Chapter 4: Describing Motion Around Us
Original notes — ready CBSE (NCERT, English Medium) • Class 9 • Science • Book: Exploration
Chapter 4: Describing Motion Around Us — EG's own notes, in simple words
Book: Exploration (CBSE, Class 9) | Chapter 4 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): A car seen from above on a straight road, with an arrow showing its direction of motion. A general diagram for study, not a figure copied from the book.
Motion all around us — running, cycling and travelling. A general picture about describing motion, not a book scene.
What the chapter is about
Motion is all around us: a bus moving down a road, a ball rolling, the hands of a clock. An object is said to be in motion when its position changes with time. To describe motion properly, we need to know how far something moved, in which direction, and how quickly.
Distance is the total path travelled, while displacement is the straight-line change in position from start to finish, with direction. Speed tells us how fast something moves, and velocity is speed in a stated direction. When velocity changes, the object accelerates. Motion can be uniform, with equal distances in equal times, or non-uniform, when the speed keeps changing.
Graphs help us see motion clearly. A distance–time graph shows how distance grows with time, and its slope gives speed. A velocity–time graph shows how velocity changes, and the area under it gives the distance travelled. With equations of motion, we can also calculate distance, velocity and acceleration.
What you should learn
Notice the difference between distance and displacement, and between speed and velocity — and how graphs turn motion into a picture you can read.
Key Terms
- Distance — The total length of the path travelled by a moving object.
- Displacement — The straight-line change in position from the starting point to the finishing point, with direction.
- Speed — Distance travelled per unit time.
- Velocity — Speed in a stated direction.
- Acceleration — The rate of change of velocity with time.
Remember
- Distance has no direction; displacement has direction.
- Speed is distance over time; velocity is speed with direction.
- Acceleration means velocity is changing, not just that something is fast.
- The slope of a distance–time graph gives speed.
Exam Tip / Common Mistake
Common mistake: using distance and displacement as if they were the same. If a runner goes around a track and returns to the start, the distance is the full lap but the displacement is zero — direction and endpoints matter.
Chart / Table: Motion quantities at a glance
| Point | Detail |
|---|---|
| Distance | Total path length; no direction |
| Displacement | Straight-line change of position; has direction |
| Speed | Distance per unit time |
| Velocity | Speed in a stated direction |
| Acceleration | Change of velocity per unit time |
Practice set — try it yourself
- A car travels 60 km in 2 hours. What is its average speed?
Hint: Think about distance divided by time. - How is displacement different from distance?
Hint: Think about the straight line from start to finish. - What does a flat line on a distance–time graph mean?
Hint: Think about whether distance is changing. - A bus speeds up from rest. Is it accelerating? Why?
Hint: Think about what acceleration means. - Give one example of uniform motion and one of non-uniform motion.
Hint: Think about steady speed versus changing speed.
Questions with Answers
- What is motion?
Motion is the change in position of an object with time, measured against a fixed reference point. - What is the difference between speed and velocity?
Speed is distance travelled per unit time and has no direction. Velocity is speed in a stated direction, so it includes direction as well. - What does acceleration mean?
Acceleration is the rate of change of velocity with time. An object accelerates when it speeds up, slows down, or changes direction. - What does the slope of a distance–time graph tell us?
The slope of a distance–time graph gives the speed of the object. A steeper slope means a higher speed, and a flat line means the object is at rest. - What is uniform motion?
Uniform motion is motion in which an object covers equal distances in equal intervals of time, moving at a steady speed in a straight line.
How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.