NCERT Solutions for Class 9th Science Chapter 4 Describing Motion Around Us
Updated on 2026-09-19
About this chapter
Position is the distance and direction of an object from a chosen reference point at a given instant. An object is in motion if its position changes with time; at rest if it does not. Total distance travelled counts the whole path; displacement is only the net change in position, and it needs a direction as well as a magnitude. Magnitude of displacement ≤ total distance travelled, always. Average speed = total distance travelled / time interval. Average velocity = displacement / time interval. Both have the SI unit m s⁻¹, but only velocity carries a direction. Average acceleration = change in velocity / time interval, SI unit m s⁻². It points along the velocity when the speed is rising and opposite to it when the speed is falling. On a position–time graph the slope is velocity: a straight
- Chapter opener
- Distance travelled and displacement
- Average speed and average velocity
- Average acceleration
- Plotting graph
- Position-time graphs
- Velocity-time graphs
- Kinematic equations
- Uniform circular motion
- End-of-chapter questions
Quick revision
| Quantity | What it means | SI unit | Key point |
|---|---|---|---|
| Position | Distance and direction from the reference point at an instant | m | Needs a fixed reference point and a sign convention |
| Distance travelled | Total length of the path actually covered | m | Never decreases; no direction |
| Displacement (s) | Net change in position between two instants | m | Has direction; magnitude ≤ distance travelled |
| Average speed | Total distance travelled ÷ time interval | m s⁻¹ | No direction; zero only if the object never moves |
| Average velocity (vav) | Displacement ÷ time interval | m s⁻¹ | Can be zero even when the average speed is not |
| Average acceleration (a) | Change in velocity ÷ time interval | m s⁻² | Negative sign = acceleration opposite to the velocity |
| Slope of position–time graph | Rate of change of position | m s⁻¹ | Gives the velocity |
| Slope of velocity–time graph | Rate of change of velocity | m s⁻² | Gives the acceleration |
| Area under velocity–time graph | Velocity × time interval | m | Gives the displacement |
| Kinematic equations | v = u + at, s = ut + ½at², v² = u² + 2as | — | Valid only when the acceleration is constant |
| Uniform circular motion | Circular path at constant speed | — | Accelerated, because the direction of velocity keeps changing |
| Speed in one revolution | vav = 2πR/T | m s⁻¹ | Average velocity over one full revolution is zero |
Exercises
- Chapter opener — Think It Over Page 48
- In-text Questions — Distance travelled and displacement Page 51
- Activity 4.1: Let us analyse — Distance travelled and displacement Page 51
- Pause and Ponder — Distance travelled and displacement Page 51
- Pause and Ponder — Average speed and average velocity Page 53
- Activity 4.2: Let us calculate — Average acceleration Page 55
- Activity 4.3: Let us plot a graph — Plotting graph Page 57
- In-text Questions — Position-time graphs Page 59
- Activity 4.4: Let us calculate — Position-time graphs Page 59–60
- In-text Questions — Velocity-time graphs Page 61–62
- Bridging Science and Society — Kinematic equations Page 65
- In-text Questions — Uniform circular motion Page 66
- Activity 4.5: Let us investigate — Uniform circular motion Page 67
- End-of-chapter questions — Revise, Reflect, Refine Page 68–70
- Projects and extension work — The Journey Beyond Page 71