9th Class Physics Chapter 2 Notes PDF 2026 – Kinematics

Definition

Scalar: magnitude only (distance, speed, mass). Vector: magnitude and direction (displacement, velocity, acceleration).

Distance: total length of the path travelled. Displacement: shortest straight-line distance from start to end, in a given direction.

Remember

Over a round trip, displacement is zero (start = end) even though distance travelled is not — this is why average velocity can be zero while average speed is not.

Average speed $= \frac{distance}{time}$ , average velocity $= \frac{displacement}{time}$

Acceleration $a = \frac{v - u}{t}$ ( $u$ = initial velocity, $v$ = final velocity)

Example 1 — acceleration

A truck starts from rest and reaches $90$ km/h in $5$ s. Find its acceleration.

Convert: $90$ km/h $= 25$ m/s.
$a = \frac{v - u}{t} = \frac{25 - 0}{5}$ .

Answer: $5$ m/s $^{2}$

Rule

Gradient of a distance–time graph $=$ speed.

Gradient of a speed–time graph $=$ acceleration.

Area under a speed–time graph $=$ distance travelled.

$v = u + a t$

$S = u t + \frac{1}{2} a t^{2}$

$2 a S = v^{2} - u^{2}$

Example 2 — using the equations of motion

A body accelerates from rest to $144$ km/h in $20$ s. Find the distance covered.

$144$ km/h $= 40$ m/s; $u = 0$ .
$S = u t + \frac{1}{2} a t^{2}$ needs $a$ first: $a = \frac{40 - 0}{20} = 2$ m/s $^{2}$ .
$S = 0 + \frac{1}{2} (2) (20)^{2} = 400$ .

Answer: $400$ m

Example 3 — free fall

A stone is dropped from $45$ m. How long does it take to reach the ground? ( $g = 10$ m/s $^{2}$ )

$u = 0$ , $S = 45$ m, $a = g = 10$ m/s $^{2}$ .
$S = u t + \frac{1}{2} a t^{2} \Rightarrow 45 = \frac{1}{2} (10) t^{2} = 5 t^{2}$ .
$t^{2} = 9$ .

Answer: $t = 3$ s

Quick Recap

●Scalar: size only. Vector: size + direction
●Distance/speed use the path travelled; displacement/velocity use the straight-line change
●Gradient of distance–time graph = speed; gradient of speed–time graph = acceleration; area under speed–time graph = distance
● $v = u + a t$ , $S = u t + \frac{1}{2} a t^{2}$ , $2 a S = v^{2} - u^{2}$ — free fall uses $a = g$

9th Class Physics Chapter 2 Notes

Scalars, Vectors, Distance and Displacement

Speed, Velocity and Acceleration

Distance–Time and Speed–Time Graphs

Equations of Motion

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