• Introduction

In everyday life, we see some objects at rest and others in motion. Birds fly, fish swim, blood flows through veins and arteries and cars move. Atoms, molecules, planets, stars and galaxies are all in motion.

Point Object

In mechanics, a particle is a geometrical mass point or a material body of negligible dimensions.  It is only a mathematical idealization. In practice, the nearest approach to a particle is a body, whose size is much smaller than the distance or the length measurements involved.

Describing Motion

Let us assume that Electrical Guru School in a village is 2 km north of the railway station. Here, we have specified the position of the school with respect to the railway station. So, the railway station is the reference point. Therefore, we can say that to describe the position of an object we need to specify a reference point called the origin.

While describing motion, we use reference point or origin w.r.t. which the motion of other bodies are observed. We can use any object as reference point.  For example, a car at rest or in motion can be used as reference point.

When you travel in a bus or train you can see the trees, buildings and the poles moving back. To a tree, you are moving forward and to you, the trees are moving back. Both, you and the trees, can serve as reference point but motion can not be described without reference point.

Motion

An object is said to be in motion if it changes its position with respect to its surroundings and with time. Examples:

1.  Moving cars, buses, trains, cricket ball, etc.

2.  All the planets revolving around the Sun.

3.  Molecules of a gas in motion above 0 K.

Motion can be of different types depending upon the type of path by which the object is going through.

(i)   Circulatory motion/Circular motion – In a circular path.

(ii)  Linear motion – In a straight line path.

(iii) Oscillatory/Vibratory motion – To and fro path with respect to origin.

Rest

An object is said to be at rest if it does not change its position with respect to its surroundings and with time. Examples: Mountains, Buildings, etc.

Rest and motion are relative terms. An object may appear to be moving for one person and stationary for some other. For the passengers sitting in a moving train are at rest w.r.t. each other  but they are also in motion at the same time w.r.t. the objects like trees, buildings, etc. Physical quantity

It is the name of a characteristic or property or phenomenon in our nature which can be measured. All the quantities in terms of which laws of physics are described, and whose measurement is necessary are called physical quantity. Physical quantities can be classified into two groups – fundamental quantities and derived quantities.

Fundamental quantity: It is the one which cannot be expressed in terms of any other quantity. There are basically seven fundamental quantities − Mass, Length, Time, Temperature, Electric current, Luminosity & Number of particles.

There two more quantities which were discovered later and added to the list of fundamental quantities − Plain angle & Solid angle.

Derived quantity: It is that characteristic, which can be expressed in terms of many fundamental quantities.

Scalar Quantity : There are certain quantities which are described by specifying only their numerical values. The numerical value of a physical quantity is its magnitude. Those physical quantities which have only magnitude but no direction is called scalar quantity. For examples: distance, speed, length, mass, time, speed, energy, etc.

Vector Quantity : There are certain quantities which are described by specifying both numerical values & direction. Those physical quantities which have magnitude as well as direction is called vector quantity. For examples: displacement, velocity, acceleration, momentum, Force, etc.

Motion along a Straight Line

The simplest type of motion is the motion along a straight line. When an object moves along a straight line, the motion of the object is called rectilinear motion. For example; motion of a vehicle along a straight road.

This can be represented graphically by plotting a graph between the position of the body and the time taken by it. This is called position-time graph.

Position Measurement of an Object 1. The distance measured to the right of the origin of the position axis is taken positive and the distance measured to the left of the origin is taken negative.

2. The origin for position can be shifted to any point on the position axis.

3. The distance between two points on position-axis is not affected due to the shift in the origin of position-axis.

Time Measurement of an Object 1. The time measured to the right of the origin of the time-axis is taken positive and the time measured to the left of the origin is taken negative.

2. The origin of the time-axis can be shifted to any point on the time-axis.

3. The negative time co-ordinate of a point on time-axis means that object reached that point a time that much before the origin of the time-axis, i.e., t = 0.

4. The time interval between two points on time-axis is not affected due to the shift in the origin of time-axis.