The Physics of Submarines
A submarine is a ship that controls its own density. Flood the ballast tanks and it sinks; blow them out with compressed air and it rises; trim it just right and it hangs motionless — neutrally buoyant — in the dark.
Controlled buoyancy
Floating needs average density below water's; sinking needs above. Ballast tanks adjust the submarine's total mass without changing its volume, sliding its density across the neutral-buoyancy point with precision.
Pressure with depth
Pressure grows as P = P₀ + ρgh — about 1 extra atmosphere every 10 m. At 300 m the hull bears ~30 atmospheres (~3 MPa), which is why military hulls are thick high-strength steel or titanium cylinders.
Sonar and sound in water
Sound travels ~1,480 m/s in water — 4× faster than in air. Active sonar pings and times echoes; passive sonar just listens. Temperature layers (thermoclines) bend sound rays, creating hiding zones submariners exploit.
Silent running
Every pump and propeller radiates sound. Modern boats isolate machinery on rafts and shape propellers to delay cavitation — because in the deep, whoever hears first wins.
Try this yourself
- Make a Cartesian diver: a ketchup packet in a sealed water bottle — squeeze to sink it, release to float it.
- Compute the pressure at 100 m depth and compare it to a car tire (~2.5 atm).