MECHANICAL PROPERTIES OF SOLIDS

RIGID BODY 

A rigid body means a body which its ralative position of the constituent particles does not change when an external force is applied on it. A rigid body generally have a definite shape and size. In case of solid, it can be deformed when a sufficiently large external force is applied on it; i.,e., solid bodies are not perfectly rigid an also in practice, no body is perfectly rigid. 

ELASTICITY

The property of a body which tends to regain its original size and shape, when the external applied force is removed is known as elasticity. 

In a solid, the constituent paricles like atoms or molecules have thier definite ralative positions due to its interatomic or intermolecular forces, that means they are in a stable equilibrium position. When the solid is under the action of an external applied force, the atoms or molecules are displaced from thier equilibrium positions. If the external force or the deforming force is removed, these atoms or molecules start to back them at thier orignal relative positions due to its interatomic or intermolecular forces,i.e., a restoring force which tends to restore its original relative positions was developed. 

 PERFECTLY ELASTIC BODY

If a body can be restored to its original shape and size after removing the  deforming force, then such a body is said to be a perfectly elastic. 

PERFECTLY PLASTIC BODY 

If a body gets a permanantly deformed though after removing the external applied force, then such a body is said to a perfectly plastic body. 

STRESS 

The restoring force per unit area is known as stress.  

or When a body is subjected to a deforming force, the deveeloped restoring force per unit area is known as stress. The restoring force is equal in magnitude but opppsite in direction to the applied external force.  

If F is the force applied and A is the area of cross section of a body, then the magnitude of the stress is given by 

         Stress = F / A 

The SI unit of stress is Newton per square meter or Pascal. 

(a) Tensile stress 

The restoring force per unit area 

(a) Longitudinal strain

The ratio of the change in length to the original length after a body had deformed is known as its longitudinat strain.

i.,e., longitudinal strain = dl/L

(b) Tangbential strain or shearing strain

The ratio of the relative displacement of the faces to the length of the cylinder is known as shearing stress. 

i.,e., shearing stress = dx/L