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

10.5 Pressure and Stress —Force Acting Over an Area 269

10.5 Pressure and Stress–Force Acting Over an Area

Pressureprovides a measure of intensity of a force acting over an area. It can be defined as the

ratio of force over the contact surface area:

(10.12)

To better understand what the magnitude of a pressure represents, consider the situations shown

in Figure 10.19. Let us first look at the situation depicted in Figure 10.19(a), in which we lay

a solid brick, in the form of a rectangular prism with dimensions of 21.6 6.4 10.2 cm

that weighs 28 N (6.4 lb), flat on its face. Using Equation (10.12) for this

orientation, the pressure at the contact surface is

Note that one newton per squared meter is called one pascal (1 N/1 m

2

1 Pa). Now, if we

were to lay the brick on its end as depicted in Figure 10.19(b), the pressure due to the weight

of the brick becomes

It is important to note here that the weight of the brick is 28 N, regardless of how it is laid.

But the pressure that is created at the contact surface depends on the magnitude of the contact

surface area. The smaller the contact area, the larger the pressure created by the same force.

You already know this from your everyday experiences —which situation would create more

pain, pushing on someone’s arm with a finger or a thumbtack?

pressure

force

area



28 N

1 0.064 m 21 0.102 m 2

 4289

N

m

2 4289 Pa

pressure

force

area



28 N

1 0.216 m 21 0.102 m 2

 1271

N

m

2 1271 Pa

18

1

2 ^2

1

2 4 in.^2

pressure

force

area

(a) (b)

■Figure 10.19

An experiment demonstrating the

concept of pressure. (a) A solid

brick resting on its face. (b) A

solid brick resting on its end. In

position (b), the block creates a

higher pressure on the surface.

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