Atmospheric Pressure
Simple Explanation
Atmospheric pressure is the force per unit area exerted by the weight of the air in Earth's atmosphere pressing down on everything at the surface. At sea level, it averages about 101,325 Pa (roughly 100,000 Pa).
Why Do We Need It?
Atmospheric pressure affects countless everyday phenomena, from why drinking straws work to why high-altitude cooking takes longer — understanding it as a real, measurable force is the foundation for the rest of this chapter.
Formula
Pressure
P = F / A
Pressure is the amount of force acting perpendicular to a surface, spread over the area of that surface.
- P
- — Pressure, in pascals (Pa), where 1 Pa = 1 N/m²
- F
- — Force acting perpendicular to the surface, in newtons
- A
- — Area over which the force is spread, in m²
When to use it: Use whenever you need to relate a force to the area it acts over — the same force spread over a smaller area produces much greater pressure.
Worked Example
Finding force from atmospheric pressure
Atmospheric pressure is about 100,000 Pa. Find the total downward force it exerts on a table surface of area 1.5 m².
Why Does This Work?
The air above us has weight, and that weight, spread over the entire surface of the Earth, produces a continuous downward (and, since air is a fluid, also sideways and upward) pressure — we don't notice it because it acts equally in all directions and our bodies are adapted to it.
Real-Life Example
Why a drinking straw works
Sucking on a straw draws liquid up into your mouth.
Sucking reduces the air pressure inside the straw — since atmospheric pressure on the liquid's surface outside the straw is now greater than the pressure inside, it pushes the liquid up the straw and into your mouth. The liquid isn't being 'pulled'; it's being pushed by the now-unbalanced atmospheric pressure.
Practice
Atmospheric pressure is 100,000 Pa. Find the force it exerts on a window of area 2 m².
MediumCommon mistake
Thinking we would feel crushed by atmospheric pressure — since it acts equally in all directions (including from inside our bodies, which contain gases and fluids at similar pressure), the net effect on us is balanced, which is why we don't notice it under normal conditions.
Quick Review
- Atmospheric pressure ≈ 100,000 Pa at sea level, from the weight of the air above.
- P = F/A relates pressure, force, and area.
- We don't feel crushed because pressure acts equally in all directions, balanced from inside and out.