CBSE CLASS IX, MOTION, PHYSICS NOTES PART II

 

CBSE CLASS 12, MOTION, PHYSICS NOTES-(PART II)

MOTION

According to the CBSE Syllabus 2025-26

CBSE Class 9 Science Chapter 8 Motion Notes

In Class 9 Science Chapter 8 Motion, students learn to describe the motion of objects along a straight line and express such motions through simple equations and graphs. The chapter also discusses ways of describing circular motion.

TOPICS IN THE CHAPTER

• Introduction


• Distance and Displacement


• Uniform and Non-uniform motion


• Speed


• Velocity


• Accelerated and Decelerated motion


• Equations of motion


• Graphical representation of motion


• Uniform circular motion


SPEED

The measurement of distance travelled by a body per unit time is called speed.

• If a body is executing uniform motion, then it will be at a constant speed or uniform motion.

 If a body covers a certain distance in a certain amount of time, its speed is given by,

 Speed=Distance / Time

 Speed (v) = Distance Travelled / Time Taken = s / t

 Its SI unit = m/s (meter/second)

If a body is travelling with non-uniform motion, then the speed will not remain uniform but will have different values throughout the motion of such a body.

 INSTANTANEOUS SPEED

 The instantaneous speed is the speed of an object at a particular moment in time.

 AVERAGE SPEED

Average speed is stated as the distance covered by the object within a period of time.

Average speed = Total distance travelled / Total time taken

DIFFERENCE BETWEEN AVERAGE SPEED AND INSTANTANEOUS VELOCITY

AVERAGE SPEED 

INSTANTANEOUS SPEED

It is defined as the total distance travelled divided by the total time elapsed.

It is defined as the speed at a particular instant of time.

It is constant.

It is not constant.

Measured by calculating the speed for an entire journey.

It is measured by a speedometer.

Example: A car travelling at a speed of 60 kmph. Thus, the average speed of the car is 60 km an hour.

Example: A car travelling at a certain speed at an instant of time can be given by a speedometer.

UNIFORM MOTION


Average Velocity

Instantaneous Velocity

Average velocity is defined as the displacement (∆x) divided by the time interval (∆t) in which the displacement occurs.

Instantaneous velocity is the rate of change of position for a time interval which is very small, i.e. almost zero.

Average velocity is calculated by dividing the rate of displacement by the time elapsed.

Instantaneous velocity is calculated by dividing displacement by time at that instant.

If Jack took a total of 1 hour to travel 10 km from his house to school, then his average velocity will be 10 km/hr.

In Jack’s case, on his way to school, while he is sitting and waiting for the train to pass, his instantaneous velocity will be zero. Though the instantaneous velocity was zero for a small part of the journey, the average velocity will not be zero.

UNIFORM MOTION AND NON-UNIFORM MOTION


When a body travels an equal distance in an equal interval of time, then the motion is said to be uniform motion.

 Examples of Uniform Motion.

i. Movement of the ceiling fan’s blades.

ii. Motion of Earth around the Sun

iii. Pendulum with equivalent amplitude on either side

 NON-UNIFORM MOTION

When an object covers unequal distances in equal intervals of time, it is said to be in non-uniform motion.

 Examples of Non-Uniform Motion.

i. Bouncing ball                                                                                                                                       

ii. Running horse                                                                                                                               

iii. Moving train

 Two types of non-uniform-motion

 (i) Accelerated Motion:

 When the motion of a body increases with time.

 (ii) De-accelerated Motion:

 When motion of a body decreases with time.

VELOCITY

The Rate of change of displacement is velocity. It is a vector quantity. Here the direction of motion is specified. Velocity is a vector quantity. Its value changes when either its magnitude or direction changes.

Velocity = Displacement / Time

where,  

u = initial velocity,

v = final velocity

t = Time

SI unit of velocity = ms-1

 INSTANTANEOUS VELOCITY

 Instantaneous velocity is the rate of change of position for a time interval which is very small, i.e. almost zero. In simpler words, the velocity of an object at a given instant of time is known as instantaneous velocity.

AVERAGE VELOCITY

 Average velocity is defined as the displacement (∆x) divided by the time interval (∆t) in which the displacement occurs.

Average Velocity = Initial Velocity + Final Velocity / 2

 vavg =  u + v / 2

 Where,

vavg = Average Velocity

u = Initial Velocity

v = Final Velocity

 For non-uniform motion in a given line, average velocity will be calculated in the same way as done in average speed.

DIFFERENCE  BETWEEN AVERAGE VELOCITY AND INSTANTANEOUS VELOCITY

 ACCELERATION

 The rate of change of velocity is called acceleration. It is a vector quantity. In non-uniform motion, velocity varies with time, i.e., the change in velocity is not 0. It is denoted by “a”

 Acceleration(a) = Change in Velocity / Time

(OR)        a = v − u / t

Where, t = time taken,

v = final velocity,

u = initial velocity.

If v > u, then ‘a’ will be positive (+ve).

 RETARDATION / DEACELERATION

 Deaceleration is seen in non-uniform motion during the decrease in velocity with time. It has the same definition as acceleration.

 Deaceleration = Change in velocity / Time

 OR,  a  = v - u  / t

Here, v < u, then ‘a’ = negative (-ve).

EQUATIONS OF MOTION

The motion of an object moving at uniform acceleration can be described with the help of three equations, namely

(i) v = u + at

(ii) v2 – u2 = 2as

(iii) s = ut + (1/2)at2

Where,

 u is the initial velocity,

v is the final velocity,

t is the time,

a is the acceleration

s is the displacement.

 DERIVATION OF VELOCITY-TIME RELATION BY GRAPHICAL METHOD



 

Velocity – Time Graph

 A body starts with some initial non-zero velocity at A and goes to B  with constant acceleration a.

From the graph BD = v (final velocity) – DC = u (initial velocity)…………..(eq 1).

BD = BC – DC……………..(eq 2)

We know acceleration = slope = BDAD

or AD = OC = t (time taken to reach point B)

Therefore,  BD = at………………….(eq 3)

Substitute everything we get: at = v – u

Rearrange to get v = u + at.

DERIVATION OF POSITION-TIME RELATION BY GRAPHICAL METHOD




Velocity – Time Graph

 A body starts with some initial non-zero velocity at A and goes to B  with constant acceleration a

Area under the graph gives Displacement as follows:

A (ΔABD) +  A (OADC)  =  1 / 2  (AD×BD)  +  (OA×OC)  ….  eqn(1)

OA = u , OC = t and BD = at

Substituting in (eq 1) we get 

s= ut+1 / 2at2

DERIVATION OF POSITION-VELOCITY RELATION BY GRAPHICAL METHOD


Velocity – Time Graph

 A body starts with some initial non-zero velocity at A and goes to B  with constant acceleration a

Displacement covered will be the area under the curve, which is the trapezium OABC.

We know the area of a trapezium is 

s = 2 (OA + BC)2 × OC

 OA = u, BC = v, and OC = t

Therefore,s = (v+u) 2 × t……………(eq1)

 We also know that, t = (v−u) a……………..(eq2)

 Substitute (eq 2) in (eq 1) and arrange to get

v2−u2=2as

UNIFORM CIRCULAR MOTION

If an object moves in a circular path with uniform speed, its motion is called uniform circular motion. In such a motion, the speed may be the same throughout the motion, but its velocity (which is tangential) is different at each and every point of its motion. Thus, uniform circular motion is an accelerated motion.

i. Velocity changes as direction keeps changing.

ii. Acceleration is constant.

iii. The uniform circular velocity is given by the following formula:

V =2πr / t

Uniform Circular Motion Examples

I. The motion of artificial satellites around the Earth is an example of uniform circular motion.

ii. The motion of electrons around its nucleus.

iii. The motion of the blades of the windmills. 

iv. The tip of the second hand of a watch with a circular dial shows uniform circular motion.


 


Comments

Post a Comment

please do not enter any spam link in the comment box

Popular posts from this blog

FORCE AND LAWS OF MOTION, Class IX, Part 3

Image
                                         In order to understand Newton’s second law of motion, we should first know the meaning of the term momentum of a moving body (or moving object). We know that a cricket ball is much heavier than a tennis ball. Suppose we throw a cricket ball and a tennis ball, both with the same speed or velocity. It will be found that more force is required to stop the cricket ball (Which has more mass) and less force is required to stop the tennis ball (which has less mass). We conclude that the force required to stop a moving body is directly proportional to its mass. If we throw two cricket balls of the same mass at different speeds or velocities, it will be found that more force is required to stop that cricket ball which is moving with higher velocity and less force is required to ...
Read more

FORCE AND LAWS OF MOTION, Class IX, Part 4

Image
  If a boy wearing roller skates stands facing a wall and pushes the wall with his hands, the boy finds himself moving backward, away from the wall. It appears as if the wall also pushes the boy away. When the boy exerts a force on the wall by pushing it with his hands, then the wall exerts an equal force on the boy in the opposite direction . Since the boy is wearing roller skates, the opposite force exerted by the wall makes him backward, the boy on roller skates is exerting force on the wall towards the left side. The wall exerts an equal force on the boy towards the right side. Due to this, the boy moves in the backward direction. From this discussion, we conclude that when a boy exerts a force on the wall, the wall exerts an equal and opposite force on the boy . This is just an illustration of Newton’s third law of motion. Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the second body. The force exerted by the firs...
Read more

CBSE CLASS XII PHYSICS NOTES PART I

Image
  CBSE CLASS 12 PHYSICS NOTES   CHAPTER 1 ELECTRIC CHARGES AND FIELDS   Here are the notes for CBSE Class 12 Physics Chapter 1 Electric Charges and Fields. This chapter explains the basics of electric charges and the electric fields they create. It includes key concepts like Coulomb's Law, which describes the force between two charged objects, and the superposition principle, which helps calculate the total electric field from multiple charges. You'll also learn about electric field lines, their properties and electric flux. These concepts are essential for understanding how electric forces work. You can find the full notes through the link provided for more details. INTRODUCTION Electric charges and fields are fundamental concepts in the study of electrostatics, a branch of physics that deals with the forces exerted by charges at rest. This study note will break down these concepts into digestible sections, providing clear explanations, examples, and important ...
Read more