# Pressure – The calculation of a force applied over a given area

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Pressure – The calculation of a force applied over a given area
SYMBOL: p FORMULA: p = F/A UNITS: Metric – N/m2 or Pascals (Pa) (1 Pa = 1 N/m2) (A Pascal is very light. It’s about equivalent to the pressure of a dollar bill resting on a table.) English – lb/in2 or psi

Pressure Calculation EXAMPLE: For a certain individual the area of each foot that makes contact with the floor is 135 cm2 (.0135 m2) when this person is standing upright. If this individual’s mass is kg, what is the pressure exerted on the floor by both feet? (Assume both feet have equal contact area.)

Pressure Calculation F = weight = mag = 60.55 kg*9.81 m/s2 = 594 N
A = 2*135 cm2 (Since there are two feet, the contact area is doubled.) = 270 cm2 (or m2) p = F/A = 594 N / 270 cm2 = 2.2 N/cm2 (or 594 N/.0270 m2 = Pa or 22 kPa) SEE PRACTICE PROBLEM ON TEXT p 67

Mechanical Load – Force or weight applied to a structure
There are three types of load Compression – A pressing or squeezing force. Bones and fibrocartilage are designed to resist compression. Tension – A pulling or stretching force. Tendons and ligaments are designed to resist tension. Shear – A cutting force (a force which tends to separate material along parallel planes). Shear loading is normally seen in joints.

The three types of mechanical loads
Shear Tension Compression

Mechanical Stress The distribution of a load or loads on a structure.
Each category of mechanical load produces an associated category of stress. Compressive Stress Tensile Stress Shear Stress

The combined structure of the vertebrae and the intervertebral discs is designed to withstand compressive stress. When a muscle contracts it produces tensile stress in a tendon. Shear stress occurs in joints.

Note that in the vertebral column the vertebrae and discs become larger as you get closer to the bottom. This is to accommodate the increase in weight that must be carried by the lower vertebrae and discs. Adequate anteroposterior growth of the vertebral bodies is dependent on weight bearing activities (sitting and standing upright), especially in the first two years of life.

Mechanical Stress Stress, like pressure, is calculated as a load distributed over an area. The units are the same as the units for pressure. SYMBOL: s (The Greek letter sigma) EXAMPLE: A tendon has cross sectional area of .8 cm2. If the muscle attached to it generates a force of 200 N, calculate the tensile stress in the tendon. s = load/area = 200 N/.8 cm2 = 250 N/cm2 SEE PRACTICE PROBLEM TEXT p 73

Bending Stress Produces compressive stress on one side of a structure and tensile stress on the other.

Torsion Stress which occurs when a structure twists around its long axis. This type of stress creates spiral fractures in long bones like the tibia.

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