Ships, submarines, and ocean gliders all depend on buoyancy, the upward force that water exerts on objects placed in it. A steel ship floats because its large hollow hull makes its average density less than the density of seawater. A submarine can sink or rise by changing how much water is in its ballast tanks.
An ocean glider uses the same idea in a smaller, more efficient way to travel through the ocean without a propeller.
An ocean glider has an internal buoyancy engine that pumps oil or another fluid in and out of an external bladder. When the bladder expands, the glider displaces more water, its average density decreases, and it rises. When the bladder contracts, the glider displaces less water, its average density increases, and it sinks.
Wings convert this up and down motion into forward motion, so the glider moves in a long zigzag path while collecting data on temperature, salinity, depth, and currents.
Key Facts
- Buoyant force equals the weight of displaced fluid: F_b = rho_fluid g V_displaced.
- An object floats when its average density is less than the fluid density: rho_object < rho_fluid.
- An object sinks when its average density is greater than the fluid density: rho_object > rho_fluid.
- Density is mass divided by volume: rho = m / V.
- A glider rises by increasing displaced volume while keeping nearly the same mass, which lowers its average density.
- A glider dives by decreasing displaced volume, which raises its average density above seawater.
Vocabulary
- Buoyancy
- Buoyancy is the upward force exerted by a fluid on an object in the fluid.
- Displacement
- Displacement is the volume of fluid pushed aside by an object.
- Density
- Density is the amount of mass in a given volume of a substance.
- Ballast
- Ballast is material or water added to a vessel to change its weight, stability, or buoyancy.
- Buoyancy engine
- A buoyancy engine is a device that changes a vehicle's volume or internal fluid distribution to make it sink or rise.
Common Mistakes to Avoid
- Thinking heavy objects always sink is wrong because floating depends on average density, not just weight. A large steel ship can float if it displaces enough water.
- Forgetting that buoyant force depends on displaced fluid volume is wrong because F_b = rho_fluid g V_displaced. Changing the glider's external volume changes the upward force.
- Assuming ocean gliders use propellers is wrong because most gliders move forward by converting vertical motion into horizontal motion with wings. Their pump mainly changes buoyancy, not thrust.
- Confusing mass and density is wrong because a glider can keep nearly the same mass while changing its volume. That volume change changes its average density and determines whether it sinks or rises.
Practice Questions
- 1 A glider displaces 0.045 m^3 of seawater with density 1025 kg/m^3. What buoyant force acts on it? Use g = 9.8 m/s^2.
- 2 A small underwater vehicle has a mass of 52 kg and a volume of 0.050 m^3. Its average density is what, and will it sink or rise in seawater of density 1025 kg/m^3?
- 3 Explain how an ocean glider can move forward without a propeller. Include the roles of the pump, changing density, and the wings.