Here we come with Motion for CBSE Class 9. It is very important and base chapter of Motion for CBSE Class 9. Without having clear idea about Motion we cannot move forward to further chapters. So before starting, have a look on table of content which we learn hear.

**Table of Content**

- What is motion?
- Types of motion
- Mechanics
- Concept of a point object, rest and motion
- Scalar and vector quantities
- Distance and displacement
- Uniform and non-uniform motion
- Speed and Velocity
- Acceleration

**What is motion?**

If any objects change its position with reference to its immediate surrounding then it said to be the object is in motion. In other words, the movement of an object is known as the motion of the object.

If we think about the example of motion then definitely we should mention that Cars, motorcycles, cycles, scooters, buses, rickshaws, trucks etc. running on the road, Birds flying in the sky, Fish swimming in water, as small like movement of atom or as big like movement of planet. All these objects are in motion.

**Types Of Motion**

As per syllabus of Motion for CBSE Class 9, we can categorize motion in three tipes.

- Linear motion
- Circular (or rotational) Motion
- Vibratory motion

**Linear motion :** We now linear means straight line or path. So A body has linear motion if it moves in a straight line or path.

Ex. Motion of a moving motorcycle on a straight road.

**Circular (or rotational) Motion :** A body has circular motion if it moves around a fixed point. A vertical passing through the fixed point around which the body moves is known as axis of rotation.

Ex. Motion of an electric fan. Axis of rotation Object

** Vibratory motion :** As name suggest vibratory means there should be some vibration. A body has vibratory motion if it moves to and fro about a fixed point.

Ex. Motion of a pendulum of a wall clock.

**Mechanics : **

If we want to know what is mechanics? Then we have to know that the branch of physics which deals with the motion of non-living objects in everyday life is called mechanics or Classical mechanics. It is of two types. (i) Statics and (ii) Dynamics

**STATICS :** Statics means bodies at rest under the effect of different forces.

**DYNAMICS :** Dynamics means the bodies in motion. It is further of two types:

**(i) Kinematics :** Kinematics, which is derives from a Greek word kinema meaning motion,is a branch of Physics, which deals with the motion of a body without taking into account the cause of motion.

**(ii) Dynamics proper :** Dynamics proper, which is derived from a Greek word dyna meaning power it is a branch of Physics, which deals with the motion of bodies by taking into account the cause of motion (force).

**Concept Of A Point Object, Rest And Motion**

##### Point object :

in physics the concept of point objects is very important. Because every law of physics is created to treat the object as point object. So an extended object can be treated as a point object when the distance travelled by the object is much greater than its own size.

**“A point object is one, which has no linear dimensions but possesses mass.”**

Ex. Study of motion of a train travelling from Kolkata to New Delhi.

##### Rest :–

A body is said to be at rest when its position does not change with time respect to the observer or its surrounding.

##### Motion :–

A body is said to be in motion when its position changes with time respect to the observer or its surrounding.

##### Describe motion :

When a hut, is observed by a man sitting on a bench, the hut is at rest. This is because position of the hut is not changing with respect to that man.

Now, When the same hut is observed by a man sitting in a car moving with a velocity ν, then the hut is moving with respect to that man because the position of hut is changing with respect to the man.

**Rest and motion are relative terms: **

As per syllabus of Motion for CBSE Class 9 ,there is nothing like absolute rest. This means that an object can be at rest and also in motion at the same time i.e. all objects, which are stationary on earth, are said to be at rest with respect to each other, but with respect to the sun are making revolutions at 30 kmh^{–1}. In order to study motion, therefore, we have to choose a fixed reference position or point with respect to which the motion has to be studied. Such a point or fixed position is called a reference point or the origin. In order to describe the motion of an object we need to keep in mind three things.

- (i) The distance of the body from a reference point. This reference point is called the origin of the motion of the body.
- (ii) The direction of motion of the body.
- (iii) The time of motion.

**Scalar And Vector Quantities**

**Scalar Quantity :–** A quantity that has only magnitude no direction is called a scalar quantity.

Ex. mass, time, distance, temperature etc.

**Vector Quantity :–** The physical quantity that has magnitude as well as direction are called vector quantity.

Ex. velocity, acceleration, force, displacement etc.

#### **Distance And Displacement**

##### Distance :

The length of the actual path between the initial or starting and the final position of a moving object in the given time interval is known as the distance traveled by the object.

Distance = total length traveled that is AB shown in figure.

Distance is a scalar quantity.

Unit In SI system : **metre (m)**

In CGS system : **centimetre (cm)**

Large unit **Kilometre (km)**

##### Displacement:

The shortest distance between the initial or starting position and the final position of a moving object in the given time interval is known as the displacement of the object.

Displacement of an object may also be defined as the change in position of the object in a particular direction.

That is,

Displacement of an object = **Final position – Initial position of the object.**

Displacement of an object may be zero but the distance traveled by the object in never zero.

Distance traveled by an object is either equal or greater than the magnitude of displacement of the object.

Displacement = Minimum distance between A to B shown in dotted line in figure. it is a vector quantities.

Units In SI system :** metre (m)**

In CGS system : **centimetre (cm)**

**Uniform And Non-Uniform Motion**

A moving body may cover equal distances in equal intervals of time or different distances in equal intervals of time. On the basis of above assumption, the motion of a body can be classified as uniform motion and non-uniform motion.

##### Uniform motion:

When a body covers equal distances in equal time intervals however small may be time intervals, the body can describe as a uniform motion.

**Example of uniform motion –**

- (i) An aeroplane flying at a speed of 600 km/h
- (ii) A train running at a speed of 120 km/h

##### Non-uniform motion:

When a body covers unequal distances in equal intervals of time, the body is in a non-uniform motion.

**Example of non-uniform motion –**

- (i) An aeroplane running on a runway before taking off.
- (ii) A freely falling stone under the action of gravity.

#### SPEED

Speed of a body is the distance travel by the body per unit time. In other word speed is the rate of change of motion.

Speed = D**istance ****traveled**** /Time taken**

If a body covers a distance S in time t then speed,

V=

Unit: In SI system :** m/s or ms ^{–1}**

In CGS system :

**cm/s or cms**

^{–1}Other

**km/h or kmh**

^{–1} **Uniform speed :-** When a body covers equal distance in equal intervals of time, the body is to be moving with a uniform speed or constant speed.

Ex. A train running with a speed of 120 km/h

**Non-uniform speed :-** When a body covers unequal distances in equal intervals of time, the body is said to be moving with non-uniform speed or variable speed.

Ex. A car running on busy road.

Average speed :- The average speed of the body in a given time interval is defined as the total distance traveled, divided by the time interval.

Average speed = **Total distance traveled /Total time taken**

#### **Velocity**

The velocity of a body is the displacement of a body per unit time.

Velocity =

The displacement covered by a body per unit time or the speed of a body in specified direction is called the velocity.

Unit: In SI system : **m/s or ms ^{–1}**

In CGS system :

**cm/s or cms**

^{–1}Other km/h or kmh–1, km/min.

##### Uniform velocity

When a body covers equal displacement in equal interval of time, the body can said to be moving with a uniform velocity.

**Conditions for uniform velocity :–**

(i) The body must cover equal displacement in equal intervals of time.

(ii) The direction of motion of the body should not change.

Ex. A train running towards south with a speed of 120 km/h.

##### Non-uniform velocity/variable velocity :

When a body covers unequal displacement in equal intervals of time, the body can said to be moving with variable velocity.

We can explain variable velocity in another way.

When a body covers equal distance in equal intervals of time, but its direction changes, then the body is said to be moving with variable velocity.

**Conditions for variable velocity :–**

(i) It should cover unequal displacement in equal intervals of time.

(ii) It should cover equal distances in equal intervals of time but its direction must change.

Ex. (i) A car running towards north on a busy road has a variable velocity as the displacement covered by it per unit time changes with change in the road condition.

##### Average velocity :

An average velocity is a calculation of Total displacement divided by total time.

Average velocity = **Total displacement / Total time taken**

**Acceleration**

The rate of change of velocity of a moving body with time is called acceleration.

Acceleration = **Change in velocity /Time taken for change .**

But we know that change in velocity = **Final velocity – Initial velocity**. So we can formulate acceleration in other way.

Acceleration = **Final velocity – Inital velocity / Time taken for change**

Ie, a =

** If body moves with uniform velocity, then v = u and then acceleration is zero i.e. a = o.

**Unit of accelration**

In SI system is m/s2 or ms–2

In CGS system is cm/s2 or cms–2

**Positive Acceleration :** If the velocity of an object increases with time in the direction of the motion of the object, then the acceleration of the body is known as positive acceleration.

In this case, the object pick up the speed in a particular direction (i.e., velocity). For example, if an object starts from rest and its velocity goes on increasing with time in the direction of its motion, then the object has positive acceleration. The direction of positive acceleration is in the direction of motion of the object.

**Negative Acceleration :**If the velocity of an object decreases with time, then the acceleration of the object is known as negative acceleration.

It is written as **(– a)**

For example, if an object moving with certain velocity is brought to rest then the object is said to have negative acceleration.

##### Let see how you learn Motion for Class 9 CBSE ?

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`Question 1 of 5`

`1. Question`

`A body goes from A to B with a velocity of 20 m/s and comes back B to A with a velocity of 30 m/s. The`

average velocity of the body during the whole journey is`Correct`

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`2. Question`

`Distance travelled by a freely falling body is proportional to :-`

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`3. Question`

`Which of the following is not vector quantity?`

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`4. Question`

`If the time-displacement graph of a particle is parallel to the time-axis, then velocity of the particle is :-`

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`5. Question`

`The motion in animals is called locomotion. Mechanics deals with the motion of non living object.`

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