# Chapter 8 – Motion Class 9 Notes

## Chapter 8 Motion Class 9 Notes

Topics to be discussed –

Here is the list of topics that we are going to study in this Motion Chapter of Class 9.

1. What is Motion?
2. Distance and Displacement
3. Physical Quantities
4. Types of Motion
5. Speed
6. Acceleration
7. Distance time Graph
8. Equations of uniform accelerated Motion
9. Derivation of Equations of Motion by Graphical Method
10. Uniform Circular Motion

### Motion

What is Motion?

A body is said to be in motion, if its position changes with time, with respect to a reference point.

What is Distance?

Distance between two points is the total path length covered in travelling between these two points.

Example- Let say AB = 4 m and BC = 7 m, then distance is the total path travelled/covered, i.e., AB+BC = 4 m + 7 m = 11 m.

• SI Unit of Distance is metre (m).

What is Displacement?

It is th estraight line path between initial and the final positions. It is the lenght od shortest line between initial and final position.

• SI Unit of displacement is metre (m).

Physical Quantities

There are two types of physical quantities –

1. Scalar Quantities: defined by only magnitude. Example- distance
2. Vector Quantites: defined by both magnitude and direction. Example- displacement.

### Types of Motion

• Uniform Motion
• Non-Uniform Motion

#### Uniform Motion

What do you understand by Uniform Motion?

When an object covers equal distance is covered in equal interval of time, it is said to be in uniform motion.

What do you understand by Non-Uniform Motion?

When unequal distances are covered in equal interval of time.

What is Speed?

The distance covered per unit time is called speed.

Example- distance is 10 cm, and time taken is 2 seconds.

Speed = Distance/time = 10/2 = 5 m/s.

• The SI Unit of Speed is metre/second (m/s).
• Speed is a scalar quantity.

What is Average Speed?

The average speed of a body is the total distance travelled divided by total time taken to cover this distance.

What is Velocity?

Velovity of a body is the distance travelled per unit time in a given direction.

or

Velocity of a body is the displacement produced per unit time .

Velocity = Displacement/time taken

What is an Odometer?

Odometer is a device or an instrument that measures the distance covered by a vehicle.

What is a Speedometer?

Speedometer is a device or an instrument that measures speed.

Average Velocity = (Initial Velocity + Final Velocity)/2

What is Acceleration?

Acceleration of a body is defined as the rate of change of its velocity with time.

or, Acceleration = Change in velocity/time-taken

a = (final velocity- initial velocity)/time = (v-u)/t

• Acceleration is a vector quantity.
• SI Unit of accelration is metre/second² (m/s²).

### Graphical Representation of Motion

In the graphical representation we have to study two types of graphs- displacement-time graph and velocity-time graph.

In these graphs we will come to know about two terms i.e., slope and Area under graph.

The slope will help us to find velocity in case of displacement-time graph, and the slope and area in case of velocity-time graph will help us to find acceleration and displacement respectively.

#### Distance-Time Graphs

Slope- will give velocity

Area- nothing

Important Points:

• The distance time grpah of a body moving at uniform speed is always a straight line.
• It can be used to calculate the speed of a body.
• If the speed of a body is non-uniform, then the graph between distance travelled and time is a curved line (parabola).
• If the displacement time graph is drawn, then it will specifically repreesnt velocity.
• The speed-time graph is also known as velocity time graph.

#### Velocity-Time Graph

Slope- It will give acceleration

Area- It will provide displacement.

### Equations of Uniformly Accelerated Motion

First Equation of Motion

The first eqaution of Motion is – v = u + at

where, v = Final velocity, u = Initial Velocity, t = time, and a = acceleration

• It gives the velocity acquired by a body in time t.

Second Equation of Motion

It gives the distance travelled by body in time ‘t’.

The 2nd equation of motion is – s = ut + ½at²

Third Equation of Motion

The third equation of motion is – v² = u² + 2as

• It gives the velocity acquired by a body in travelling a distance ‘s’.

Note: For Derivation Watch the videos below

### Derivation of Equations of Motion by Algebraic Method

Let us now discuss the derivation of equations of motion. In class 9 Motion we will study two methods to derive the equations of motion.

• Algebraic Method
• Graphical Method

In the higher classes you will be studying another method called the Calculus method.

The videos which discuss the derivation of equations of motion, are in hindi and English language (mixed). I request the students to click the subtitle if not able to understand hindi.

#### Derivation of Equations of Motion by Algebraic Method

Watch The video for Derivation.

### Uniform Circular Motion

What is Uniform Circular Motion?

When a body moves in a circular path with a uniform speed (constant speed), its motion is called uniform circular motion.

#### Examples of Uniform Circular Motion

• Artificial satellites move in a uniform circular motion around the earth.
• The moon moves in uniform circular motion around the earth.
• The earth moves around the sun in uniform circular motion.
• An athelete (or cyclist) moving on a circular track with a constant speed exhibits uniform circular motion.
• The tip of a seconds’ hand of a watch exhibits uniform circular motion on the circular dial of the watch.