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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:
 Translatory motion
 Rotatory motion
 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 backandforth manner along a fixed spot is described as vibrational motion.
Motion:
Uniform and Nonuniform motion:
Car A and Car B cover the following distances with respect to time.
Car A :
Time (s)  0  5  10  15  20  25  30  35 
Distance (m)  0  10  20  30  40  50  60  70 
Car B :
Time (s)  0  5  10  15  20  25  30  35 
Distance (m)  0  10  15  20  30  60  65  75 
Uniform motion is the one in which equal distances are covered in equal intervals of time, like in Car A. Car B shows nonuniform 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.
Nonuniform velocity: Nonuniform 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 yaxis against time on the xaxis. The following graphs are observed in the following cases:
1)Increasing acceleration:
 Uniform acceleration
 Nonuniform acceleration
2) Decreasing acceleration:
 Nonuniform retardation
 Uniform retardation
3) Zero acceleration
A VT 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.
SpeedTime Graph
The steps involved in calculating a moving object’s distance using a speedtime graph are as follows:
 The area under the speedtime graph, between the speed axis (yaxis) and the time (xaxis) is covered for calculation.
 For nonuniform motion, there are smaller areas, which are then taken collectively to find the area under the speedtime 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 speedtime 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 speedtime 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 = 0t6.
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= vut
vu = 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=vut0
a = vut
vu = 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 velocitytime graph, gives the distance travelled.
Distance travelled = Area of the trapezium
= area of rectangle + area of triangle
= t u + 12(vu) t
= ut + 12(vu) t
v = u +at
S = ut + 12at t = ut + 12at2
 Third Equation of Motion
From
v = u+at
vu=at ……..(1)
Average velocity = St ………(2)
Average velocity = u +vt ……….(3)
Thus,
u +vt = St
Multiplying (1) to this
(vu)(v+u) = at 2St
(vu)(v+u) = 2as
(v2–u2) = 2aS
Graphically,
Let ‘u’ be the initial velocity
‘a’ be the acceleration
Then the area inside the velocitytime graph, gives the distance travelled.
S = Area of trapezium
= 12(b1+b2) h=12(u+v)t
But, a=vut or t=vua
Thus,
S = 12(u+v)(vu)a
(v+u)(vu) = 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 nonuniform motion. Examples are :
 A turn taken by a vehicle at a constant speed
 Spinning of the hammer before throwing it, by a hammerthrow athlete
 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 nonuniform
 Speed and velocity : uniform and variable
 Acceleration and its relation to velocity
 Graphs, like the VT graph and the SpeedTime graph
 Laws of Motion, and the three equations of motion
FAQs (Frequently Asked Questions)
1. Define Distance and Displacement
 Distance refers to the total length of the path travelled by any object
 Displacement is the net change in position of the object.
2. Differentiate between Distance and Displacement
Distance  Displacement 
The total length of the path an object travels  The shortest distance between the initial and final positions of an object in travel 
It is a scalar quantity, so only magnitude is considered  It is a vector quantity and has both magnitude and direction 