If the position of a material system as measured by a particular observer changes with respect to time, that system is said to be in motion with respect to the observer. Absolute motion, has no significance, and only relative motion may be defined.
There are various kinds of motion – one dimensional, two-dimensional and three-dimensional.
Movement of lift from ground floor to sixth floor or an apple, dropped from the branch of a tree before it strikes the ground are examples of motion in one dimension.
A boat sailing on a lake over distances which are small compared to the earth’s radius; a carrom coin or a billiards ball in motion; these are examples of motion in two dimensions over a plane. Objects moving on the surface of the earth over distances comparable to the earth’s radius are examples of motion in two dimensions, but not over a plane. Finally, the most general motions we can consider are of objects moving in space, involving all the three dimensions.
Movement of birds in the sky, helicopter or flying of kite in windy day are examples of motion in three dimension.
The actual length of path travelled by a body is called the distance (S) travelled or covered by the body. It is a scalar quantity and is measured in metres in SI units.
The shortest distance from the initial to the final position of a body is called displacement of the body. Displacement vector is the straight line joining the initial and final position and does not depend on the actual path undertaken by the object between the two positions.
A person starts his journey from point A and moves along ABCDA.
For path AB, distance = 2 km
and displacement = 2 km
For path A-B-C, distance = 3 km
and displacement = 22 + 12 = 15 km For path A-B-C-D distance = 5 km
and displacement =
For path A-B-C-D-A distance =5 km
and displacement = 1 km
For path A-B-C-D-A distance =6 km
and displacement = ZERO
When initial and final position coincide the displacement becomes zero.
Speed is the time rate of change of position. It is measured in metre per second, centimetre per second, kilometre per hour etc. Average speed is the total distance traveled divided by the time taken.
The time rate of displacement (change of position vector) of the particle is called velocity. Thus
The velocity is a vector quantity. Its unit is m/s.
Is is of two types:
(a) Uniform Velocity: If a body covers equal distances in a particular direction in equal intervals of time, it is said to move with a uniform velocity.
(b) Variable Velocity: If a body covers unequal distances in a particular direction in equal intervals of time, however small, it is said to move with a variable velocity.