Fluids, Pressure, and Buoyancy
Pressure in Fluids
Definition of pressure
Pressure is force per unit area: . In fluids at rest, pressure increases with depth according to .
What happens to pressure in a stationary fluid as depth increases?
Deeper points support more fluid above them, so pressure increases with depth.
Correct answer: It increases
Why do sharp objects cut more easily than blunt ones?
Smaller contact area means greater pressure for the same force.
Correct answer: They concentrate force over smaller area
Buoyancy and Archimedes’ Principle
Buoyant force
An object in a fluid experiences an upward buoyant force equal to the weight of the displaced fluid. This is Archimedes’ principle and it explains floating, sinking, and apparent weight.
Floating conditions
| Condition | Outcome |
|---|---|
| Object density < fluid density | Floats |
| Object density > fluid density | Sinks |
| Object density = fluid density | Neutral buoyancy |
Application
Ships float because their overall average density is reduced by the large air-filled volume inside the hull.
The buoyant force on an object equals what?
Archimedes’ principle states that buoyant force equals the weight of displaced fluid.
Correct answer: The weight of displaced fluid
Fluid flow idea
In real systems, moving fluids involve pressure differences, viscosity, and energy losses. These effects matter in pipes, blood flow, and aerodynamic design.
Why can a heavy steel ship float?
The ship displaces enough water that the buoyant force balances its weight before it sinks.
Correct answer: Its average density can be less than water