Neutral buoyancy is the condition in which an object in water neither rises nor sinks. For ships and submarines, it matters because floating, diving, and hovering all depend on the balance between upward buoyant force and downward weight. A submarine at neutral buoyancy can stay at a chosen depth with little vertical motion.
This makes it useful for navigation, research, and safe underwater operation.
A submarine changes its buoyancy by controlling how much water and air are in its ballast tanks. Taking in seawater increases its mass and weight, helping it sink, while pushing water out with compressed air decreases its mass and helps it rise. At neutral buoyancy, the buoyant force equals the submarine’s weight, so the net vertical force is zero.
The submarine may still move forward with its propeller, but it does not accelerate up or down.
Understanding Ships and Submarines: Neutral Buoyancy
A submarine does not become neutral all at once. Its crew makes small adjustments because water pressure, fuel use, cargo movement, and changing water conditions can alter its balance. As the submarine travels, it burns fuel or uses stored supplies.
This slightly reduces its mass. Water may enter small spaces, and equipment can be moved inside.
Each change affects the depth the submarine tends to hold. Crews watch depth instruments closely and use carefully measured amounts of water in special control tanks to correct the balance.
Pressure becomes much greater at depth because of the water above. This pressure can compress air spaces inside the submarine, including parts of its tanks. When air is compressed, it takes up less volume.
The submarine can then displace a little less water, reducing the upward push from the sea. A vessel that was balanced near the surface may therefore become slightly heavier relative to the water it displaces deeper down.
This is one reason depth control requires attention throughout a dive. The strong hull protects the crew, but changes in air volume still matter for buoyancy.
Neutral buoyancy does not mean that a submarine is fixed in one place. Currents can carry it sideways, and its propeller can move it forward. Its diving planes act like underwater wings.
When the submarine moves forward, these angled surfaces can create an upward or downward force. This helps it change depth smoothly without making a large ballast adjustment. A pilot must coordinate speed, plane angle, and tank control.
If the submarine loses speed, the diving planes become less effective. Then the balance of water and air in the tanks becomes especially important.
People meet the same idea in swimming and diving. A scuba diver uses a buoyancy control device to add or release air. A full breath can make the diver rise slightly because the lungs occupy more space and displace more water.
Breathing out has the opposite effect. Divers learn to make slow changes because rapid rising can be dangerous when pressure changes. Fish use a gas filled swim bladder for similar control, though not every fish has one.
When learning this topic, separate mass from volume in your thinking. Adding water mainly changes mass.
Compressing an air space mainly changes volume. Both can change the overall density of the object, which determines whether its depth stays steady.
Key Facts
- Neutral buoyancy occurs when F_b = W.
- Buoyant force is given by F_b = ρ_fluid g V_displaced.
- Weight is given by W = mg.
- If F_b > W, the object rises; if F_b < W, the object sinks.
- A submarine controls buoyancy by changing mass with ballast tanks, not by changing the density of seawater.
- At neutral buoyancy, net vertical force is zero: ΣF_y = F_b - W = 0.
Vocabulary
- Buoyant force
- The upward force a fluid exerts on an object that is partly or fully submerged.
- Weight
- The downward gravitational force on an object, equal to its mass times gravitational acceleration.
- Neutral buoyancy
- The state in which an object’s buoyant force equals its weight, so it does not rise or sink.
- Ballast tank
- A tank in a submarine that can be filled with water or air to change the submarine’s mass and buoyancy.
- Displaced water
- The volume of water pushed aside by an object submerged or floating in it.
Common Mistakes to Avoid
- Thinking neutral buoyancy means no forces act on the submarine is wrong because both buoyant force and weight still act, but they are equal and opposite.
- Using the submarine’s total volume incorrectly is wrong if part of an object is above water, because buoyant force depends on the volume of fluid displaced.
- Forgetting that ballast water changes the submarine’s mass is wrong because adding water increases weight and can make the submarine sink.
- Assuming a motionless submarine has no engine activity is wrong because it may use propulsion to move horizontally while maintaining zero vertical acceleration.
Practice Questions
- 1 A submarine displaces 2.0 x 10^6 kg of seawater. What buoyant force acts on it if g = 9.8 m/s^2?
- 2 A small underwater vehicle has a mass of 800 kg and displaces 0.78 m^3 of seawater with density 1025 kg/m^3. Using g = 9.8 m/s^2, determine whether it rises, sinks, or is nearly neutrally buoyant.
- 3 A submarine at neutral buoyancy takes a small amount of seawater into its ballast tanks without changing its volume. Explain what happens to its vertical motion and why.