Glossary
Absolute Pressure (P)
Absolute pressure is the total pressure at a specific point, which includes the atmospheric pressure acting on the surface of the fluid plus any gauge pressure.
Example:
When a submarine dives, the absolute pressure on its hull is the sum of the water pressure at that depth and the atmospheric pressure at the surface.
Area (A)
Area is the extent or measurement of a surface. In pressure calculations, it's the surface over which a force is distributed.
Example:
To calculate the pressure exerted by a book on a table, you need to know the area of the book's bottom cover.
Atmospheric Pressure (P_0)
Atmospheric pressure is the pressure exerted by the weight of the air column above a given point on Earth's surface. It is the standard reference for gauge pressure.
Example:
At sea level, the atmospheric pressure is approximately 101,325 Pascals, which is why your ears might pop when you drive up a mountain.
Compressible Fluid
A compressible fluid is a fluid (typically a gas) whose volume and density can change significantly when subjected to pressure changes.
Example:
Air is a compressible fluid; you can easily squeeze a balloon filled with air, reducing its volume.
Depth (h)
Depth is the vertical distance from the surface of a fluid down to a specific point within the fluid. Fluid pressure increases linearly with depth.
Example:
The deeper a diver goes in the ocean, the greater the depth, and consequently, the higher the pressure they experience.
Fluid Density (ρ)
Fluid density is a measure of a fluid's mass per unit volume. It is a key factor in determining fluid pressure at a given depth.
Example:
Oil has a lower fluid density than water, which is why it floats on top of water.
Force (F)
Force is a push or a pull that can cause an object to accelerate or deform. It is a vector quantity, possessing both magnitude and direction.
Example:
The force of gravity pulls an apple downwards from a tree.
Gauge Pressure (P_gauge)
Gauge pressure is the pressure measured relative to the surrounding atmospheric pressure. It represents the 'extra' pressure above or below the ambient air pressure.
Example:
The pressure reading on a car tire gauge is the gauge pressure, indicating how much pressure is inside the tire above the outside atmospheric pressure.
Gravity (g)
Gravity is the acceleration due to gravity, approximately 9.8 m/s² near Earth's surface, which causes objects to fall and contributes to fluid pressure.
Example:
The gravity on the Moon is much weaker than on Earth, meaning an astronaut would weigh less there.
Incompressible Fluid
An incompressible fluid is a fluid (typically a liquid) whose volume and density remain constant, even when subjected to significant pressure changes.
Example:
Water is considered an incompressible fluid because its volume doesn't noticeably change when you squeeze it in a bottle.
Pressure (P)
Pressure is the measure of how concentrated a force is over a given area. It quantifies the 'push' spread out on a surface.
Example:
When a ballerina stands on her toes, the immense pressure on the small area of her pointe shoes allows her to balance gracefully.
Scalar Nature of Pressure
Pressure is a scalar quantity, meaning it only has magnitude and no specific direction. In a fluid, pressure acts equally in all directions.
Example:
When you inflate a balloon, the scalar nature of pressure means the air pushes outwards with the same magnitude in every direction on the balloon's inner surface.