Mechanical Properties of Fluids
Easy Overview
Ever noticed how a needle can float on water even though it's metal? Or why honey pours slower than water? That's fluids doing weird, wonderful things. This chapter is about how liquids and gases behave — under pressure, when flowing, and at surfaces.
Pressure and Pascal's Law
Pressure is just force spread over an area. A sharp needle pierces skin easily because all the force is concentrated on a tiny tip. Pascal's law says if you push on a fluid in a closed container, that pressure spreads everywhere equally. That's how hydraulic lifts work — a small force on a small piston lifts a heavy car on a big piston. It's force multiplication for free.
Viscosity
Viscosity is a fancy word for how sticky a fluid is — how much it resists flowing. Water has low viscosity, honey has high. Imagine stirring a bucket of water vs a bucket of oil. The oil fights back more. That resistance comes from layers of fluid sliding past each other. The faster you try to stir, the more it fights. That's why thick fluids flow slowly through pipes.
Surface Tension
Surface tension is why water forms droplets and why some insects can walk on water. The molecules at the surface of a liquid are pulled inward by the molecules below, creating a sort of elastic skin. It's like a stretched trampoline — it wants to shrink to the smallest possible area. That's why droplets are spherical. Soap reduces surface tension, which is why soap bubbles form easily.
Bernoulli's Principle
Bernoulli noticed something weird — when a fluid flows faster, its pressure drops. Think of air rushing over an airplane wing. The top is curved, so air moves faster there, creating lower pressure. Higher pressure below pushes the wing up. That's lift. It's also why a shower curtain gets sucked inward when you turn the water on — fast-moving water creates low pressure nearby.
Key Points
- •Pascal's law: Pressure applied to an enclosed fluid transmits equally in all directions.
- •Viscosity is the internal friction between fluid layers. It depends on temperature — heating oil makes it less viscous.
- •Surface tension makes liquid surfaces behave like stretched membranes.
- •Bernoulli: Faster flow = lower pressure. Slower flow = higher pressure.
- •Stokes' law gives the viscous drag force on a sphere moving through a fluid.
- •The terminal velocity of a falling object in a fluid is when weight = buoyant force + drag.
Practice Questions
- Explain why a needle floats on water despite being denser than water.
- State Bernoulli's principle and give two real-life applications.
- Derive the expression for terminal velocity of a sphere falling through a viscous fluid.
- What is the difference between laminar and turbulent flow? What is Reynolds number?