Pressure & Fluids

Why do your ears pop when traveling to Namche Bazaar? Why does a sharp needle pierce more easily than a blunt stick? Why do some objects float? The answer is pressure.

What is Pressure?

Pressure = Force / Area. Unit: Pascal (Pa) where 1 Pa = 1 N/m^2

Smaller area means greater pressure for the same force. That is why sharp knives cut better.

Example: A 60 kg person (weight 600 N) on shoes with area 0.03 m^2: Pressure = 600/0.03 = 20,000 Pa

Atmospheric Pressure

The atmosphere exerts about 101,325 Pa at sea level. At higher altitudes, pressure decreases. Kathmandu (1,400 m) has lower pressure than the Terai, and Everest Base Camp even less.

Pressure in Liquids

P = h x density x g where h = depth

Pressure increases with depth. That is why dams are thicker at the bottom.

Pascal's Law

Pressure applied to a confined fluid transmits equally in all directions.

Application: Hydraulic brakes and jacks -- a small force on a small piston creates a large force on a large piston.

Archimedes' Principle

An immersed object experiences an upward buoyant force equal to the weight of fluid displaced.

Floating and Sinking:

• Object density less than fluid: floats
• Object density greater than fluid: sinks
• Equal densities: remains suspended

Example: Wood (density about 500 kg/m^3) floats on water (1,000 kg/m^3). Iron (7,800 kg/m^3) sinks.

Did You Know? Water boils at lower temperatures at high altitudes due to reduced pressure. In Kathmandu, water boils at about 95 degrees C instead of 100 degrees C. That is why pressure cookers are so popular in Nepali kitchens!

Key Takeaways

• Pressure = Force/Area; smaller area means greater pressure
• Atmospheric pressure decreases with altitude
• Liquid pressure increases with depth
• Objects float when their density is less than the fluid's density