Page 27 - 9-phy-7 Properties of Matter

7. Properties of Matter eLearn.Punjab

Look at the pictures in figure 7.22. What happens to the objects due to the forces acting on them.
The applied force that changes shape, length or volume of a substance is called deforming force.
In most of the cases, the body returns to its original size and shape as soon as the deforming force
is removed.

The property of a body to restore to its original size and shape as the deforming force ceases to
act is called elasticity.

STRESS

Stress is related to the force producing deformation. It is defined as:

The deforming force acting on unit area at the surface of a body is called stress.

In SI, the unit of stress is newton per square metre (Nm-2 ).

STRAIN

When stress acts on a body, it may change its length,
volume, or shape. A ratio of such a change caused by the
stress with the original length, volume or shape is called
as strain. If stress produces a change in the length of an
object then the strain is called tensile strain.

Strain has no units as it is simply a ratio between two Figure 7.22 (a) A spring is stretched by a force
similar quantities. (b) A rod is twisted by the torque produced by a

couple (c) A strip is bent by a force.

27

v: 1.1