Class 9 Physics | Punjab Curriculum and Textbook Board Syllabus 2025
An external force applied on an object can change its size or shape. Such a force is known as a deforming force.
Elasticity is the property of solids by which they come back to their original shape when deforming force ceases to act.
Most of the materials are elastic up to a certain limit known as elastic limit. Beyond the elastic limit, the change becomes permanent, and the object does not regain its original shape or size even after the removal of the deforming force.
Some materials such as clay dough or plasticine do not return to their original shape after the removal of the deforming force. They are known as inelastic materials.
Within the elastic limit of a helical spring, the extension or compression in it is directly proportional to the applied force. This is known as Hooke's law.
If force F is applied on a spring to stretch or compress it, the extension or compression x has been found directly proportional to the applied force within the elastic limit. Thus,
\[ F \propto x \]
\[ F = kx \]
\[ k = \frac{F}{x} \]
Here, k is the constant of proportionality and is known as the spring constant.
Spring constant is defined as the ratio of applied force to the change in length of spring. Spring constant written as
\[ k = \frac{F}{x} \]
Density of a substance is defined as its mass per unit volume.
\[ \text{Density} = \frac{\text{mass of substance}}{\text{volume of that substance}} \]
\[ \rho = \frac{m}{v} \]
SI unit of density is kilogram per cubic meter (kgm-3). Other unit also in use is gcm-3.
Pressure is defined as the force exerted normally on unit area of an object.
\[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} \]
\[ P = \frac{F}{A} \]
Pressure is a scalar quantity. In SI units, the unit of pressure is Nm-2 also called pascal (Pa).
\[ \text{Nm}^{-2} = 1 \text{Pa} \]
The atmosphere exerts pressure on the surface of the Earth and on everything on the Earth. This pressure is called atmospheric pressure. Standard value at sea level: 1.013×105 Pa.
The pressure at depth h in a liquid is:
\[ P = \rho gh \]
where ρ is liquid density and g is gravitational acceleration.
When pressure is applied at one point in an enclosed fluid, it is transmitted equally to all parts of the fluid without loss.
Atmospheric pressure is usually measured by the height of mercury column which it can support. Instruments which measure the atmospheric pressure are called barometers.
Atmospheric pressure is measured by the height of a mercury column it supports:
\[ P = \rho gh \]
At sea level: 760 mm Hg = 1.013×105 Pa
A simple manometer consists of a U-shaped glass tube containing mercury. Initially, the atmospheric pressure at both open ends is the same, so the mercury level in both arms remains equal.
A system where a small force F1 on a small piston (area A1) creates a larger force F2 on a larger piston (area A2):
\[ F_2 = F_1 \times \left( \frac{A_2}{A_1} \right) \]
Hydraulic Brakes work on Pascal's law. Pressure from brake pedal is transmitted equally to all wheels, creating friction to slow the vehicle.