When an external deforming force is applied on the body it produces change in volume of body which is known as volume or bulk stress.

The formula for bulk or volume stress is given as,

Bulk Stress or Volume Stress = Change in magnitude of Force / Area

We know that Force can also be expressed as Force = Pressure x Area

Bulk Stress or Volume Stress = Change in Pressure / Area

Bulk Stress or Volume Stress = Change in Pressure (Δ P)

Bulk Stress or Volume Stress = Δ P

When an external deforming force applied on the body produces change in length of body then it is longitudinal or tensile stress.
The formula for longitudinal or tensile stress is given as,

Tensile Stress = Force / Area

Longitudinal Stress is often seen in cylinder, pipe , wire etc. So we can also write it as,

Longitudinal Stress = Force/ Area

Longitudinal Stress = F / π r ²

we know that force can also be defined as Force = mass x acceleration due to gravity

Longitudinal Stress = m g/ π r ²

where m = unit length of mass, g= acceleration due to gravity and r= radius.

Internal restoring force per unit area of the body on which it acts is stress. Deformation happens (change in shape or in size or both) when an external force is acting on an elastic body. There is equal and opposite force developed inside the body to bring the body back to its original shape. This force is known as the restoring force.

The formula for stress is given as,

Stress = Applied Force / Area

Stress = F /A

The unit for stress is newton metre^-2 with symbol N m^-2 or ‘pascal’ with symbol  ‘Pa’.