Tamil Nadu Class 9 Science Motion — Samacheer Kalvi

Quick Summary

Motion is a fundamental concept in physics. Class 9 covers types of motion, the distinction between scalar and vector quantities, equations of motion, and graphical representation of motion. This chapter has 3-mark and 5-mark questions regularly in board exams and numerical problems are very common.

Key Definitions

• Motion: Change in position of an object with respect to time and surroundings.
• Rest: An object that does not change its position with time.
• Distance: Total path length covered (scalar).
• Displacement: Shortest distance from initial to final position with direction (vector).
• Speed: Distance covered per unit time (scalar). Unit: m/s.
• Velocity: Displacement per unit time (vector). Unit: m/s.
• Acceleration: Rate of change of velocity. Unit: m/s².
• Uniform Motion: Equal distances in equal intervals of time.
• Non-uniform Motion: Unequal distances in equal intervals of time.

Equations of Motion

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

Types of Motion

Based on Path

• Linear motion: Along a straight line (e.g., car on a straight road).
• Circular motion: Along a circular path (e.g., satellite orbiting Earth).
• Oscillatory motion: Back and forth about a fixed point (e.g., pendulum).
• Random motion: No fixed path (e.g., Brownian motion of gas particles).

Based on Velocity

• Uniform motion: Constant velocity (zero acceleration).
• Non-uniform motion: Changing velocity (non-zero acceleration).

Graphical Representation

Distance–Time Graph

• Straight line through origin → uniform motion.
• Curve → non-uniform motion.
• Slope of line = speed.

Velocity–Time Graph

• Straight line parallel to x-axis → uniform velocity (zero acceleration).
• Straight line with positive slope → uniform acceleration.
• Area under graph = displacement.
• Slope = acceleration.

Important Formulae

• Speed = Distance / Time
• Velocity = Displacement / Time
• Acceleration = (v − u) / t
• For a freely falling body: a = g = 9.8 m/s² (downward)

Important Questions

One Mark

• Define acceleration. State its SI unit.
• What type of motion does a satellite show?
• A car moves 100 m in 10 s. Calculate its speed.
• What does the slope of a velocity–time graph represent?
• State whether velocity is a scalar or vector quantity.

Short Answer (3 marks)

• Distinguish between speed and velocity with examples.
• A body starts from rest and attains a velocity of 20 m/s in 4 seconds. Find the acceleration and the distance covered.
• Explain the difference between uniform and non-uniform motion with examples.

Long Answer (5 marks)

• Derive the three equations of motion graphically using a velocity–time graph.
• A train starts from rest and accelerates at 2 m/s². Find (a) velocity after 5 s, (b) distance covered in 5 s, (c) velocity after covering 100 m.

Common Mistakes

• Confusing speed (scalar) with velocity (vector) — direction matters for velocity.
• Using distance instead of displacement in the equations of motion.
• Forgetting to check units — always convert to m/s and m before substituting in equations.

Revision Checklist

• [ ] Know all definitions: distance, displacement, speed, velocity, acceleration
• [ ] Memorised all three equations of motion with variable meanings
• [ ] Can draw and interpret distance–time and velocity–time graphs
• [ ] Practised at least 5 numerical problems
• [ ] Understand types of motion with examples