Stress and Pressure: Definition, Differences & Examples

Both the words stress and pressure are frequently interchanged. Pressure is defined as the degree of force exerted per unit area. Stress, on the other hand, corresponds to the amount of force exerted per unit area by a material. The difference between stress and pressure can help us better understand the fundamentals and their analogies.

The SI unit of stress & pressure is Pascal (represented as Pa) which is equal to one newton per square metre (N/m^2 or kg m^(-1)s^(-2)). It is named after the mathematician and scientist Blaise Pascal. Read this article to get complete details of the stress and pressure.

What is Stress?

When the deforming force is applied to an object, the object deforms. Moreover, an opposing force will be developed inside the object to return it to its original shape and size. This restoring force will be equivalent in magnitude and direction to the deforming force applied. The amount of restoring force generated per unit area of material is referred to as Stress.

Stress is thus stated as “the restoring force per unit area of a material.” It is a tensor quantity.

Formula & Unit of Stress in Physics

The following formula is used to compute stress:

Stress = Force / Cross-sectional Area

Stress is denoted by the Greek letter σ. It is measured using Pascal or N/m^2.

Mathematically expressed as –

σ= F / A

Where,

F is the restoring force measured in Newton or N.
A is the area of cross-section measured in m^2.
σ is the stress measured using N/m^2 or Pa.

Example of Stress

When somebody pulls on the spring, it develops stress and strives to return to its previous position. We connect stress with material engineering, where it is applied to wires and sheets.

When we apply force to the body, it attempts to change its initial shape by adjusting its interatomic distance. However, the body has a tendency to reject external forces to keep its original shape. As a result of this tendency, an opposite force develops inside the body. This opposing force developed per unit is termed as Stress.

What is Pressure?

‘Pressure’ is defined as the amount of force exerted (thrust) on a surface per unit area. It can alternatively be defined as the force-to-area ratio (over which the force is acting).

Pressure is commonly utilised inside a fluid. At a point inside the fluid, it is the same in all directions. Pressure is used in mechanical circumstances when a certain amount of pressure is applied to a hydraulic machine. It is a scalar quantity, which means it has a magnitude, but not a direction.

Formula & Unit of Stress in Physics

When a force of ‘F’ Newton is applied perpendicularly to a surface area ‘A’, then the pressure exerted on the surface by force is equal to the ratio of F to A.

The formula for pressure (P) is:

Pressure (P) = Thrust/ Area

Mathematically expressed as –

P = F / A

Units of Pressure

The SI unit of pressure is the Pascal (Pa).

A pascal is defined as a force of one newton exerted over a one-meter square surface area.

Example of Pressure

Holding a knife to a piece of fruit is a basic example of pressure. The surface will not be sliced if you hold the flat section of the knife against the apple. The force is distributed over a vast area (low pressure). When the blade is turned so that the cutting edge is pressed into the fruit, the same force is given over a significantly smaller surface area (vastly increased pressure), and the surface cuts smoothly.

Another example of pressure is hammering a sharp nail is easier than hammering a blunt nail. It is due to the fact that the area at the tip of a sharp nail is smaller than the area at the tip of a blunt nail. It causes an increase in pressure, allowing you to effortlessly hammer the sharp nail.

Difference between Stress and Pressure

The key differences between stress and pressure are:

Parameters	Stress	Pressure
Definition	Stress is defined as the internal resistive force to deformation per unit area.	Pressure is defined as the total amount of force applied per unit area.
Representation	Stress can be denoted as (Strain) / (Young’s modulus).	Pressure can be mathematically denoted as (force) / (area).
Characteristic	Stress is a material characteristic.	Pressure is a distinctive feature of thermodynamics or physics.
Sign	Stress can be both positive and negative force.	Pressure is always a positive force.
Measurement	There is no stress measurement instrument (not a measurable quantity). Strain or elongation are used to calculate stress.	Pressure gauges, manometers, and other pressure measuring equipment or instruments are used to physically measure (measure) the pressure.
Types	– Normal Stress – Tangential Stress or Shearing Stress – Hydraulic Stress	– Atmospheric Pressure – Absolute Pressure – Differential Pressure – Gauge Pressure
Magnitude	The magnitude of stress at a point in a different direction varies.	The magnitude of pressure at a point remains constant in all directions.
Scalar / Vector	Stress is measured as a vector quantity. It implies that stress has both magnitude and direction. However, one more term is added to this physical quantity, and that is the point of application. Stress is referred to as the second-degree tensor.	The quantity of pressure is a scalar quantity. It means that we don’t know whether the pressure acts along the x, y, or z-axis (direction), but we only know the magnitude of the pressure in bar or atm units.
Area dependency	The stress changes as the surface area changes.	The pressure is not dependent on the contact surface area. It remains constant and does not change while the surface area changes.
Application	Stress develops internally.	The pressure exerts externally.

Task to Do

As we study the examples given above, the difference between pressure and stress becomes clear. So, try to think of 5 more real-life examples to understand the pressure stress difference better and let us know in the comment section below.

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