Fluid pressure is the force a fluid exerts on each unit of area it touches. It matters in swimming, diving, plumbing, dams, weather, and the design of submarines and storage tanks. Unlike a solid block pushing in one direction, a fluid at rest presses on surfaces in every direction.
The basic definition is P = F / A, with pressure measured in pascals, where 1 Pa = 1 N/m².
In a liquid of constant density, pressure increases with depth because deeper points support the weight of more fluid above them. This increase is described by P = P0 + ρgh, where P0 is the pressure at the surface, ρ is density, g is gravitational field strength, and h is depth. The pressure difference between two depths is ΔP = ρgΔh, which explains why dams are thicker at the bottom and why divers feel greater pressure as they descend.
At the same depth in the same connected fluid, the pressure is the same no matter the container shape.
Key Facts
- Fluid pressure is force per unit area: P = F / A.
- The SI unit of pressure is the pascal: 1 Pa = 1 N/m².
- Pressure in a fluid at rest acts in all directions, not just downward.
- Gauge pressure in a liquid is caused by depth: Pgauge = ρgh.
- Absolute pressure includes surface pressure: P = P0 + ρgh.
- At the same depth in the same static fluid, pressure is equal: ΔP = ρgΔh depends only on vertical height difference.
Vocabulary
- Pressure
- Pressure is the force applied perpendicular to a surface divided by the area of that surface.
- Fluid
- A fluid is a substance, such as a liquid or gas, that can flow and take the shape of its container.
- Pascal
- A pascal is the SI unit of pressure and equals one newton per square meter.
- Gauge pressure
- Gauge pressure is the pressure above the surrounding atmospheric or surface pressure.
- Absolute pressure
- Absolute pressure is the total pressure measured relative to a perfect vacuum.
Common Mistakes to Avoid
- Using volume instead of area in P = F / A is wrong because pressure depends on how force is spread over surface area, not how much space the object occupies.
- Forgetting atmospheric pressure in absolute pressure problems is wrong because P = P0 + ρgh includes the pressure already acting at the fluid surface.
- Thinking pressure only pushes downward is wrong because a fluid at rest exerts pressure perpendicular to every surface in all directions.
- Assuming container shape changes pressure at a given depth is wrong because in a connected static fluid, pressure depends on depth, density, and gravity, not the shape of the container.
Practice Questions
- 1 A force of 120 N is applied evenly over an area of 0.30 m². What pressure is produced in pascals?
- 2 A diver is 8.0 m below the surface of freshwater with density 1000 kg/m³. Using g = 9.8 m/s², find the gauge pressure at that depth.
- 3 Two holes are made in the side of a water tank, one near the top and one near the bottom. Explain which water stream travels farther and why.