Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

10.5 Pressure and Stress —Force Acting Over an Area 277

has a highervapor pressurethan water. Thus, under the same conditions, fluids with low vapor

pressure, such as glycerin, will not evaporate as quickly as those with high values of

vapor pressure. Stated another way, at a given temperature the fluids with low vapor pressure

require relatively smaller surrounding pressure at their free surface to prevent them from

evaporating.

Blood Pressure We all have been to a doctor’s office. One of the first things that a doctor or a

nurse will measure is our blood pressure. Blood pressure readings are normally given by two

numbers, for example 115/75 ( mmHg). The first, or the higher, number corresponds to sys-

tolic pressure, which is the maximum pressure exerted when our heart contracts. The second,

or the lower number, measures the diastolic blood pressure, which is the pressure in the arter-

ies when the heart is at rest. The pressure unit commonly used to represent blood pressure is

millimeters of mercury ( mmHg). You may already know that blood pressure may change

with level of activity, diet, temperature, emotional state, and so on.

Hydraulic Systems

Hydraulic digging machines and bulldozers perform tasks in hours that used to take days or

weeks. Today, hydraulic systems are readily used in many applications, including the braking

and steering systems in cars, the aileron or flap control system of an airplane, and as jacks to

lift cars. To understand hydraulic systems, you must first fully understand the concept of pres-

sure. In an enclosed fluid system, when a force is applied at one point, creating a pressure at that

point, that force is transmitted through the fluid to other points, provided the fluid is incom-

pressible. When you press your car’s brake pedal, that action creates a force and subsequently a

pressure in the brake master cylinder. The force is then transmitted through the hydraulic fluid

from the master cylinder to the wheel cylinders where the pistons in the wheel cylinder push

Measuring blood pressure.

Source:© Stockbyte /Corbis

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