Class 9 Motion Notes

Class – 9 CBSE

Chapter –  Motion 

Class 9 Physics Chapter 1 notes

1. Motion:
   An object is said to be in motion if it changes its position with respect to its surroundings.
   Example: A car moving on the road or a fan rotating.
2. Rest:
   A body is at rest if it does not change its position with respect to its surroundings.
   Example: A book lying on a table.
3. Displacement:
   The shortest distance between the initial and final positions of an object.
   SI Unit: metre (m)
   Example: If you walk from your home to a friend’s house in a straight line, that’s displacement.
   
4. Vector:
   Physical quantities having both magnitude and direction.
   Example: Velocity, force.
5. Scalar:
   Physical quantities having only magnitude and no direction.
   Example: Speed, distance, time.
   
6. Uniform Motion:
   A body covers equal distances in equal intervals of time.
   Example: A car moving at a constant speed of 60 km/h on a highway.
7. Non-uniform Motion:
   A body covers unequal distances in equal time intervals or equal distances in unequal time intervals.
   Example: A car stuck in traffic keeps changing speed.
   
8. Speed:
   Speed = Distance / Time
   SI Unit: m/s
   Example: A train covers 300 km in 3 hours, so speed = 100 km/h.
9. Average Speed:
   Average Speed = Total Distance Travelled / Total Time Taken
10. Locomotion:
    Movement of animals from one place to another.
    Example: A dog running, a bird flying.
11. Velocity:
    The displacement covered per unit time in a given direction.
    Velocity = Displacement / Time
    SI Unit = m/s
12. Uniform Velocity:
    Equal displacement in equal intervals of time in a particular direction.
13. Variable Velocity:
    Displacement changes in unequal intervals of time or direction keeps changing.
    Example: A bus moving in a city street with turns and stops.
14. Acceleration:
    Rate of change of velocity.
    Acceleration = (Final Velocity – Initial Velocity) / Time
    SI Unit = m/s²
15. Retardation or Deceleration:
    Negative acceleration; when the velocity of an object decreases.
    Example: A bicycle slowing down before stopping.
16. Uniform Acceleration:
    Velocity changes by equal amounts in equal intervals of time.
    Example: Free fall under gravity.
17. Variable Acceleration:
    Velocity changes by unequal amounts in equal time intervals.
    Example: A car speeding up irregularly in traffic.

Equations of Motion (For Uniform Acceleration)

1. v = u + at
2. s = ut + ½ at²
3. v² = u² + 2as
   Where:

• s = Distance
• u = Initial velocity
• v = Final velocity
• a = Acceleration
• t = Time

Graphs

18. Distance-Time Graph:

1. • Shows time vs distance.
   • Slope = Speed
     Example: A straight line shows uniform speed.

19. Velocity-Time Graph:

1. • Shows time vs velocity.
   • Slope = Acceleration
   • Area under the graph = Distance covered

Circular Motion Concepts

20. Uniform Circular Motion:
    When a body moves in a circular path with uniform speed.
    Example: A satellite orbiting the Earth.
21. Angular Displacement:
    Angle swept by the radius of a circular path in a given time.
22. Angular Velocity (ω):
    Angular Velocity = Angle swept / Time = θ / t
    Units: rad/s
23. Time Period (T):
    Time taken to complete one full revolution.
24. Frequency (f):
    Number of revolutions per unit time.
    f = 1 / T

 

Relation Between Linear and Circular Motion

25. Linear Velocity (v):
    v = 2πr / T = rω
26. Relation Between Linear Velocity and Time Period:
    v = Circumference / Time period

 

Notations of Physical Quantities

Quantity	Symbol
Distance	s
Speed	v
Time	t
Acceleration	a
Initial Velocity	u
Final Velocity	v
Average Velocity	v̅

Useful Formulas and Equations

1. v = u + at
2. s = ut + ½ at²
3. v² = u² + 2as
4. Average Velocity (v̅) = (u + v) / 2
5. s = v̅ × t = [(u + v)/2] × t

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