Pressure and Stress—Force Acting Over an Area

3 Pressure and Stress—Force Acting Over an Area 

Pressure provides a measure of intensity of a force acting over an area. It can be defined as the ratio of force over the contaa surface area  

An experiment demonstrating tie 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 pressore on the snrfoce. 

Note that one newton per squared meter is called one pascal (1 N/1 = 1 Pa). Now, if we 

It is important to note here that the weigjhit of the brick is 28 N, regardless of how it is laid. But the pressure that is created at the contact sur ce 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  

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One way to calculate the stresses is to imagine that the cellular contents are removed, leaving only the cell wall, which has a uniform hydrostatic pressure acting perpendicular to its inside surface. The projection of this P over the appropriate area gives the force acting in a certain direction, and the reaction to this force is an equal force in the opposite direction in the cell wall. By dividing the force in the cell wall by the area over which it occurs, we can determine the cell wall stress. 

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