Class 9 Science Chapter 4 Describing Motion Around Us – Revision Notes

Quick revision notes for Class 9 Science Chapter 4 (Describing Motion Around Us). These Class 9 Science Chapter 4 notes are ideal for quick revision just before exams.

Distance vs Displacement

• Distance: total path length, scalar, always positive.
• Displacement: shortest straight-line distance with direction, vector, can be zero.

Speed vs Velocity

• Speed = distance/time (scalar).
• Velocity = displacement/time (vector).

Acceleration

• Rate of change of velocity.
• Positive: speeding up; Negative (retardation): slowing down; Zero: uniform velocity.

Graphs

• Position-time graph slope = velocity.
• Velocity-time graph slope = acceleration; area under graph = displacement.

Equations of Motion

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

Uniform Circular Motion

• Constant speed, continuously changing direction → accelerated motion, directed toward centre.

One-Line Summary

Motion is described using distance/displacement, speed/velocity, and acceleration, visualised through position-time and velocity-time graphs, and summarised in three equations of motion; even constant-speed circular motion counts as accelerated motion because direction keeps changing.

Written by Satish

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