CBSE Class 9 Science Revision Notes Chapter 8

CBSE Class 9 Science Revision Notes Chapter 8 – Motion

The concept of motion in Physics explains the relative positions of rest and motion for bodies that have the ability to move. These concepts form a solid base for Physics and include certain laws that apply to the occurrence of motion. Extramarks offers revision notes that are meticulously prepared to provide a clear understanding of this topic. Referring to these notes during quick revision sessions gives students the confidence they require to attempt related questions in the examinations. The Class 9 Science Chapter 8 Notes are easily accessible from the website. 

Access CBSE Class 9 Science Chapter 8 – Motion Notes

Introduction

The branch of physics that deals with moving objects is called mechanics. The two subparts of mechanics are kinematics (which focuses on the motion more than the cause) and dynamics (which focuses on the force being the source of motion).

Motion and Rest:

• If the surroundings of an object change during a given time frame, it is in motion.
• If the surroundings of an object stay stationary during a given time, it is at rest.
• Both these definitions of rest and motion are taken against a frame of reference. The object is compared to this frame of reference. 

The moving object moves with reference to an unmoving object, which in this case, is the frame of reference.

Types of Motion:

These are as follows:

1. Translatory motion
2. Rotatory motion
3. Vibratory motion

Translatory Motion:

• An object in translatory motion may follow either a straight line or a curved path, such that there is a change in the position of the object from one point in space to another. 
• If the object moves along a straight line, it is in rectilinear motion.
• If the object moves on a curved route, it is in curvilinear motion. 

Rectilinear Motion: A car on a straight road is in rectilinear motion. 

Curvilinear Motion: A car turning on a curve shows curvilinear motion.

Rotatory Motion: When concentric circles around the object’s axis of motion are formed, the object shows rotatory motion.

Vibrational Motion: The movement of particles of an object in a back-and-forth manner along a fixed spot is described as vibrational motion. 

Motion:

Uniform and Non-uniform motion:

Car A and Car B cover the following distances with respect to time. 

Car A :

Car B :

Uniform motion is the one in which equal distances are covered in equal intervals of time, like in Car A. Car B shows non-uniform motion as unequal distances are covered in equal intervals of time. 

 Speed:

To determine if one person or object is faster than the other, both objects can be made to cover the same distances in the same amount of time. This gives the value of the speed of the object, which is 

Speed =  distancetime = St

S.I Unit of speed is millimetres per second (m/s). It is a scalar quantity.

Uniform Speed:

A certain object is said to have uniform speed if it travels the same distance in the same period of time. In uniform speed, the force of friction or resistance is not considered. 

If a ball travels a distance of 10m every 2 secs between different points on a path, then it has a uniform speed of 5m/s, which is a constant speed. 

Variable Speed:

If the speed of the object per unit of time decreases due to the decreasing values of distance covered by the object, then it moves at a uniform speed. 

For example, a bouncing ball decreases the distances covered by it with every bounce. This is why the speed of the ball also varies.  

Average Speed: 

Average speed can be calculated by dividing the total distance travelled by the object by the total time required for the object’s journey. 

If the object covers S1 distance in time t1 , S2 in t2 , S3 in t3 etc. then,

Average speed = S1+S2+S3+ …+ Snt1+t2+t3 +….+tn

Instantaneous speed: 

The instantaneous speed of a moving body is the speed at which it is travelling at any given moment.

Velocity:

Raju can take different roads to reach his school. If driving at an average of 60 km/hour is taken every day, the time taken to reach is calculated on the basis of this information. 

Speed = Distance / Time

Just giving the speed of a moving object does not help determine the definitive position of that object, which is why the magnitude and direction of the object are crucial. Velocity can be introduced here. 

The distance travelled in a given direction in a given time, specified with both magnitude and direction gives the velocity. 

Velocity = distance travelled in a particular directiontime taken (t)

Velocity is the rate at which a displacement changes. 

Uniform velocity: When an object moves with equal velocity, it travels similar distances in the same period of time and the same direction.

Non-uniform velocity: Non-uniform velocity, also known as variable velocity, occurs when different distances are travelled in the same amount of time. It has to change velocity if the direction changes.

Acceleration:

The rate at which the velocity of a body changes with respect to time is called the acceleration of that body. A car can start moving from a position of rest, i.e zero speed and then increase its speed as it moves further. Again, if the car comes to a stop, it lowers its speed to zero. Acceleration hence implies the change in the speed, if it increases, but also focuses on the fact that the speed may decrease, remain constant or even become zero.

A change in the object’s speed, the direction of motion, or both may be referred to as acceleration. Thus, it is a vector quantity, having the units m/s2.

If v is the final velocity and u is the initial velocity, in a given time t, 

Then 

Acceleration = v – ut

Types of acceleration are positive acceleration, negative acceleration, zero acceleration, uniform acceleration and variable acceleration.

V – T Graph:

Velocity time graphs display the results for velocity on the y-axis against time on the x-axis. The following graphs are observed in the following cases:

1)Increasing acceleration:

• Uniform acceleration
• Non-uniform acceleration

2) Decreasing acceleration:

• Non-uniform retardation
• Uniform retardation

3) Zero acceleration  

A V-T graph is used to find the acceleration of an object and to deduce the equations of motion. It can also provide information about the distance travelled by a moving object. 

Speed-Time Graph

The steps involved in calculating a moving object’s distance using a speed-time graph are as follows:

1. The area under the speed-time graph, between the speed axis (y-axis) and the time (x-axis) is covered for calculation.
2. For non-uniform motion, there are smaller areas, which are then taken collectively to find the area under the speed-time graph.

For example, if a vehicle is moving at a constant speed of 60 km/hr, for 5 hours, then the distance covered by the vehicle would be: 

S = v x t

S = 60 X 5

= 300 km, which is the distance covered by the car. 

Find the region bounded by the time axis and the speed-time graph to determine the distance travelled by a moving object. When an object moves in an irregular manner, its distance travelled grows incrementally as its speed rises. The speed is constant from 0 – t1, t1 – t2, t2 – t3, etc.

The figure below shows the motion of an object travelling at a varied speed.

An object moving at a variable speed is depicted on a speed-time graph.

Measurement of Distance:

The overall distance covered by the object throughout the period.

Area of rectangle 1 + Area of rectangle 2 +…+ Area of rectangle 6 = 0-t6.

Motion

Equations of Motion:

The interrelationship between speed, time, distance covered, acceleration and other variables is given using the equations of motion. 

Three equations of motion are

• v = u + at

• S = ut + 12at2

• v2–u2=2as

Derivations:

• First Equation of Motion

A particle is moving in a straight path with constant acceleration ‘a’ ; Let its position at t=0 be A, having initial velocity u and final velocity v at t=t

Then, 

a= v-ut

v-u = at

v = u + at

Graphically :

An object moving in a straight line with a uniform velocity u, has u as its initial velocity. It is given a uniform acceleration at t=0. As time becomes t=t, the velocity increases and acceleration to v (final velocity) increases. Distance covered by the object is S in time t.

The slope of the graph depicts the acceleration.

AB = BCAC=v-ut-0

a = v-ut

v-u = at

v=u+ at

• Second Equation of Motion 

We know, 

Average Velocity = Total distance travelled / Total Time taken = S / t

also,

Average velocity = u + v2

Thus, 

St= u +v2

Substituting, v = u + at in place of v

St= u + u + at2= (2u + at)t2

S = ut + 12at2

Graphically :

Let 

u  be the initial velocity 

‘a’ be the acceleration 

S is the distance covered in time t 

Then the area inside the velocity-time graph, gives the distance travelled. 

Distance travelled = Area of the trapezium

= area of rectangle + area of triangle

= t u + 12(v-u) t

= ut + 12(v-u) t

v = u +at

S = ut + 12at t = ut + 12at2

• Third Equation of Motion

From 

v = u+at

v-u=at                                  ……..(1)

Average velocity = St        ………(2)

Average velocity = u +vt   ……….(3)

Thus, 

u +vt   = St 

Multiplying (1) to this

(v-u)(v+u) = at 2St

(v-u)(v+u) = 2as

(v2–u2) = 2aS

Graphically,

Let ‘u’  be the initial velocity 

‘a’ be the acceleration 

Then the area inside the velocity-time graph, gives the distance travelled.

S = Area of trapezium

= 12(b1+b2) h=12(u+v)t

But, a=v-ut or t=v-ua 

Thus,

S = 12(u+v)(v-u)a

(v+u)(v-u) = 2as

v2–u2=2as

Examples of Uniform Circular Motion:

Since an object’s direction changes at every instant on a circular path, an object in uniform circular motion is an example of a non-uniform motion. Examples are :

1. A turn taken by a vehicle at a constant speed 
2. Spinning of the hammer before throwing it, by a hammer-throw athlete
3. A loop created by the aeroplane. 

Expression for Linear Velocity

If a circular path of radius ‘r’ is covered in t seconds, then the velocity v can be determined as :

V = distance travelledtime

Which would be equal to the circumference of the circular path = 2r

Linear velocity = 2rt

Class 9 Science Chapter 8 Revision Notes – Free Download

Contents of Class 9 Science Motion Notes

Class 9 Science introduces students to many concepts that form the core foundation for young analytical scientific minds. Class 9 Science Chapter 8 – Motion explores core terms such as speed, velocity, distance, displacement and the equations of motion. The chapter begins with an introduction to acceleration and concludes with circular motion. Extramarks curates revision notes for students to prepare well before their exams. These notes are easily accessible and are a great source to enhance the experience of learning. 

Subtopics Covered in Class 9 Chapter 8 Science Notes

Many concepts are covered in Extramarks’ Class 9 Science Chapter 8 Notes. A few of these are given below.

• Concepts of motion and rest
• Motion and its types : translational, vibrational, rotatory
• Distance and displacement, the difference between the two
• Types of motion : uniform and non-uniform
• Speed and velocity : uniform and variable
• Acceleration and its relation to velocity
• Graphs, like the V-T graph and the Speed-Time graph
• Laws of Motion, and the three equations of motion