Motion Class 9 Notes

 

📘 MOTION – COMPLETE NOTES (Class 9)

1. 🔹 What is Motion?

An object is said to be in motion if its position changes with respect to time and a reference point.

👉 Example: A car moving on a road.

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2. 🔹 Types of Motion

1. Rectilinear Motion – Motion in a straight line
2. Circular Motion – Motion in a circular path
3. Periodic Motion – Motion that repeats after equal intervals of time

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3. 🔹 Distance and Displacement

Distance	Displacement
Total path covered	Shortest path between two points
Scalar quantity	Vector quantity
Always positive	Can be positive, negative, or zero

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4. 🔹 Speed

Speed tells how fast an object moves.

Formula:

$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$

Types of Speed:

• Uniform Speed – Equal distance in equal time
• Non-uniform Speed – Unequal distance in equal time

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5. 🔹 Velocity

Velocity is speed in a given direction.

Formula:

$\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$

👉 Velocity is a vector quantity

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6. 🔹 Acceleration

Acceleration is the rate of change of velocity.

Formula:

$a = \frac{v - u}{t}$

Where:

• u = initial velocity
• v = final velocity
• t = time

👉 Types:

• Positive acceleration (speed increases)
• Negative acceleration / Retardation (speed decreases)

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7. 🔹 Equations of Motion (Important)

First Equation:

$v = u + at$

Second Equation:

$s = ut + \frac{1}{2}at^2$

Third Equation:

$v^2 = u^2 + 2as$

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8. 🔹 Graphs of Motion

(a) Distance–Time Graph

• Straight line → Uniform motion
• Curve → Non-uniform motion

(b) Velocity–Time Graph

• Straight line → Uniform acceleration
• Area under graph = Distance

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9. 🔹 Uniform Circular Motion

Motion in a circular path at constant speed.

👉 Even if speed is constant, direction changes → velocity changes → acceleration exists

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10. 🔹 Important Units

• Distance → meter (m)
• Time → second (s)
• Speed/Velocity → m/s
• Acceleration → m/s²

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11. 🔹 Key Points to Remember

• Distance ≥ Displacement
• Speed ≥ Velocity (in magnitude)
• If velocity is constant → acceleration = 0
• Motion equations apply only for uniform acceleration

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12. 🔹 Quick Formula Summary

• Speed = Distance / Time
• Velocity = Displacement / Time
• Acceleration = (v − u) / t
• v = u + at
• s = ut + ½at²
• v² = u² + 2as