Study Guide

# Fluids - Common Mistakes

## Common Mistakes

### States of Matter

We sometimes get confused because these three states appear to be so different. A water molecule is fundamentally two hydrogen atoms bonded to an oxygen atom. Yet the same H2O molecule appears to be different at different temperatures: a rock below 0 oC, like a liquid at room temperature, and like hot steam at 100 oC. But water is water, no matter its state of matter. No rhyme intended.

### Mechanics of Fluids

Forces can't be added just any old way: they're vectors. The normal force balances the force of gravity for a flat horizontal surface as Fg = FN.

Oh, and pounds aren't kilograms: pounds are a weight, while kilograms are a mass. That said, on planet Earth, there are 2.2 pounds per kilogram since the acceleration of gravity is constant.

### Density

On the scale of water having a density of , densities less than 1 float, and higher than 1 sink. That's the bottom line. Don't drop it. Oh, and watch those units. Compatibility is the key, and don't forget that converting cubic meters to cubic centimeters requires multiplying by 100,000, not 100.

### Pressure

Since Pascal's Law involves two ratios, we have to mind our p's and q's, figuratively speaking. Fractions and their values get turned upside down sometimes when numerators and denominators change, just think of versus . The first yields 1, but the second is equal to 0.1. A smaller denominator means a larger number.

### Hydrostatic Equilibrium

Any time there is a displaced volume of fluid, the buoyant force is present. And any time the object is supported by the surface tension of a fluid (on a surface, or surface area), the force from surface tension is present.

We think of volume as the entire space occupied by a 3D object, and the surface area as the boundary that defines the object's space. Visualize the surface area of a single drop of a fluid by picturing the object wrapped in a thin layer of plastic. If we peel off the plastic and stretch it out, we're left with a 2D surface area.

### Archimedes' Principle

Any time there is a displaced volume of fluid, the buoyant force is present. And any time the object is supported by the surface tension of a fluid (on a surface, or surface area), the force from surface tension is present. We think of volume as the entire space occupied by a 3D object, and the surface area as the boundary that defines the object's space. Visualize the surface area of a single drop of a fluid by picturing the object wrapped in a thin layer of plastic. If we peel off the plastic and stretch it out, we're left with a 2D surface.

### Fluid Dynamics

Let's repeat this one more time: The fluid in a flow tube stays always in the flow tube.