NCERT Solutions for Class 9 Science Chapter 4: Describing Motion Around Us – Free PDF Download

Chapter 4 of Class 9 Science Exploration is Describing Motion Around Us, the first physics chapter of the book. It builds up the mathematical language used to describe how objects move — distance, displacement, speed, velocity, acceleration, and graphs of motion — along with uniform circular motion. These Class 9 Science Chapter 4 solutions are also useful as quick revision notes before exams.

Distance and Displacement

Distance is the total path length covered by a moving object, always positive. Displacement is the shortest straight-line distance from the initial to the final position, along with direction — it can be zero even if distance travelled is not (e.g. a runner returning to the starting point).

Speed and Velocity

Speed = distance / time (a scalar quantity, magnitude only). Velocity = displacement / time (a vector quantity, with direction). Uniform velocity means equal displacements in equal time intervals in a fixed direction; average velocity = total displacement / total time.

Acceleration

Acceleration is the rate of change of velocity with time. It is positive when velocity increases, negative (retardation/deceleration) when velocity decreases, and zero for uniform velocity.

Graphs of Motion

Position–time graphs: the slope gives velocity; a straight line means uniform velocity, a curve means changing (non-uniform) velocity. Velocity–time graphs: the slope gives acceleration, and the area under the graph gives the displacement covered.

Kinematic (Equations of Motion)

Using the graphical method (from a velocity–time graph), three key equations relate initial velocity (u), final velocity (v), acceleration (a), time (t), and displacement (s):

• v = u + at
• s = ut + ½at²
• v² = u² + 2as

Uniform Circular Motion

When an object moves at constant speed along a circular path, it is in uniform circular motion. Even though speed is constant, velocity keeps changing because direction changes continuously — so the motion is accelerated, directed toward the centre of the circle.

Frequently Asked Questions

Can displacement be zero while distance travelled is not zero?
Yes — for example, if a person walks around a circular track and returns to the starting point, distance travelled is the full circumference, but displacement is zero since the start and end positions coincide.

What does the slope of a position-time graph represent?
The velocity of the object.

Why is uniform circular motion considered accelerated motion even though speed is constant?
Because velocity is a vector (speed + direction), and direction keeps changing continuously as the object moves along the circle, even though the speed (magnitude) stays constant — a changing velocity means there is acceleration.

Written by Satish

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