We See different things in motion- people and vehicles move on roads, trains move on railway tracks, aeroplanes and bird fly, our teeth go up and down while we eat, the blades of a fan move when the fan is switched on (though the fan remains at the same place), raindrops fall, the sun moves from east to west as seen from the earth, and soon.
Things are in motion inside our body too. For example, blood moves through our blood vessels, the heart pumps blood and our muscles move when we work or play. We do not see these motions, but know about them from their effects.
The different kinds of motion around us appear to be very different. But they all follow the same set of rules. The motion of a body starts or changes when a force acts on it. We shall learn about the relation between force and motion in some other article. But first let us understand the meaning of motion more precisely.
Difference between the motion of the living and the non-living
The major difference between the motion of the living and the non-living is that the living objects move because of the energy from within whereas the non-living objects require energy from some outside source to move.
What is Motion?
Motion is change in position of an object with time.
The branch of physics which deals with the study of the motion of the object is known as mechanics.
Mechanics consists of three branches:
(i) Statics
(ii) Kinematics and
(iii) Dynamics
(i) Statics: The branch of mechanics which deals with the study of the objects at rest.
(ii) Kinematics: The branch of mechanics which describe motion without going into the cause of motion.
(iii) Dynamics: The branch of mechanics which deals with the study of the motion of the objects by
taking into the account of the force which cause the motion in the objects.
Reference Point
All motions are studied with respect to reference point. Such a reference point is called the origin. The motion of a body is studied with respect to this origin. Thus, a reference point is a fixed point with respect to which a body changes its position.
Rest
An object is said to be at rest, if it does not change its position with respect to the surroundings
with the passage of time.
Motion
An object is said to be in motion if it change its position with respect to the surroundings with
the passage of time.
Rest and motion are relative terms
There is nothing like absolute rest. This means that an object can be at rest and also in motion at the same time e.g., all objects, which are stationary on earth, are said to be at rest with respect to each other, but with respect to the sun are making revolutions at 30 kms–
In order to study motion, therefore, we have to choose a fixed position or point with respect to which the motion has to be studied. Such a point or fixed position is called a reference point or the origin. In order to describe the motion of an object we need to keep in mind three things-
- The distance of the body from a reference point. This reference point is called the origin of the motion of the body.
- The direction of motion of the body.
- The time of motion.