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In such a motion an object move along a straight line path. If an object changes its position with respect to its surroundings with time, then it is called in motion. It is change in position of an object over time.

Basic Requirement

  • Class 11 Student

Skills Covered

  • Speed & Velocity

  • Finding Instant Velocity

  • Acceleration

Expert Review

Class 11 Notes Physics Chapter 3: Motion in a Straight Line


Physics deals with motion extensively. The universe is filled with motion. Movement happens, though it may be slow and very very small. Although you may appear to be standing still, the Earth is moving around the sun, and the sun is moving around our galaxy.

The position of an object changes with time if it is in motion.

Motion is an abstract concept and a body that is moving relative to one reference system may be at rest relative to another.

In physics, there are two branches that study motion.

(i) Kinematics: It describes the motion of objects without taking into account their cause.

(ii) Dynamics: It explains objects' motion in terms of the forces that cause them.

Point Object

If the objects cover a large distance relative to their size, they are considered point objects.

Reference Systems

It is always possible to describe the motion of a particle with respect to a reference system. A reference system is created by taking an arbitrary point as the origin and attaching a coordinate system to it. The coordinate system chosen for a given problem is its reference system. The most common reference system is a co-ordinate system attached to the earth.

Total Path Length (Distance)

The total length of the path traveled by the particle between its initial and final positions is called its 'total path length' or 'distance covered'.

Types of Motion

For an object's motion to be described completely, its position must be specified. This can only be done by knowing its coordinates. Three position co-ordinates are required in some cases, whereas two or one position co-ordinate is necessary in others.

Motion can be classified as follows:

(i) One-dimensional motion. In one-dimensional motion, a particle moves along a straight line or a path. Trains traveling along straight lines, bodies falling naturally under gravity, etc.

(ii) Two-dimensional motion. The motion of a particle in a plane is said to be two-dimensional.

 The motion of a shell fired by a gun, the movement of carrom board coins, etc.

(iii) Three-dimensional motion. When a particle moves in space, it is considered to undergo three-dimensional motion. You can think of a kite flying in the sky, an airplane flying, etc.


A particle's displacement over a given period of time is the change in the position of the particle in a particular direction during that period. It is given by a vector drawn from its initial position to its final position.

Factors Distinguishing Displacement from Distance

(i)Displacement is direction-based. Distance is not direction-based.

(ii)Positive and negative displacements are both possible.

(iii)The distance is always positive. The distance does not decrease over time.

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Course creator

                                 Yogesh P

Yogesh P

I Am School Teacher having more than 12 years of experience subject of Physics in ICSE and CBSE Schools. Also, I have done M.Sc., B.Ed. And presently pursuing post-graduation diploma in school leadership and Management.