These comprehensive RBSE Class 9 Science Notes Chapter 8 Motion will give a brief overview of all the concepts.

## RBSE Class 9 Science Chapter 8 Notes Motion

→ A body is said to be in motion when it changes its position with respect to a reference point.

→ A reference point is used to describe the location of an object. An object can have many reference points.

→ An object is said to be at rest if it does not change its position with respect to its surroundings or reference point.

→ Types of motion : Motion can be of different types according to the type of path by which object goes through –

(i) Linear motion : If a body moves in a straight line or path then it is called linear motion. Examples – Moving car on a straight road, Ball dropped from the roof of a building.

(ii) Circulatory/rotational motion : If a body moves around a fixed point or in circular path, it is known as circular motion.

Examples – Motion of an electric fan, merry-go-round.

(iii) Oscillatory/vibratory motion : A body has oscillatory motion if it moves to and fro about a fixed point.

Example – Motion of a pendulum of wall clock.

→ Distance : It is the length of the actual path between the initial and final position of a moving object in a given time interval i.e. the total path length covered by an object during a given time is known as distance. It is a scalar quantity which requires magnitude but no direction.

→ Displacement: It is the shortest distance measured from initial position to final position. Displacement is a vector quantity because it requires both magnitude and direction.

→ Uniform Motion : A body is said to be in uniform motion when an object covers equal distance in equal time intervals. Example – A train running at a speed of 120 km/h.

→ Non-uniform Motion : The motion is said to be non-uniform motion, when the body covers unequal distances in equal intervals of time. Examples – An object thrown vertically upwards, A freely falling stone under the action of gravity.

→ Speed : The distance travelled by the body per unit time is the speed of a body. Its unit is m/s or ms^{-1}.

→ There are mainly three types of speed –

(i) Uniform speed or Constant speed : An object is said to be moving with uniform speed if it covers equal distances in equal intervals of time. For example – Let a bus start travelling with a constant speed of 60 km/h. It means that the bus is covering a distance of 60 km in every one hour.

(ii) Non-uniform speed or Variable speed : If an object covers unequal distances in equal intervals of time or equal distances in unequal intervals of time, then the speed of the object is said to be non-uniform speed. For example – Moving car in traffic, falling ball from certain height etc.

(iii) Instantaneous speed : The speed of a moving body at a particular instant during its motion is called instantaneous speed. Average Speed : The average speed of the body is the total distance travelled by the total time taken to cover this distance.

→ Conversion of speed units :

Change from km/hr to m/s → 1 km/hr = \(\frac{5}{18}\) m/s

Change from m/s to km/hr → 1 m/s = \(\frac{18}{5}\) km/hr.

→ Velocity : The speed of an object moving in a particular direction is known as velocity. Its S.I. unit is m/s. It is a vector quantity.

It can be positive, negative or zero.

→ Average Velocity : For non-uniform motion, average velocity will be calculated in the same way as done in average speed.

In case of uniform motion, average velocity can be calculated as follows :

→ Acceleration : It is the measure of the change in the velocity of an object per unit time. It is seen in non-uniform motion. Its S.I. unit is m/s^{2} or ms^{-2}.

It is a vector quantity.

→ Uniform Acceleration : An object is said to have a uniform acceleration if it travels along a straight path and its velocity changes by equal amounts in equal time intervals.

Example – Motion of stone falling freely from the top of cliff.

→ Non-uniform Acceleration : An object is said to have a non-uniform acceleration if the velocity of an object changes by unequal amount in equal intervals of time. Example – Motion of a railway train or a bus from one township to another.

→ The direction of acceleration is same if velocity is increasing in the same direction, such acceleration is called positive acceleration.

→ The direction of acceleration becomes opposite as that of velocity if velocity is decreasing in a direction, such acceleration is called negative acceleration. It is also called De-acceleration or retardation.

→ Graphical representation of motion :

(i) Distance-time (s/t) graph :

(ii) Velocity-time (v/t) graph :

→ Equations of uniformly accelerated motion :

The equations of motion are:

(i) v = u + at (ii) s = ut + \(\frac{1}{2}\) at^{2} (iii) v^{2} = u^{2} + 2as

Where, u = initial velocity, v = final velocity

s = distance travelled by the object

a = with uniform acceleration

t = time.

→ Uniform Circular Motion : The motion of an object along a circular path around a fixed point with uniform speed is known as the circular motion. In this, the speed of a moving object remains constant but velocity of the object continuosly changes due to change in its direction of motion.

The time taken by an object, undergoing uniform circular motion, to complete one revolution along the circular path is called revolution period.

## Leave a Reply