In physics, Stress is the force acting on the unit area of a material. Effect of stress on a body is named as strain. Stress can deform the body. How much force material experience can be measured using stress units. Stress can be categorised into three categories depending upon the direction of the deforming forces acting on the body. Let us study them one by one.
What is Stress?
When the deforming force is applied to an object. The object deforms. In order to bring the object back to the original shape and size, there will be an opposing force generated inside the object.
This restoring force will be equal in magnitude and opposite in direction to the applied deforming force. The measure of this restoring force generated per unit area of the material is called Stress.
Thus, Stress is defined as “The restoring force per unit area of the material”. It is a tensor quantity. Denoted by Greek letter σ. Measured using Pascal or N/m2. Mathematically expressed as –
Where,
 F is the restoring force measured in Newton or N.
 A is the area of cross section measured in m2.
 σ is the stress measured using N/m2 or Pa.
Stress Units
Stress can be expressed using multiple units. Refer to the table given below for Stress units.


Fundamental units 
Kg.m^{1}.s^{2} 
SI (derived units) 
N.m2 
SI (derived units) 
Pa or pascal 
SI (mm)(derived units) 
M.Pa or N/(mm)^{2} 
US unit (ft) 
lbf/ft^{2} 
US unit (inch) 
Psi (lbf/inch^{2}) 
Types of Stress
There are several different types of stress in physics but mainly it is categorized into two forms that are Normal Stress and Tangential or Shearing Stress. Some stress types are discussed in the points below.
Normal Stress:
As the name suggests, Stress is said to be Normal stress when the direction of the deforming force is perpendicular to the crosssectional area of the body. The length of the wire or the volume of the body changes stress will be at normal. Normal stress can be further classified into two types based on the dimension of force
 Longitudinal stress
 Bulk Stress or Volumetric stress
Longitudinal Stress:
Consider a cylinder. When two crosssectional areas of the cylinder are subjected to equal and opposite forces the stress experienced by the cylinder is called longitudinal stress.
Longitudinal Stress = Deforming Force / Area of cross section = F/A
As the name suggests, When the body is under longitudinal stress
 The deforming force will be acting along the length of the body.
 Longitudinal stress results in the change in the length of the body, Hence thereby it affects slight change in diameter.
The Longitudinal Stress either stretch the object or compress the object along its length. Thus it can be further classified into two types based on the direction of deforming force
 Tensile stress
 Compressive stress
Tensile Stress
If the deforming force or applied force results in the increase in the object’s length then the resulting stress is termed as tensile stress. For example: When a rod or wire is stretched by pulling it with equal and opposite forces(outwards) at both ends.
Compressive Stress
If the deforming force or applied force results in the decrease in the object’s length then the resulting stress is termed as compressive stress. For example: When a rod or wire is compressed/squeezed by pushing it with equal and opposite forces(inwards) at both ends.
Bulk Stress or Volume Stress
When the deforming force or applied force acts from all dimension resulting in the change of volume of the object then such stress in called volumetric stress or Bulk stress. In short, When the volume of body changes due to the deforming force it is termed as Volume stress.
Shearing Stress or Tangential Stress
When the direction of the deforming force or external force is parallel to the crosssectional area, the stress experienced by the object is called shearing stress or tangential stress. This results in the change in the shape of the body
Summary
In short, stress can be visualised as –
Practice Questions For Stress
Q1: What is Stress?
Ans: Stress is the measure of restoring force per unit area.
Q2:What is the unit of Stress?
Ans: units of stress is pascal or N/m2.
Q3: Is stress is a vector quantity?
Ans: Yes.
Stress is a vector quantity.
Q4: What is the effect of deforming force?
Ans: The deforming force can change the shape or volume or size of the object.
Q5: What is the direction of the deforming force in the case of shearing stress?
Ans: The deforming force is parallel to the area of crosssection.
Q6: What is the nature of the restoring force?
Ans: The restoring force is equal in magnitude and opposite in direction to deforming force or external force.
Q7: Name the types of normal stress.
Ans: Longitudinal stress and bulk or volume stress are two types of normal stress.
Q8: What is the direction of the deforming force in the case of longitudinal stress?
Ans: The deforming force is perpendicular to the area of crosssection.
Q9: Name the types of longitudinal stress.
Ans: Tensile stress and compressive stress are the two types of longitudinal stress.
Q10: Define longitudinal stress.
Ans: Stress experienced by an object along its length due to the presence of equal and opposite deforming forces perpendicular to the area of cross section is called longitudinal stress.
Q11: What does Bulk stress do to an object?
Ans: Bulk stress results in a change in the volume of the object.
Q12:What does tangential stress do to an object?
Ans: Tangential stress results in a change in the shape of the object.
Q13: Define tangential or shear stress
Ans: When the direction of the deforming force or external force is parallel to the crosssectional area, the stress experienced by the object is called shearing stress or tangential stress.
Q14: Give the expression for stress and explain the terms.
Ans: The expression for stress is given by \(\sigma =\frac{F}{A}\)
Where,
 F is the restoring force.
 A is the area of crosssection.
 σ is the stress.
Q15:A rod is stretched by pulling at both the ends. Name the type of stress experienced by the rod.
Ans: Tensile stress.
Hope you have understood about Stress by learning what is stress? It’s formula, Stress units, Types of stress like Normal stress, shear stress or tangential stress, longitudinal stress, bulk stress or volume stress, tensile stress, compressive stress.
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