## Importance of the study of motion

The universe is in a state of continual motion. We move over the earth surface; the earth moves in its orbit around the sun. The sun and earth are also in a state of continuous motion. Similarly, in every piece of matter, atoms are in a state of never-ending motion. Take any physical process, it may involve some kind of motion. Therefore, study of motion has a huge importance in Physics.

## Motion and Rest

Before we say something about the states of rest and motion, let’s elaborate on few important concepts.

### Relative and absolute measuremets

Let’s take some examples.
• You are in a car moving with a speed of 90 km/hr. Another car which moves with a speed of 120 km/hr, overtakes you. What is the rate of separation between the two vehicles? Sure, neither 90 km/hr nor 120 km/hr. It is 30 km/hr. So, the other car is moving with a speed of 30 km/hr as you measure it. However, a person standing on the roadside measures the speed of the other car as 120 km/hr.
• Similarly, if your friend is sitting with you in your car, his/her distance is not changing ‘relative’ to you and it seems you are at rest relative to one another. But the person standing on the roadside will note that you are moving with a speed of 90 km/hr.

This means that motion and rest depends upon the state of the observer. (‘State’ means whether the observer itself is in motion or rest). So, if the two observers are in different states of motion, they might reach to different conclusions about the speed of a single event!

Such measurements are called relative measurements.

Now consider another situation; speed of light. Light will reach you, the person in the other car or the one standing on the bank or road at the same time, irrespective of their position or state motion. This means that speed of light is same, seen by any observer, no matter how fast is he moving with respect to the source of light. Such measurements are called absolute measurements.

Position and position vector

Position of a moving object is given by its position vector. Position vector is determined by two parameters; magnitude of the vector and the angel θ which the vector makes with x-axis. Angle θ, in fact, shows the direction of the vector.

Position vector is drawn from a reference point ‘O’, usually the place of the observer, to the place in coordinate axes, where the object exists at a certain time. If the object is moving in a straight line then only the magnitude of the vector is changing, the angle θ remains constant. If the object is moving in a circle, only the angle θ is changing and the magnitude of the position vector remains constant. If the body is moving on a zig-zag path, both magnitude and direction of the position vector change.

Now, let’s define few terms used in the study of motion.

## Definition of Motion

If a body is continuously changing its position with respect to the observer with the passage of time, it is said to be in motion. For example, a passenger in a car is in motion with respect to the observer on the roadside, as its position continuously changes with respect to the observer.

## Definition of Rest

If, with the passage of time, a body does not change its position with respect to an observer then it is said to be in the state of rest. For example, a passenger in a car is at rest with respect to his fellow passenger as the position vector between does not change with time.