# Fluid Statics

## Pressure


### Learning Objectives

By the end of this section, you will be able to:
1. Define pressure.
2. Explain the relationship between pressure and force.
3. Calculate force given pressure and area.
You have no doubt heard the word pressure being used in relation to blood (high or low blood pressure) and in relation to the weather (high- and low-pressure weather systems). These are only two of many examples of pressures in fluids. Pressure  is defined as
where  is a force applied to an area  that is perpendicular to the force.
A given force can have a significantly different effect depending on the area over which the force is exerted, as shown in . The SI unit for pressure is the pascal, where 
In addition to the pascal, there are many other units for pressure that are in common use. In meteorology, atmospheric pressure is often described in units of millibar (mb), where
Pounds per square inch  is still sometimes used as a measure of tire pressure, and millimeters of mercury (mm Hg) is still often used in the measurement of blood pressure. Pressure is defined for all states of matter but is particularly important when discussing fluids.

      
The force exerted on the end of the tank is perpendicular to its inside surface. This direction is because the force is exerted by a static or stationary fluid. We have already seen that fluids cannot withstand shearing (sideways) forces; they cannot exert shearing forces, either. Fluid pressure has no direction, being a scalar quantity. The forces due to pressure have well-defined directions: they are always exerted perpendicular to any surface. (See the tire in , for example.) Finally, note that pressure is exerted on all surfaces. Swimmers, as well as the tire, feel pressure on all sides. (See .)

### Test Prep for AP Courses


### Section Summary

1. Pressure is the force per unit perpendicular area over which the force is applied. In equation form, pressure is defined as
    
2. The SI unit of pressure is pascal and .

### Conceptual Questions


### Problems & Exercises
