Class 9 Physics | Punjab Curriculum and Textbook Board Syllabus 2025
Dynamics deals with the forces that produce changes in the motion of bodies.
A force is a push or pull that can start, stop, or change the magnitude and direction of a body's velocity.
A contact force is a force that is exerted by one object on the other at the point of contact. Examples: Friction, drag, thrust, normal force, air resistance, tension force, elastic force.
It is the force that resists motion when the surface of one object comes in contact with the surface of another.
The drag force is the resistant force caused by the motion of a body through a fluid. It acts opposite to the relative motion of any object moving with respect to surrounding fluid.
It is an upward force exerted by a liquid on an object immersed in it. For example, when we try to immerse an object in water, we feel an upward force exerted on the object.
It is the force of reaction exerted by the surface on an object lying on it. This force acts outward and perpendicular to the surface.
It is the resistance (opposition) offered by air when an object falls through it.
It is the force experienced by a rope when a person or load pulls it.
It is a force that brings certain materials back to their original shape after being deformed. Examples include rubber bands, springs, and trampolines.
A non-contact force is defined as the force between two objects which are not in physical contact. Examples: Gravitational force, electrostatic force, magnetic force, strong and weak nuclear forces.
Gravitational Force: An attractive force between masses.
Electromagnetic Force: Interaction between charged particles.
Strong Nuclear Force: Binds protons and neutrons in a nucleus.
Weak Nuclear Force: Responsible for beta decay.
A diagram showing all external forces acting on an object, represented by arrows indicating magnitude and direction.
A body continues its state of rest or of uniform motion in a straight line unless acted upon by some external force.
The property of a body to maintain its state of rest or of uniform motion in a straight line. For example, when a tablecloth is pulled abruptly from under dishes, the objects remain in their original positions due to inertia.
If a net external force acts upon a body, it accelerates the body in the direction of force. Mathematically:
\[ F = ma \]
For every action, there is always an equal and opposite reaction.
When two bodies interact, if body A exerts a force on body B (action), then body B exerts an equal and opposite force on body A (reaction).
Newton's laws of motion have limitations in certain conditions:
High Velocities: When dealing with objects moving at speeds close to the speed of light, Newton's laws fail to provide accurate results. In such cases, relativistic mechanics developed by Albert Einstein is used.
Elementary Particles: Newton's laws are not suitable for describing the motion of elementary particles at atomic or subatomic scales.
One newton is the force which produces an acceleration of 1 ms^(-2) in a body of mass 1 kg. From equation F=ma:
\[ 1N = 1 kg \times 1 ms^{-2} \]
Mass of a body is the quantity of matter in it. It determines the magnitude of acceleration produced when a force acts on it. Mass of a body does not vary. It is a scalar quantity and its unit is kilogram (kg).
The weight of an object is equal to the force with which the Earth attracts the body towards its centre. (SI unit: N).
The gravitational field is the space around a mass where another mass experiences a force due to gravitational attraction.
The gravitational field strength is defined as the gravitational force acting on a unit mass. For a mass m on the surface of the Earth, the force it experiences (its weight) is given by:
\[ w = mg \]
where g is the gravitational field strength, with a standard value of 10 N kg⁻¹.
A device with a rigid beam and pans to compare masses using standard weights.
A precise instrument displaying mass digitally without standard weights.
A force meter is a scientific instrument used to measure force. It is also called a newton meter or a spring balance. A digital force meter measures the weight of an object directly in newtons.
Friction is a dissipative force that opposes motion, causing energy to be wasted as work is done against it. The lost energy appears in the form of heat.
The resisting force between the two surfaces before the motion starts is called the static friction.
The friction during motion is called kinetic friction.
The maximum value of the static friction is called a limiting friction.
The constant velocity attained when air resistance balances gravity.
Friction when an object rolls over a surface (lesser than sliding friction).
The following methods are used to reduce friction:
The parts which slide against each other are highly polished.
Since the friction of liquids is less than that of solid surfaces, oil or grease is applied between the moving parts of the machinery.
As rolling friction is much less than the sliding friction, sliding friction is converted into rolling friction by the use of ball bearings in the machines and wheels under heavy objects.
High-speed vehicles, airplanes, and ships face friction from air or water. Their bodies are made streamlined (pointed front) to allow smooth airflow and reduce resistance.
The momentum of a moving body is the product of its mass and velocity. Mathematically:
\[ p = mv \quad \text{(SI unit } kg\,ms^{-1} = Ns\text{)} \]
When a large force F acts on an object for a short time △t, the product F×△t is called Impulse, which is equal to the total change in momentum of the object. Mathematically:
\[ \text{Impulse} = F \times \triangle t \]
If no external force acts on an isolated system, the final total momentum of the system is equal to the initial total momentum of the system.