Pressure and strain are fundamental concepts in fields from engineering to geology. They describe the relationship between a force applied to an object and the resulting deformation of that object.
Defining Pressure
Pressure is the measure of a force acting perpendicularly on a surface, calculated by dividing the force by the area over which it is applied. The same force can produce different pressures depending on the contact area. For example, a stiletto heel concentrates body weight onto a tiny point, creating high pressure, while wide tractor tires distribute weight to reduce pressure on soft ground. This principle is also why a sharp knife cuts more effectively than a dull one.
The standard international unit for pressure is the Pascal (Pa), or one Newton per square meter (N/m²). Since a Pascal is a small unit, measurements are often in kilopascals (kPa) or, in the United States, pounds per square inch (psi). Atmospheric pressure, the force exerted by the weight of the air, is another common example.
Defining Strain
Strain measures an object’s deformation in response to an applied force. It quantifies the change in an object’s shape relative to its original dimensions and is expressed as a dimensionless ratio or a percentage. This is calculated by dividing the change in a dimension, like length, by the original dimension. Unlike pressure, which measures force, strain describes a physical change.
Deformations lead to different types of strain. Tensile strain occurs when an object is stretched, like a rubber band. Compressive strain is the opposite, happening when an object is shortened, such as a squeezed foam block. A third type is shear strain, which involves internal layers sliding past one another, similar to pushing the top of a deck of cards.
The Relationship Between Pressure and Strain
The connection between pressure and strain is one of cause and effect. When external pressure is applied to an object’s surface, it generates internal forces distributed over the material’s cross-sectional area, known as stress. It is this internal stress that causes the material to deform, a phenomenon measured as strain.
The amount of strain resulting from a given pressure depends on a material’s intrinsic properties, like its stiffness. For instance, applying the same pressure to a steel block and a rubber block will cause the rubber to exhibit much greater strain because it is less stiff.
This response leads to two types of deformation. Elastic deformation is temporary, and the material returns to its original shape once the pressure is removed, like a stretched spring. Plastic deformation is permanent; if enough pressure is applied to bend a metal bar, it will remain bent.
Real-World Examples of Pressure and Strain
In engineering, bridge and building design depends on how materials respond to loads. The weight of vehicles on a bridge creates pressure, leading to compressive strain in support columns and tensile strain in the cables of a suspension bridge. Engineers calculate these forces to ensure the strain does not exceed material limits.
In geology, the movement of tectonic plates exerts pressure on rock formations. This pressure causes strain to build in the Earth’s crust, and when the strain is too great, the rocks fracture and slip, releasing energy as an earthquake. This process is also responsible for forming mountain ranges.
The human body offers another example. Blood pressure is the force of circulating blood on artery walls. This pressure causes the flexible walls to expand and contract with each heartbeat, a form of strain. Understanding this is necessary for diagnosing conditions like hypertension, where high blood pressure causes excessive, long-term strain on arteries.