Motion Class 9 Chapter Notes
These CBSE Class 9 Physics Motion notes help you revise the full chapter quickly: basic terms, scalar-vector differences, speed, velocity, acceleration, distance-time graphs, velocity-time graphs and the three equations of uniformly accelerated motion. Motion is a scoring chapter because most questions are formula-based, graph-based or definition-based, but marks are lost when students confuse distance with displacement or speed with velocity. Use these notes as a last-night revision sheet: first memorise the formula box and key points, then practise the mini-examples, and finally test yourself using the quick-check questions. For faster recall, you can turn the definitions into YoLearn AI Flashcards, create a Mind Map of motion terms, use the Quiz tool for formula practice, and summarise graph rules with the YoLearn Summarizer before exams.
Key points
- Motion is always described with respect to a reference point or observer. The same object may be at rest for one observer and in motion for another.
- Distance is the actual path length travelled; displacement is the shortest straight-line change in position from initial point to final point.
- Distance can never decrease and is always positive or zero; displacement can be positive, negative or zero depending on direction.
- Speed = distance/time and velocity = displacement/time. Speed is scalar; velocity is vector.
- Acceleration = change in velocity/time = (v - u)/t. Its SI unit is m/s².
- Uniform motion means equal distances are covered in equal intervals of time. Non-uniform motion means unequal distances in equal intervals or equal distances in unequal intervals.
- For uniformly accelerated motion: v = u + at, s = ut + 1/2 at², and v² - u² = 2as.
- Slope of distance-time graph gives speed; slope of velocity-time graph gives acceleration; area under velocity-time graph gives displacement.
- If velocity decreases, acceleration is negative and is called retardation or deceleration.
- In circular motion at constant speed, velocity changes continuously because direction changes, so it is accelerated motion.
Important definitions
- Motion
- An object is said to be in motion if its position changes with time with respect to a chosen reference point.
- Rest
- An object is at rest if its position does not change with time with respect to a chosen reference point.
- Distance
- The total length of the actual path travelled by an object. It is a scalar quantity and its SI unit is metre.
- Displacement
- The shortest straight-line distance from the initial position to the final position in a specified direction. It is a vector quantity.
- Speed
- Distance travelled per unit time. Formula: speed = distance/time.
- Velocity
- Displacement per unit time in a specified direction. Formula: velocity = displacement/time.
- Acceleration
- Rate of change of velocity with time. Formula: a = (v - u)/t.
- Uniform acceleration
- Acceleration is uniform when velocity changes by equal amounts in equal intervals of time.
- Retardation
- Negative acceleration produced when the speed or velocity of a body decreases with time.
Core idea: describing motion correctly
In Class 9 Physics, motion is not just movement; it is change in position with time relative to a reference point. For example, a student sitting in a moving bus is at rest with respect to the bus but in motion with respect to trees on the roadside. This is why motion and rest are relative terms. While solving numericals, first identify the initial velocity u, final velocity v, acceleration a, time t and displacement s. Then check whether acceleration is uniform; only then use the three equations of motion. Also remember that distance depends on the path actually followed, while displacement depends only on initial and final positions. In graph questions, the shape and slope matter: a straight line in a distance-time graph shows uniform speed, while a horizontal line shows rest. In a velocity-time graph, the area under the graph gives displacement, even if the graph is triangular or trapezium-shaped.
Formula sheet for Motion
Common contrasts: do not mix these
| Aspect | Details |
|---|---|
How to solve Motion numericals
- List the given values —
- Convert units first —
- Choose the correct equation —
- Check sign of acceleration —
- Write final answer with unit —
Quick worked examples
- {"title":"Example 1: Average speed","bodyMarkdown":"A car travels 120 m in 10 s. Average speed = distance/time = 120/10 = 12 m/s."}
- {"title":"Example 2: Acceleration","bodyMarkdown":"A scooter speeds up from 5 m/s to 25 m/s in 4 s. a = (v - u)/t = (25 - 5)/4 = 5 m/s²."}
- {"title":"Example 3: Displacement using equation","bodyMarkdown":"A body starts from rest and accelerates at 2 m/s² for 5 s. s = ut + 1/2 at² = 0 + 1/2 × 2 × 25 = 25 m."}
Graph reading: diagram-friendly revision
For a distance-time graph, put time on the x-axis and distance on the y-axis. A straight sloping line means uniform speed; a steeper line means greater speed because slope = distance/time. A horizontal line means the object is at rest because distance is not changing with time. For a velocity-time graph, put time on the x-axis and velocity on the y-axis. A horizontal line above the time axis means constant velocity. A sloping straight line means uniform acceleration, and its slope gives acceleration. The area under the velocity-time graph gives displacement. If the graph is a rectangle, area = length × breadth. If it is a triangle, area = 1/2 × base × height. If it is a trapezium, area = 1/2 × sum of parallel sides × height.
Exam tips and common mistakes
Write formulas before substitution to secure method marks. Do not use equations of motion unless acceleration is uniform. Never say velocity and speed are the same; velocity needs direction. In displacement questions, check if the object returns to the starting point: displacement may be zero even when distance is not zero. In graph questions, mention slope or area clearly because CBSE marking often awards marks for the correct graph property. Always include units; acceleration without m/s² can lose marks.
Quick revision checks
- Q: When is displacement zero but distance not zero? A: When an object returns to its starting point, such as completing one round of a circular track.
- Q: What does the slope of a velocity-time graph represent? A: Acceleration.
- Q: Which equation is used when time is not given? A: v² - u² = 2as.
- Q: Convert 72 km/h into m/s. A: 72 × 5/18 = 20 m/s.
Frequently Asked Questions
What are the three equations of motion for Class 9?
The three equations are v = u + at, s = ut + 1/2 at², and v² - u² = 2as. They are used only when acceleration is uniform.
What is the difference between distance and displacement?
Distance is the total path covered and is a scalar quantity. Displacement is the shortest straight-line change from initial to final position and is a vector quantity.
How do I remember graph rules in Motion?
Use the memory line: DT slope gives speed, VT slope gives acceleration, VT area gives displacement. Here DT means distance-time and VT means velocity-time.
Can speed be constant when acceleration is present?
Yes, in uniform circular motion speed can remain constant but velocity changes because direction changes continuously. Since acceleration is change in velocity per unit time, acceleration is present.
When should acceleration be taken negative?
Acceleration is taken negative when the object slows down or when acceleration acts opposite to the chosen positive direction of motion. This is also called retardation or deceleration.