New NCERT Exploration · Class 9 Science · Chapter 4: Describing Motion Around Us
This chapter describes motion in a straight line. It explains position, distance and displacement, average speed, average velocity and acceleration. It also shows how to read position-time and velocity-time graphs and use the three equations of motion.
From the Class 9 Science Short Notes, made as per the new NCERT book Exploration · Pages 24–28 of the book.
Key points
- Motion in a straight line is called linear motion, like a car on a straight road or a ball falling vertically.
- To describe position we choose a reference point (origin) and give distance and direction from it.
- Distance travelled is the total path covered; it is a scalar and can never be negative.
- Displacement is the shortest straight-line change in position from start to end; it is a vector and can be zero, positive or negative.
- The SI unit of both distance and displacement is metre (m); the SI unit of speed and velocity is m/s.
- Average speed depends on the path taken; average velocity depends only on the initial and final positions.
- If an object moves from A to B in a straight line without changing direction, average speed equals average velocity.
- Acceleration tells how quickly velocity changes with time; it can act in the direction of motion (speeding up) or opposite to it (slowing down).
- On a position-time graph, a straight line means constant velocity, a curve means accelerated motion, and a horizontal line means rest.
- The slope of a velocity-time graph gives acceleration, and the area under it gives displacement.
- For constant acceleration there are three kinematic equations of motion.
Formulas
| Average speed | Average speed = d ÷ t d = total distance travelled (m), t = total time taken (s) |
| Average velocity | Average velocity = s ÷ t s = displacement (m), t = total time taken (s) |
| Average acceleration | a = (v − u) ÷ t u = initial velocity, v = final velocity, t = time interval |
| Velocity from position-time graph | v = slope = x₁ ÷ t₁ For a straight-line position-time graph |
| First equation of motion | v = u + at |
| Second equation of motion | s = ut + ½at² |
| Third equation of motion | v² = u² + 2as Use when time is not known |
Key terms
- Linear motion: Motion of an object in a straight line.
- Reference point: The origin from which the position of an object is described.
- Distance: Total path covered by an object; a scalar quantity.
- Displacement: Shortest straight-line change in position from initial to final point; a vector quantity.
- Acceleration: The rate at which velocity changes with time.
- Kinematic equations: Three equations used for straight-line motion with constant acceleration.
Diagrams to learn
- Examples of linear motion: car, train, swimmer, falling ball
- Distance travelled vs displacement between points A and B
- Comparison table of average speed and average velocity
- Car speeding up and slowing down with direction of acceleration
- Position-time graphs: straight line, curve, horizontal line
- Velocity-time graphs: constant, increasing and decreasing velocity
- Table of the three equations of motion with motion diagrams
- Distance can never be negative, but displacement can be zero, positive or negative.
- If time is known, use equation 1 or 2; if time is not known, use equation 3.
- Slope of a velocity-time graph gives acceleration; the area between the graph and the time axis gives displacement.
A real page from the book
Get the full chapter in the book
This page is a summary. The Class 9 Science Short Notes has the complete chapter in colour, with every diagram and worked example.
Frequently asked questions
What is the difference between distance and displacement?
Distance is the total path covered and is a scalar, so it has no direction and is never negative. Displacement is the shortest straight-line change in position from start to end; it is a vector and can be zero, positive or negative.
What are the three equations of motion in Class 9?
For constant acceleration: v = u + at, s = ut + ½at², and v² = u² + 2as. Here u is initial velocity, v is final velocity, a is acceleration, t is time and s is displacement.
What does the slope of a velocity-time graph show?
The slope of a velocity-time graph gives acceleration. The area between the graph and the time axis gives displacement.
← Chapter 3: Tissues in ActionChapter 5: Exploring Mixtures and their Separation →
