CONCEPT OF REST AND MOTION AND THEIR TYPES

A person sitting in a moving bus is at rest with respect to a fellow passenger but is in motion with respect to a person outside the bus. The concepts of rest and motion have meaning only with respect to some reference frame. To understood rest and motion we need a convenient fixed reference frame.

FRAME OF REFERENCE:
If we imagine a coordinate system and the position of an object is described relative to it, then such a coordinate system is called frame of reference.

POINT MASS
To explain the motion of an object which has finite mass, the concept of “point mass” is required and is very useful. Let the mass of any object be assumed to be concentrated at a point. Then this idealized mass is called “point mass”.

It has no internal structure like shape and size. Mathematically a point mass has finite mass with zero dimension. Even though in reality a point mass does not exist, it often simplifies our calculations. It is to be noted that the term “point mass” is a relative term. It has meaning only with respect to the king of motion.

Example:
To analyse the motion of Earth with respect to Sun, Earth can be treated as a point mass. This is because the distance between the sun and Earth is very large compared to the size of the Earth.

TYPES OF MOTION

LINEAR MOTION
An object is said to be in linear motion if it moves in a straight line.

Examples:
An athlete running on a straight track.
A particle falling vertically downwards to the Earth.

CIRCULAR MOTION
Circular motion is defined as a motion described by an object traversing a circular path.

Examples:
The whirling motion of stone attached to a string.
The motion of a satellite around the Earth.

ROTATIONAL MOTION:
If any object moves in a rotational motion about an axis, the motion is called ‘rotation’. During rotation every point in the object transverses a circular path about an axis, (except the points located on the axis).

Examples:
Rotation of a disc about an axis through its centre.
Spinning of the Earth about its own axis.

VIBRATORY MOTION
If an object or particle executes a to-and-fro motion about a fixed point, it is said to be in vibratory motion. This is sometimes also called oscillatory motion.

Examples:
Vibration of a string on a guitar.
Movement of a swing