MOTION IN A STRAIGHT LINE, CBSE CLASS XI, PHYSICS NOTES PART-I

CBSE CLASS 11, MOTION IN A STRAIGHT LINE, PHYSICS NOTES-(PART I)

MOTION IN A STRAIGHT LINE

CBSE Class 10 Science Chapter 10th Motion in a Straight Line Notes

Study Material and Notes of Ch 2 Motion in a Straight Line World Class 10th Science

ACCORDING TO THE CBSE SYLLABUS 2025-26, THIS CHAPTER HAS BEEN RENUMBERED AS CHAPTER 2.

INTRODUCTION

Motion is one of the significant topics in physics. Everything in the universe moves. It might only be a small amount of movement and very, very slow, but movement does happen. Even if you appear to be standing still, the Earth is moving around the sun, and the sun is moving around our galaxy.
“An object is said to be in motion if its position changes with time”.
The concept of motion is a relative one, and a body that may be in motion relative to one reference system may be at rest relative to another.

There are two branches of physics that examine the motion of an object.

(i) Kinematics: It describes the motion of objects, without looking at the cause of the motion.
(ii) Dynamics: It relates the motion of objects to the forces which cause them.

POINT OBJECT

If the length covered by the objects is very large in comparison to the size of the objects, the objects are considered point objects.

REFERENCE SYSTEMS

The motion of a particle is always described with respect to a reference system. A reference system is made by taking an arbitrary point as the origin and imagining a co-ordinate system to be attached to it. This co-ordinate system chosen for a given problem constitutes the reference system for it. We generally choose a co-ordinate system attached to the earth as the reference system for most of the problems.

TYPES OF MOTION

In order to completely describe the motion of an object, we need to specify its position. For this, we need to know the position coordinates. In some cases, three position co-ordinates are required, while in some cases, two or one position co-ordinate is required.
Based on these, motion can be classified as:

(i) One-dimensional motion. A particle moving along a straight-line or a path is said to undergo one dimensional motion. For example, motion of a train along a straight line, freely falling body under gravity etc.

(ii) Two dimensional motion.A particle moving in a plane is said to undergo two dimensional motion. For example, motion of a shell fired by a gun, carrom board coins etc.
(iii) Three dimensional motion.A particle moving in space is said to undergo three dimensional motion. For example, motion of a kite in sky, motion of aeroplane etc.

TYPES OF RECTILINEAR/LINEAR MOTION

The Rectilinear Motion, or linear motion, can be of two types:

a. Non-uniform linear motion with non-zero acceleration or variable velocity

b. Uniform linear motion with zero acceleration or constant velocity

The simplest type of one-dimensional motion is Linear motion. As per Newton’s first law of motion, an object will either be at rest or continue to move in a straight line with a uniform velocity unless and until an external force is applied to it.

 Linear motion is often mistaken for general motion. Linear motion is a one-dimensional motion, but in general, the motion has both magnitude and direction, i.e. an object’s position and velocity are described in vector quantities.

UNIFORM MOTION IN A STRAIGHT LINE

A body is said to be in a uniform motion if it travels in a straight line, covering equal distances in equal intervals of time. A body is said to have uniform acceleration if the rate of change of its velocity remains constant.

 For example: The ball covers equal distances in equal intervals of time

NON-UNIFORM MOTION IN A STRAIGHT LINE

A body is said to have a non-uniform motion when the velocity of a body changes by unequal amounts in equal intervals of time. While in motion, the rate of change of its velocity changes at different points in time.

TOTAL PATH LENGTH (DISTANCE)

For a particle in motion, the total length of the actual path traversed between the initial and final positions of the particle is known as the ‘total path length’ or distance covered by it.

DISPLACEMENT

Displacement of a particle in a given time is defined as the change in the position of the particle in a particular direction during that time. It is given by a vector drawn from its initial position to its final position.

UNIFORM MOTION

When the displacement of an object is equal in equal time intervals, the motion is said to be uniform, it is referred to as non-uniform.

AVERAGE SPEED

Average speed is referred to as the ratio between the total path length traversed to the time taken.

INSTANTANEOUS VELOCITY

It is the velocity when the limit of the average velocity is an indefinitely smaller interval of time.

AVERAGE ACCELERATION

It is the change in velocity corresponding to the time interval within which the change has accelerated.

FACTORS DISTINGUISHING DISPLACEMENT FROM DISTANCE

Displacement has direction. Distance does not have direction.
The magnitude of displacement can be both positive and negative.
Distance is always positive. It never decreases with time.

Distance ≥ Displacement

S.No.	Distance	Displacement
1	Length of actual path covered between the initial and final positions/ points.	Length of the shortest path covered between the initial and final positions/ points.
2	Scalar quantity.	Vector quantity.
3	Can have only positive values.	Can have only positive, zero and negative values.

Both distance and displacement are measured in metres or kilometres. Their dimensions are (L).

FACTORS DISTINGUISHING SPEED FROM VELOCITY

 

S.NO.	SPEED	VELOCITY
1	The rate at which distance is covered.	The rate at which displacement is covered.
2	Scalar quantity.	Vector quantity.
3	Can have only positive values.	Can have only positive, zero and negative values.

Speed and velocity are expressed in metres per second. The dimensional formula is (LT^-1).

Velocity = final position- initial position / time = x_f –x_i / t