Motion – Class 9 Science

🟦 📝 Introduction to Motion


Motion is defined as the change in the position of an object with respect to time. Whether it’s a car on the road, the Earth revolving around the sun, or a ball rolling on the floor — everything that changes position is in motion.

🟩 📏 Distance vs DisplacementDistance

Total path covered (Scalar Quantity)
Displacement – Shortest path between two points (Vector Quantity)

✅ Example:
If you walk 10 m forward and 10 m back,

  • Distance = 20 m
  • Displacement = 0 m


Displacement can be zero even if distance is not!

  • 🟨 🚶‍♂️ Uniform and Non-Uniform Motion
  • Uniform Motion: Equal distance in equal time
    Non-Uniform Motion: Unequal distance in equal time
    🎯 Real-Life Example:
    Uniform: Train on straight track
    Non-uniform: City traffic movement

⚡ Speed, Average Speed & Velocity

TermFormulaType
SpeedDistance / TimeScalar
Average SpeedTotal Distance / Total TimeScalar
VelocityDisplacement / TimeVector

🎯 Note: Speed ≠ Velocity always. Direction matters in velocity!

🌀 Acceleration & Retardation

  • Acceleration = Change in velocity / Time
  • Uniform Acceleration: Constant change
  • Retardation: Negative acceleration (decreasing speed)

🧠 Note: Acceleration is a vector quantity.

📊 Graphical Representation of Motion

📈 Distance-Time Graphs

  • Straight Line = Uniform motion
  • Curved Line = Non-uniform motion

📉 Velocity-Time Graphs

  • Horizontal = Constant velocity
  • Sloped = Acceleration
  • Area under graph = Distance covered

🧮 Equations of Motion (Uniform Acceleration)

v = u + at
s = ut + ½at²
v² = u² + 2as

Where:
v = final velocity
u = initial velocity
a = acceleration
s = displacement
t = time

🧠 These can also be derived using velocity-time graphs!

🔁 Uniform Circular Motion

Even if speed is constant, direction keeps changing.
So velocity is changing → motion is accelerated.

Examples:

  • Earth around Sun
  • Athlete on circular track
  • Second hand of a watch

📌 Formula: Speed = 2πr / T

🧠 Practice Numericals

  • Speed problems
  • Displacement with direction
  • Graph-based numericals
  • Acceleration cases

📘 PYQ Tag: (CBSE 2020) – “A car covers 20 m in 4 s. What is its speed?”

🎯 Key Takeaways Summary

  • Motion = Change in position over time
  • Distance ≠ Displacement
  • Speed is scalar, velocity is vector
  • Acceleration can be positive or negative
  • Graphs help visualize motion
  • Equations are used in solving real-world problems

📚 Related Lessons