Both pressure and stress are related to force. The crucial difference between pressure and stress is that pressure is associated with the externally subjected force to a body or an object per unit area. As against, stress is concerned with a restoring force per unit area against deformation which is produced within a body when an external force is applied to it.
Before understanding the factors of differentiation of pressure and stress, let us understand-
What is Force?
Force in physics is defined as an act that leads to change or maintain the motion possessed by a body or an object. The quantity force is a vector in nature due to having both magnitude as well as direction. This can be said so because when a force is applied to a body then along with the magnitude of the force, the concerning direction in which force is applied also becomes important.
The various fundamental forces in nature are electromotive force, nuclear force, gravitational force, etc.
Content: Pressure Vs Stress
Comparison chart
| Basis for Comparison | Pressure | Stress |
|---|---|---|
| Basic | Acts externally | Produced internally |
| Represented by | p | σ |
| Direction of action | Normal to the surface. | Can be normal or parallel to the surface. |
| Property | It is a fundamental property of physics. | It is a material property. |
| Expressed as | External force/ Area | Internal resistive force/ Area |
| Type of quantity | Scalar | Vector |
| Dependency on area | Pressure is not dependent on the surface area where contact is made. | It shows dependency on the surface area. |
| Measurement | By devices like pressure gauge. | Not any specific measuring device. |
Definition of Pressure
Whenever we apply some amount of force on an object that the action which will take place does not only dependent on the magnitude of force but the pressure with which the force is provided is of equal importance.
Now you must be thinking that why pressure is given so much importance?
This is so because the pressure is concerned with the area of the surface when force perpendicular to it is applied.
Let us understand this by considering an activity.
Suppose we have a metallic rod and an iron nail and both are needed to be hammered on a wall adjacent to each other. What do you think will easily be inserted within the wall? Obviously a nail. This is so because though the magnitude of the applied force is a crucial parameter but the area where the force is to be concentrated is equally important.
When the rod is considered then the applied force will get distributed over a comparatively large surface area than for the same magnitude of applied force when the nail is considered. This is so because, in the case of nails, the applied force is concentrated over a comparatively smaller region.
Thus, the pressure is given by the ratio of applied force to the unit area of the surface.
P = F/A
Its unit is Pascal.
Definition of Stress
When some amount of force is applied to an object then definitely due to the applied force the object will experience deformation. This deformation can be minor or major depending upon the characteristics of the material and the magnitude of applied force. This force is said to be deforming force. But when this deforming force is applied to an object then the object also possesses some kind of restoring force to deal with the deforming force.
The restoring force generated in the unit area in response to the applied deforming force is known as stress. This restoring force has the same magnitude as that of deforming force but in the opposite direction.
Thus, stress is also given by the ratio of force and area. Hence unit of measurement is Pascal.
Key Differences Between Pressure and Stress
- The key factor of differentiation between pressure and stress exist in the region where the action of force takes place. The pressure is concerned with the application of external force on a unit area of the body. On the contrary, stress is internally generated as a restoring force due to applied external force.
- Pressure and stress are symbolically represented by p and σ respectively.
- In the case of pressure, it is noticed that its magnitude at a point where force is applied is the same for all the directions thus is said to be scalar in nature. While for stress, the magnitude is different for different directions at the point of application hence is vector.
- Pressure is applied normal to the surface thus is compressive in nature. While stress is a quantity that acts either normal or parallel to the surface hence possesses tensile as well as compressive behavior.
- Pressure is regarded as the fundamental property of physics but stress is the property of the material in which it is produced.
- There is pressure measuring devices by which applied force on a unit area can be measured such as a pressure gauge. As against, there is no such device through which stress can be measured.
Conclusion
Thus, from this discussion, we can conclude that pressure and stress are two quantities that are related through force but both act differently. When we talk about pressure then it is associated with a body that is subjected to externally applied force but stress is regarded as a restoring force that is internally generated in response to deformation. Thus, we can say, stress is an outcome of applied pressure.
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