Chapter-Describing Motion Around Us Science Exploration class 9 in english Medium CBSE Notes

Chapter 4. Describing Motion Around Us

This section explains the concepts of distance, displacement, average speed, average velocity and acceleration. These physical quantities help us describe motion accurately and compare the movement of different objects.

9. Distance Travelled

Whenever an object moves, it covers a certain path. The total length of this actual path is called the distance travelled. Distance depends only on the path followed and not on the direction of motion. 

Definition

Distance is the total length of the actual path travelled by an object between two points.

Characteristics

• Scalar Quantity – It has only magnitude.
• No Direction – Direction is not considered.
• Always Positive – Distance can never be negative.
• Depends on Path – Different paths may have different distances.

SI Unit

The SI unit of distance is metre (m).

10. Displacement

Displacement represents the shortest straight-line distance between the initial and final positions of an object. Unlike distance, it also specifies the direction of motion. 

Definition

Displacement is the shortest distance between the initial and final positions of an object in a specified direction.

Characteristics

• Vector Quantity – It has both magnitude and direction.
• Shortest Distance – Always the minimum possible distance.
• May be Zero – If an object returns to its starting point, displacement becomes zero.
• Independent of Path – Depends only on the initial and final positions.

SI Unit

The SI unit of displacement is metre (m).

11. Difference Between Distance and Displacement

Distance	Displacement
Total length of the actual path travelled.	Shortest distance between two positions.
Scalar quantity.	Vector quantity.
No direction.	Direction is essential.
Always positive.	May be positive, negative or zero.
Depends on the path followed.	Depends only on initial and final positions.

12. Speed

Speed tells us how fast an object moves. It is one of the most commonly used quantities to describe motion. 

Definition

Speed is the distance travelled by an object per unit time.

Formula

Speed = Distance ÷ Time

SI Unit

The SI unit of speed is metre per second (m/s).

Other Unit

Kilometre per hour (km/h) is commonly used for vehicles.

13. Average Speed

An object may move at different speeds during a journey. Therefore, we calculate the average speed for the entire journey. 

Definition

Average speed is the total distance travelled divided by the total time taken.

Formula

Average Speed = Total Distance Travelled ÷ Total Time Taken

Important Points

• Scalar Quantity – Has only magnitude.
• Based on Distance – Uses the total path travelled.
• Always Positive – Cannot have a negative value.

14. Velocity

Velocity describes both the speed and the direction of motion. Two objects may have the same speed but different velocities if they move in different directions. 

Definition

Velocity is the displacement of an object per unit time in a specified direction.

Formula

Velocity = Displacement ÷ Time

Characteristics

• Vector Quantity – Has magnitude and direction.
• Depends on Displacement – Not on the total path.
• May be Positive or Negative – Depends on the direction of motion.

15. Difference Between Speed and Velocity

Speed	Velocity
Based on distance.	Based on displacement.
Scalar quantity.	Vector quantity.
No direction.	Direction is necessary.
Always positive.	May be positive, negative or zero.

16. Average Velocity

When the velocity of an object changes during motion, the average velocity gives the overall rate of displacement. 

Definition

Average velocity is the total displacement divided by the total time taken.

Formula

Average Velocity = Total Displacement ÷ Total Time Taken

17. Acceleration

Sometimes an object speeds up, slows down or changes its direction. Such changes indicate acceleration. 

Definition

Acceleration is the rate of change of velocity with respect to time.

Formula

Acceleration = Change in Velocity ÷ Time Taken

SI Unit

The SI unit of acceleration is metre per second squared (m/s²).

Types of Acceleration

• Positive Acceleration – Velocity increases with time.
• Negative Acceleration (Retardation) – Velocity decreases with time.
• Zero Acceleration – Velocity remains constant.

ATP Education Tip

For CBSE examinations, remember these four formulas: Speed = Distance ÷ Time, Average Speed = Total Distance ÷ Total Time, Velocity = Displacement ÷ Time, and Acceleration = Change in Velocity ÷ Time. Also remember that Distance & Speed are scalar quantities, whereas Displacement, Velocity and Acceleration are vector quantities.

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