Schooling · Class 9 · Science · Describing Motion Around Us · Chapter on a page
CBSE Class 9 · Science · NCERT Exploration
Describing Motion Around Us: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 26 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Rest and motion are relative
- Distance is the total path length…
- Speed is distance covered per unit…
- Uniform motion
- Acceleration
- Equations of motion
- Distance-time graphs
- Formulas and facts
- Speed
- Velocity
- Acceleration
- First equation of motion
- Second equation of motion
- Third equation of motion
- SI unit of speed and velocity
- Worked examples
- Finding velocity and speed
- Using equations of motion
- Deceleration problem
- Watch out for
- Confusing distance with displacement
- Using speed and velocity…
- Forgetting to convert minutes to…
- Taking acceleration as always positive
- Misreading graph slopes
What this chapter is about
This chapter introduces you to the scientific way of describing how objects move. In everyday life, you see buses on roads, birds flying, and the Earth moving around the Sun. Physics gives you precise language and mathematical tools to describe all these movements.
Key ideas
- Rest and motion are relative: An object is at rest or in motion only with respect to a reference point. A passenger sitting in a moving train is at rest relative to another passenger but in motion relative to a tree outside.
- Distance is the total path length covered; displacement is the shortest straight-line distance from start to finish with direction. Distance is always positive; displacement can be zero, positive or negative.
- Speed is distance covered per unit time; velocity is displacement per unit time. Speed is a scalar (no direction); velocity is a vector (has direction).
- Uniform motion means equal distances are covered in equal time intervals. Non-uniform motion means the object covers unequal distances in equal time intervals.
- Acceleration is the rate of change of velocity. When velocity increases, acceleration is positive; when velocity decreases, acceleration is negative (also called retardation or deceleration).
- Equations of motion for uniformly accelerated motion along a straight line connect initial velocity (u), final velocity (v), acceleration (a), time (t) and displacement (s).
- Distance-time graphs: A straight line with slope indicates uniform speed; a curved line indicates changing speed. The slope of the graph gives speed.
- Velocity-time graphs: A horizontal line shows uniform velocity; a sloping line shows uniform acceleration. The area under the graph gives displacement.
Formulas and facts to remember
- Speed = Distance / Time → tells how fast an object covers ground, ignoring direction.
- Velocity = Displacement / Time → tells how fast and in which direction an object moves.
- Acceleration = (Final velocity − Initial velocity) / Time = (v − u) / t → tells how quickly velocity changes.
- First equation of motion: v = u + at → relates final velocity to initial velocity, acceleration and time.
- Second equation of motion: s = ut + (1/2)at² → gives displacement when initial velocity, acceleration and time are known.
- Third equation of motion: v² = u² + 2as → connects velocities, acceleration and displacement without needing time directly.
- SI unit of speed and velocity: metre per second (m/s). SI unit of acceleration: metre per second squared (m/s²).
- For an object moving in a circle and returning to the start, distance equals the circumference but displacement is zero.
Common mistakes
- Confusing distance with displacement → Remember: distance is path length (always positive); displacement is shortest straight-line separation with direction.
- Using speed and velocity interchangeably → Speed has no direction; velocity must include direction.
- Forgetting to convert minutes to seconds in problems → Always use SI units (seconds, metres) before substituting in formulas.
- Taking acceleration as always positive → Acceleration is negative when velocity decreases; include the sign in calculations.
- Misreading graph slopes → On a distance-time graph, slope gives speed; on a velocity-time graph, slope gives acceleration; area under velocity-time graph gives displacement.
Quick revision
- Rest and motion depend on the reference point chosen.
- Distance ≥ |Displacement|; they are equal only for straight-line motion without turning back.
- v = u + at; s = ut + (1/2)at²; v² = u² + 2as — memorise and practise these three equations.
- Slope of distance-time graph = speed; slope of velocity-time graph = acceleration.
- Area under velocity-time graph = displacement covered.
- Always use SI units and watch the sign of acceleration.