INTERMEDIATE PHYSICS – LESSON 2

MOTION IN A STRAIGHT LINE

1. Introduction

Motion is one of the most fundamental concepts in Physics. An object is said to be in motion if its position changes with respect to time and a chosen reference point.

This chapter deals with motion along a straight line, also called one-dimensional motion.

2. Rest and Motion

An object is said to be:

At Rest – if its position does not change with time
In Motion – if its position changes with time

Important Note: Rest and motion are relative terms. The same object may be at rest for one observer and in motion for another.

3. Reference Point and Reference Frame

To describe motion, we need a reference point. Without a reference point, motion cannot be defined.

A set of coordinates along with a clock is called a reference frame.

4. Distance and Displacement

(a) Distance

Distance is the actual path length travelled by an object.

It is a scalar quantity
It is always positive

(b) Displacement

Displacement is the shortest distance between initial and final positions of an object in a given direction.

It is a vector quantity
It can be positive, negative or zero

Example: If a person walks 5 m east and then 5 m west, Distance = 10 m, Displacement = 0

5. Speed and Velocity

(a) Speed

Speed is the distance travelled per unit time.

Speed = Distance / Time

Scalar quantity
Never negative

(b) Velocity

Velocity is the displacement per unit time.

Velocity = Displacement / Time

Vector quantity
Can be positive, negative or zero

6. Average and Instantaneous Velocity

Average Velocity

Average velocity is defined as the ratio of total displacement to total time.

Average velocity = Total displacement / Total time

Instantaneous Velocity

Instantaneous velocity is the velocity of an object at a particular instant of time.

7. Acceleration

Acceleration is the rate of change of velocity with respect to time.

Acceleration = Change in velocity / Time

SI unit: m/s²
It is a vector quantity

Special Case: If velocity decreases, acceleration is called retardation.

8. Uniform and Non-uniform Motion

Uniform Motion

An object is said to be in uniform motion if it covers equal distances in equal intervals of time.

Non-uniform Motion

An object is said to be in non-uniform motion if it covers unequal distances in equal intervals of time.

9. Important Formulae

Speed = Distance / Time
Velocity = Displacement / Time
Acceleration = (v − u) / t

10. Important Points for Intermediate Exam

Learn definitions clearly (2 marks)
Difference between distance & displacement is important
Speed vs velocity questions are common
Numericals on acceleration are scoring

➡️ NEXT: STAGE–2 – INTERMEDIATE EXAM QUESTIONS & ANSWERS

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