Fluids

The One Thing

Fluids exert pressure, while immersed objects experience an upward buoyant force equal to the weight of the fluid they displace. Whether an object sinks, floats, or remains suspended depends on the balance between its weight and buoyant force, closely related to its average density compared with that of the fluid.

Reactions, Processes and Experiments

Key Terms

• Fluid: A substance that can flow and take the shape of its container; liquids and gases are fluids.
• Pressure: The normal force acting per unit area of a surface.
• Atmospheric pressure: The pressure exerted by the weight of the air surrounding Earth.
• Liquid pressure: Pressure exerted by a liquid because of its weight; it acts in all directions.
• Density: Mass contained in unit volume of a substance, given by density = mass divided by volume.
• Pascal: The SI unit of pressure; 1 pascal equals 1 newton per square metre.
• Buoyant force: The upward force exerted by a fluid on an object partly or completely immersed in it.
• Upthrust: Another name for buoyant force.
• Archimedes' principle: A body wholly or partially immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by the body.
• Flotation: The condition in which an object remains on the surface of a liquid because the buoyant force balances its weight.
• Relative density: The ratio of the density of a substance to the density of water at the same temperature; it has no unit.
• Thrust: The force acting normally, or perpendicularly, on a surface.
• Manometer: An instrument that measures the pressure of a gas or liquid, usually by comparing it with the pressure of a liquid column.
• Barometer: An instrument used to measure atmospheric pressure.

Easily Confused

• Thrust and pressure: Thrust is the force acting normally on a surface, whereas pressure is thrust per unit area.
• Buoyant force and upthrust: These are two names for the same upward force exerted by a fluid.
• Sinking and suspension: An object sinks when its weight is greater than the maximum buoyant force; it remains suspended when its weight equals the buoyant force.
• Floating and suspension: A floating object remains at the liquid surface and displaces liquid whose weight equals its own; a suspended object remains within the fluid and has average density equal to the fluid density.
• Liquid pressure and atmospheric pressure: Liquid pressure results from the weight of the liquid column, whereas atmospheric pressure results from the weight of the surrounding air.
• Manometer and barometer: A manometer measures gas or liquid pressure by comparison with a liquid column; a barometer measures atmospheric pressure.
• Density and relative density: Density is mass per unit volume and has units; relative density is the ratio of a substance’s density to the density of water and has no unit.
• Weight and buoyant force: Weight acts downward, whereas buoyant force acts upward and equals the weight of displaced fluid.

What Gets Asked

• Calculations involving pressure: Questions may require use of Pressure = thrust / area, or P = F / A. Marks are lost by confusing thrust with pressure or by failing to use area.
• Liquid-pressure calculations: Questions may use P = h rho g or P_total = P_atmospheric + h rho g. Marks are lost by omitting atmospheric pressure when total pressure in an open liquid is required.
• Comparisons of liquid pressure: Questions may ask how depth, liquid density, or container shape affects pressure. Marks are lost by claiming that pressure depends on the container’s shape or total amount of liquid.
• Applications of Archimedes’ principle: Questions may require use of Buoyant force = weight of fluid displaced, W = V rho g, or F_b = V rho g. Marks are lost by using the object’s weight instead of the displaced fluid’s weight.
• Predicting whether an object sinks, floats, rises, or remains suspended: Questions test comparison of weight with buoyant force or comparison of average density with fluid density. Marks are lost by reversing the conditions for sinking and floating.
• Relative-density and flotation applications: Questions may involve hydrometers, ships, submarines, balloons, or airships. Marks are lost by overlooking that a ship’s hollow shape lowers its average density, or that submarines change depth by taking in or expelling water from ballast tanks.