Chapter 4 — Describing Motion Around Us

Chapter 4 of the Class 9 Science NCERT textbook, "Describing Motion Around Us", covers describing motion through displacement, average speed, average velocity, average acceleration, kinematic equations, and uniform circular motion, using position-time and velocity-time graphs.

• Distance vs Displacement — Motion is described using distance and displacement, where displacement is the net change in position with direction and can be zero even when distance travelled is not, as when an athlete runs and returns to the start.
• Speed, Velocity, Acceleration — Average speed is distance over time, average velocity is displacement over time (v_av = s/t), and average acceleration is change in velocity over time, which can occur even at constant speed if direction changes.
• Kinematic Equations — For straight-line motion with constant acceleration, three kinematic equations connect the quantities: v = u + at, s = ut + ½at², and v² = u² + 2as, letting students solve for unknowns of motion.
• Graphs and Circular Motion — Position-time and velocity-time graphs represent motion, where slopes give velocity and acceleration and area under a velocity-time graph gives displacement. In uniform circular motion, changing direction means constant acceleration.

1. 01Displacement is the net change in position between two instants, requiring both magnitude and direction; it can be zero even when total distance travelled is non-zero (e.g., an athlete who runs and returns to the start)
2. 02Average speed equals total distance divided by time interval; average velocity equals displacement divided by time interval (v_av = s/t), with SI unit m s⁻¹
3. 03Average acceleration equals change in velocity divided by time interval (a = (v − u)/t), with SI unit m s⁻²; acceleration can exist even when speed is constant, if direction changes
4. 04The three kinematic equations for straight-line motion with constant acceleration are: v = u + at, s = ut + ½at², and v² = u² + 2as
5. 05On a position-time graph, the slope of the line gives the magnitude of velocity; a straight line indicates constant velocity and a curve indicates changing (accelerated) velocity
6. 06In uniform circular motion, an object moves at constant speed in a circular path, but its direction of velocity continuously changes, so it is always accelerating