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You see around you several things, like houses, trees, vehicles, people, animals and many other things moving and stationary. The house, trees, etc., are all fixed in their positions. The birds sit on the trees for sometime and then fly off to another tree or place. People may sit in a place for sometime or may keep moving around. Blood flows through the arteries and veins. Atoms, molecules, stars and galaxies are in constant motion. Air in motion is perceived only when leaves and branches move. Sunrise and sunset are caused due to the motion of the earth.


How is it we do not perceive the earth in motion?


Most motions are complex. Some objects may move in a straight line, others may take a circular path. Some may rotate and a few others may vibrate. There may be situations involving a combination of these.

In this chapter, we shall first learn to describe the motion of objects along a straight line. We shall also learn to express such motions through simple equations and graphs. Later, we shall discuss ways of describing circular motion.

From all this we can make out that there are two types of objects:

a) Objects which remain fixed in their positions and
b) Objects which keep changing their positions.

The first set of objects is said to be at rest. Their position does not change with respect to a fixed point in their surroundings.


Example : houses, trees, etc.


The second set of objects is in motion. Their position goes on changing with respect to a fixed point in their surroundings.

Example : birds flying, people moving, etc.



The states of motion and rest are relative. Why?


In order to know if an object is in motion or at rest we must have a known fixed point anywhere in space. But there is no such point, as nothing in space is at absolute rest.


How do we classify objects as being in motion or at rest?


This is done by choosing a convenient fixed point in the immediate surroundings of the body. This point is called the reference point and it can be an object or body.


if a body changes its position with respect to the reference point it is said to be in motion. If its position remains unchanged with respect to the reference point, then it is said to be at rest.


If its position remains unchanged with respect to the reference point, it is said to be at rest.

To understand this, let us take the case of passengers seated in a train. The passengers are not moving with respect to each other or the train when it is in motion. But if we consider a person outside the compartment, on the platform, as reference point, to him the passengers and the train are in motion. This shows that the same set of objects may be at rest with respect to one reference point but may be in motion with respect to another reference point.

Motions can be of various types – an object can move in a straight path or along a curved path (translatory motion); or the object can move around a fixed axis (rotational motion); or the object can move about an equilibrium position (oscillatory motion). Objects can exhibit a combination of motions. In this chapter, we will be learning about motion along a straight line, equations to express such motions and graphical representation of such motion.


Describing Motion

The position of the object can be described by specifying the following physical factors:

a. The distance of the object from the reference point. (The origin or the point of intersection of the Cartesian axes is generally taken as the fixed reference point).

b. The direction of the object – the reference axis may either be the Cartesian axes or the geographical directions.



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