Class 9 Physics | Punjab Curriculum and Textbook Board Syllabus 2025
Mechanics is the branch of physics that deals with the motion of objects and the forces that change it.
The study of motion without referring to forces.
The study of forces and their effect on motion.
A scalar is that physical quantity which can be described completely by its magnitude only.
A vector is that physical quantity which needs magnitude as well as direction to describe it completely.
In the textbook, a vector is represented by a boldface letter such as A, v, F etc. Since boldface cannot be written on paper, a vector is written with a small arrow over it, i.e., A ⃗, v ⃗, F ⃗, d ⃗. The magnitude of a vector is represented by an italic letter without an arrowhead.
A vector is graphically represented by drawing a straight line with an arrowhead at one end. The length of the line represents the magnitude of the vector according to a suitable scale, while the arrowhead indicates the direction of the vector.
The horizontal line (XX') is called the x-axis, and the vertical line (YY') is called the y-axis. The point where these axes meet is known as the origin, usually denoted by O. These axes are called reference axes.
A vector is drawn from the origin of the reference axes towards the given direction. The direction is usually given by an angle θ (theta) with the x-axis. This angle is always measured from the right side of the x-axis in the anti-clockwise direction.
A resultant vector is a single vector obtained by adding two or more vectors. It has the same effect as the combined effect of all the vectors being added.
To add a number of vectors, redraw their representative lines such that the head of one line coincides with the tail of the other. The resultant vector is given by a single vector which is directed from the tail of the first vector to the head of the last vector.
If a body does not change its position with respect to its surroundings, it is said to be at rest. For example, objects like buildings, trees, and electric poles are in a state of rest as they do not change their position.
If a body continuously changes its position with respect to its surroundings, it is said to be in motion. A moving car, running water, or a flying bird are examples of motion.
If the motion of a body is such that every particle of the body moves uniformly in the same direction, it is called translatory motion. For example, the motion of a train or a car.
If each point of a body moves around a fixed point (axis), the motion of this body is called rotatory motion. For example, the motion of an electric fan or a spinning top.
When a body repeats its to and fro motion about a fixed position, the motion is called vibratory motion. For example, a swing in a children's park.
If the body moves along a straight line, it is called linear motion. For example, a freely falling body.
If the body moves along an irregular path, the motion is called random motion. For example, the motion of a bee.
The motion of a body along a circle is called circular motion. For example, a Ferris wheel or a ball tied to a string and whirled in a circle.
The distance is the length of the actual path of the motion.
The displacement of an object is a vector quantity whose magnitude is the shortest distance between the initial and final positions of the motion. Its direction is from the initial position to the final position. We can also call this the change in position. OR The shortest distance between the initial and final positions of a body is called its displacement.
Position of any object is its distance and direction from a fixed point.
Speed is the distance covered in unit time. It tells us how fast a body is moving.
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
\[ v = \frac{S}{t} \]
Speed is a scalar quantity, and its SI unit is ms-1 or kmh-1.
The speed of a vehicle at any given instant is called instantaneous speed. It is the reading shown on a speedometer at that moment.
Since speed is not constant during a journey, we often use average speed, which is defined as:
\[ \text{Average speed} = \frac{\text{Total distance covered}}{\text{Total time taken}} \]
\[ v_{av} = \frac{S}{t} \]
Velocity is the net displacement of a body in unit time.
\[ \text{Average velocity} = \frac{\text{Displacement}}{\text{Time}} \]
\[ v_{av} = \frac{d}{t} \]
The SI unit of velocity is ms-1 or kmh-1.
The velocity is said to be uniform if the speed and direction of a moving body do not change.
If the speed or direction or both of them change, it is known as variable velocity or non-uniform velocity.
Acceleration is defined as the time rate of change of velocity. The average acceleration is given by:
\[ \text{Average acceleration} = \frac{\text{Change in velocity}}{\text{Time taken}} \]
\[ a_{av} = \frac{v_f - v_i}{t} \quad \text{(i)} \]
\[ a_{av} = \frac{\Delta v}{t} \]
The SI unit of acceleration is meter per second square (ms-2). If acceleration a is constant, then Eq (i) can be written as vf = vi + at
If the velocity is increasing, the acceleration is positive.
If the velocity is decreasing, the acceleration is negative.
If the time rate of change of velocity is constant, the acceleration is said to be uniform acceleration.
If anyone of the magnitude or direction or both changes, the acceleration is called variable or non-uniform acceleration.
A graph is a pictorial diagram in the form of a straight line or a curve that shows the relationship between two physical quantities.
The gradient is the measure of the slope of a line. In a distance-time graph, the gradient is equal to the average speed of the body.
The gradient (slope) of a speed-time graph is equal to the average acceleration of the body.
The area under speed-time graph is equal to the distance covered by an object.
When a body falls freely under the action of Earth's gravity, the acceleration acting on it is called gravitational acceleration and is denoted by g.
The speed of light is a universal constant with a value of approximately 3×10⁸ ms-1. No object with mass can achieve speeds equal to or greater than that of light.