Vidyarthi Academy

Home NCERT Solutions Chapter Notes Test Papers Contact Us
CBSE NOTES CLASS 9 SCIENCE CHAPTER 8

MOTION

Motion in a Straight Line

Distance

Displacement

Uniform Motion

Average speed

Velocity

Average velocity

Acceleration

D-T Diagram for Uniform Motion

D-T Diagram for Non-Uniform Motion

V-T graphs

Equations of Uniformly Accelerated Motion along a Straight Line

Uniform Circular Motion

CBSE NOTES CLASS 9 SCIENCE CHAPTER 8

MOTION

Motion in a Straight Line

Motion of objects along a straight line is called rectilinear motion. The object can move forward or backward, up or down etc.

Distance

The distance travelled by an object, without considering the direction, is called the path length. It is a scalar quantity.

Displacement

Displacement is the shortest distance from the initial to the final position of an object.

It is a vector quantity.

A displacement vector represents the length and direction of the straight line joining the initial and final point.

The magnitude of displacement is less than or equal to the actual distance (path length) travelled by the object in the given time interval.

Displacement ≤ Distance

The two quantities are equal only if the object does not change its direction during the course of its motion.

Vidyarthi Academy

Uniform Motion

If an object moving along a straight line covers equal distances in equal intervals of time, it is said to be in uniform motion along a straight line.

Average speed

Average speed is defined as the total distance travelled divided by the total time interval during which the motion has taken place

Average speed = Total distance travelledTotal time taken

Or v = st

Speed is a scalar quantity.

Its SI unit is meter/sec. It is never negative.

Velocity

The rate of change of position or displacement of an object, in a particular direction with respect to time is called velocity.

It is equal to the displacement covered by an object a unit time.

In case the velocity of the object is changing at a uniform rate, then average velocity is given by the arithmetic mean of initial velocity and final velocity for a given period of time.

Average velocity is defined as the change in position or displacement divided by the time intervals, in which the displacement occurs,

vav = Inital velocity + Final velocityTotal time taken = u + vΔ t

Also average velocity

vav =displacementtime interval=s2-s1t2-t1

Acceleration

The rate of change of velocity of an object with respect to time is called its acceleration.

acceleration=change in velocitytime taken

If the velocity of an object changes from an initial value u to the final value v in time from t1 to t2 , the acceleration a is,

a = v-ut2- t1

It is a vector quantity. Its SI unit is meter/sec2.

It may be positive, negative or zero.

Positive acceleration

If the velocity of an object increases with time, its acceleration is positive, that is x and a have same direction.

Negative acceleration

If the velocity of an object decreases with time, its acceleration is negative. The negative acceleration is also called retardation or deceleration.

D-T Diagram for Uniform Motion

Vidyarthi Academy

Vidyarthi Academy

Vidyarthi Academy

D-T Diagram for Non-Uniform Motion

Vidyarthi Academy

V-T graphs

Vidyarthi Academy

Vidyarthi Academy

Vidyarthi Academy

Equations of Uniformly Accelerated Motion along a Straight Line

1. v = u + at …(Velocity-Time Relation)

2. s = ut + 12a t2 …(Position-Time Relation)

3. v2 – u2 = 2as …(Position-velocity Relation)

Proofs (Graphical method)

Vidyarthi Academy

1. From the diagram,

2. Displacement s = Area under the v-t curve between instants 0 and t

3. Displacement s = Area under the v-t curve between instants 0 and t

Uniform Circular Motion

When an object moves in a circular path with uniform speed, its motion is called uniform circular motion.

v =2πrT

Where, T is the time taken by the object in circular motion to complete one round.

Although the speed is constant, the velocity is changing in direction.

The direction of velocity is along a straight line tangential to the circular path.

There is a centripetal force and centripetal acceleration acting on the object towards the centre of the circular path.

ac=v2r 

And

Fc= mv2r