives this large underwater machine a way to change how heavily it weighs against the water's upward push.
About this explanation
Starting knowledge: Compare quantities and understand that water has weight.
Sources
- Naval Sea Systems Command — Carderock Engineer Supports Stability Tests for Deep Submergence Vehicle
- Naval History and Heritage Command — Nomenclature of Naval Vessels: Ballast Tanks
- United States Navy, preserved by San Francisco Maritime National Park Association — The Fleet Type Submarine: Chapter 4, Tanks