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Rest and Motion kinds Translatory, Rotatory, Oscillatory, Periodic, Random

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Rest and Motion

With regard to motion, we identify two states : "A body being at rest or in motion".

The idea of Rest and Motion

When we are standing at a place we say that

"Rest and Motion" are relative and not absolute.

Absolute = Something that does not depends on anything else.
Relative = Something that is dependent on other things.

Whenever we express the state of a body to be at rest or in motion, we are expressing the state with respect to (in relation to) some other body which we identify as the frame of reference.

Two Persons in a Train
Consider two persons seated in a moving train. For each of them the other is at rest while the train itself is in motion. Therefore, we may say that "A person seated in a moving train is at rest".

two persons in a train are at rest with respect to each other two persons in a train are in motion with respect to a person on the ground

But would this statement be the same when they are viewed by a person on the ground. A person on the ground sees the train along with all the passengers and objects in it to be in motion. For him the two persons are in motion. Therefore, we need to modify this statement as "A person seated in a moving train is at rest in relation to any other person (body) in the same train but is in motion in relation to any other person (body) outside the train.

A Person on the platform
A person on the platform is at rest with reference to any other person on the platform and is in motion with reference to a person viewing him from a moving train.

Motion

Based on the above explanation, motion can be said to be a change in the position of a body with respect to time, as measured by a particular observer in a particular frame of reference.

Kinds of Motion

Based on the path taken by the bodies in motion we can classify the motion of bodies as translational (linear), vibrational or oscillational, rotational and random.

To understand the kinds of a motion, a body can be viewed as being made up of a number of tiny particles.

Translatory Motion

The motion of an object is said to be translatory if the position of the object is changing with respect to a fixed point or object. All the particles of a body excuting translatory motion move in the same direction traversing parallel paths.

The path traversed by the body in executing translatory motion may be either linear, circular, curve or any irregular shape. The motion will result in a change in location unless in the case where the body returns to its original location.
Eg: (1)

Rotatory Motion

The motion of an object is said to be rotatory if the motion of all the particles of body is circular (i.e. along a circular path) with respect to an imaginary line called the axis of rotation. This happens when an object spins. The axis of rotation may be internal or external to the body.

The center of the circular path that each particle traces lies on the "axis of rotation".
Eg: (1) A spinning Top
(2) The Wheel of a moving vehicle
(3) The hands of a clock

A bicycle in total executes translatory motion
A playing marble executes both rotatory motion as well as translatory motion.

Translatory Vs. Rotatory

An interesting difference to be noted between rotatory motion and translatory motion is that in translation the object as a whole keeps moving and goes somewhere, with no guarantee that it will ever come back whereas in ideal rotation the body remains in the vicinity of the axis of rotation and periodically returns to its original position.

We use the word "ideal" rotation since many a times we come across objects that undergo both translation and rotation simultaneously like in the case of the marble, bicycle above.

Oscillatory Motion

Oscillatory motion is repetitive and fluctuates between two locations. The to and fro motion of an object about a fixed point is called oscillatory motion or vibratory motion.
Eg: (1) An Oscillating Table Fan
(2) The Pendulum of a wall clock
(3) A Swinging Cradle
(4) Opening and Closing the door of a refrigerator.

Boides Executing two or more types of motion

Some of the bodies in motion may be executing two or more types of motion at the same time.
Eg: (1) A running vehicle executes both translatory as well as rotatory motion. The wheels of the vehicle execute rotatory motion, the axis of rotation being the axel of the whee. The vehicle itself executes translatory motion in moving from one place to another.
(2) A Spinning Billiards Ball executes both translatory as well as rotatory motion. In spinning it is executing rotatory motion and in moving from its original location it is executing translatory motion
(3)

Other Classification

The classification as above is used for studying motion. There are other ways motion is classified which we learn about only out of interest.

You can try identifying the nature of motion of these to gather better understanding of the classification:

 
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