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Physics XI | Rest and Motion

June 4th, 2011

Rest and Motion

What is motion? An object is said to be in motion if it changes its position continuously with respect to another object. The motion is a relative term. If we are sitting in a vehicle, a person on the ground would be in motion according to our view while according to the person who is sitting outside, we are in motion.

The study of motion or moving objects is known as Mechanics. In this section, we are studying the mechanics. Mechanics involved the study of motion whichever type it is. It also studies the objects at rest which tends to move. For example the study of a block putting on a table would be covered under in the mechanics although it is not moving. The study would be covered in mechanics because if the table is not there, the object will fall on the ground.

The branch of physics (or mechanics) which studies the motion without considering its causes is known as “kinematics”. In kinematics, we usually study about the position, displacement, velocity acceleration etc. Although kinematics in itself is a very broad subject, but for the sake of simplicity, we will only explain some types of motion up to class XII studies.

What is Rectilinear Motion?

The motion in straight line is known as rectilinear motion. For example, a car moving on a straight road would be in rectilinear motion.  To be start with, for the sake of simplicity, we start our study of mechanics or kinematics with the study of rectilinear motion. Although in real world, as we seen nothing moves in rectilinear motion. After completing the study on rectilinear motion, we will proceed to ‘motion in 2 dimension’ and ‘motion in 3 dimension’.


“Motion is common to everything in the universe. We walk, run and ride a bicycle. Even when we are sleeping, air moves into and out of our lungs and blood flows in arteries and veins. We see leaves falling from trees and water flowing down a dam. Automobiles and planes carry people from one place to the other. The earth rotates once every twenty-four hours and revolves round the sun once in a year. The sun itself is in motion in the Milky Way, which is again moving within its local group of galaxies.

Motion is change in position of an object with time. How does the position change with time? In this chapter, we shall learn how to describe motion. For this, we develop the concepts of velocity and acceleration. We shall confine ourselves to the study of motion of objects along a straight line, also known as rectilinear motion. For the case of rectilinear motion with uniform acceleration, a set of simple equations can be obtained. Finally, to understand the relative nature of motion, we introduce the concept of relative velocity. In our discussions, we shall treat the objects in motion as point objects. This approximation is valid so far as the size of the object is much smaller than the distance it moves in a reasonable duration of time. In a good number of situations in real-life, the size of objects can be neglected and they can be considered as point-like objects without much error.

In Kinematics, we study ways to describe motion without going into the causes of motion.”

Wikipedia article about motion:



Physics XI | Chapter 2 : Units and Measurements : Measurement of Large Distances

May 2nd, 2010

2.3.1 Measurement of Large Distances

Large distances such as the distance of a planet or a star from the earth cannot be measured directly with a metre scale. An important method in such cases is the parallax method.

When you hold a pencil in front of you against some specific point on the background (a wall) and look at the pencil first through your left eye A (closing the right eye) and then look at the pencil through your right eye B (closing the left eye), you would notice that the position of the pencil seems to change with respect to the point on the wall. This is called parallax. The distance between the two points of observation is called the basis. In this example, the basis is the distance between the eyes.

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