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CBSE Class 9 — Notes, Chapters & Practice Quizzes

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Science · 13 chapters
Summary, key terms, extra questions with answers and a practice quiz with AI diagnosis for each.

Chapter 4: Describing Motion Around UsClass 9 Science — summary, notes, extra questions & NCERT solutions

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Which quantity is a vector, having both magnitude and direction?

Summary

This chapter develops a precise way to describe motion in a straight line and in a circle. Position is given relative to a chosen reference point. Distance is the total path length covered (a scalar, only magnitude), while displacement is the shortest straight-line change in position from start to finish (a vector, having both magnitude and direction); displacement can be zero even when distance is not. Speed is \(\text{speed} = \dfrac{\text{distance}}{\text{time}}\) and is a scalar, whereas velocity is displacement per unit time and is a vector. For changing motion we use average speed and average velocity. Acceleration is the rate of change of velocity, \(a = \dfrac{v - u}{t}\), measured in \(\text{m s}^{-2}\); it is positive when speeding up and negative (retardation) when slowing down. For uniform acceleration the three equations of motion apply: \(v = u + at\), \(s = ut + \tfrac{1}{2}at^2\) and \(v^2 = u^2 + 2as\). Motion can be shown with distance-time graphs (slope gives speed) and velocity-time graphs (slope gives acceleration, and the area under the line gives distance). Finally, in uniform circular motion an object moves around a circle at constant speed, yet because its direction keeps changing its velocity is continuously changing, so it is accelerating; examples include a stone whirled on a string or the Moon orbiting the Earth.

Reference point and positionDistance, displacement and scalars vs vectorsSpeed, velocity and accelerationEquations of motion and motion graphsUniform circular motion

Key terms

Distance
The total length of the path actually covered by a moving object; a scalar with magnitude only.
Displacement
The shortest straight-line change in position, from initial to final point; a vector with magnitude and direction.
Velocity
The displacement of an object per unit time; a vector that tells both how fast and in which direction it moves.
Acceleration
The rate of change of velocity with time, \(a = \dfrac{v-u}{t}\), measured in \(\text{m s}^{-2}\).
Equations of motion
For uniform acceleration: \(v = u + at\), \(s = ut + \tfrac{1}{2}at^2\), and \(v^2 = u^2 + 2as\).
Uniform circular motion
Motion along a circle at constant speed; the velocity keeps changing direction, so the motion is accelerated.

Extra questions & answers

Think It Over

Activity 4.1: Let us analyse

Pause and Ponder

Activity 4.2: Let us calculate

Activity 4.5: Let us investigate

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The Journey Beyond

Explore interactively

Launch a projectile and trace its path
Angle45°
Speed20 m/s
Range
40.82 m
Max height
10.2 m
Time of flight
2.89 s
Position–time graph of motion
initial speed u2
acceleration a1
y = u*x + 0.5*a*x*x — x = time (s), y = position (m)
Key-term flashcards
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Distance
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The total length of the path actually covered by a moving object; a scalar with magnitude only.
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Practice quiz · Describing Motion Around Us

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