12.1. Pressure in Fluids http://www.ck12.org
12.1 Pressure in Fluids
Objectives
The student will:
- Understand density and be able to solve problems with density.
- Understand pressure and be able to solve problems with pressure.
Vocabulary
- density: The ratio of an object’s mass divided by its volume.
- pressure: Force per unit area.
Introduction
In the language of physics,densityis defined as the ratio of the mass of an object (or a material)mdivided by the
volumeV. It is represented by the Greek letter rho – writtenρand pronounced “row”.
ρ=
m
VThis is different than in everyday English, where we tend to use “dense” only to describe heavy or thick objects. In
physics, a massive object may have a small density, and a very light object may have a large density, depending on
volume. For example, an empty aluminum soft drink can is very light compared to a full one. However, aluminum
is 2.7 times denser than water. The typical mass of an empty soda can is about 15 grams, but the soft drink in it
has a mass of about 336 grams. So, a small amount of a denser material can easily have a smaller mass than a large
amount of a less dense material.
The typical units for density are kilograms per meter cubed kg/m^3 or grams per centimeter cubed g/cm^3.
In the language of physics,pressureis defined as force per unit area.
P=
F
A
This is also different than in everyday English, where we often use “pressure” to mean the same as “force” (i.e. “he
was pressured to sign” is like “he was forced to sign”). The SI unit of pressure is called the pascal, abbreviated
Pa and defined as one Newton per square meter,mN 2. It is named after the French mathematician and physicist Blaise
Pascal (1623-1662), pictured in theFigure12.1. We will use bothmN 2 and Pa in this text, but they have exactly the
same meaning, so 1mN 2 =1 Pa.