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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

RestA person is at rest if the distance between that person and ourself is constant.

MotionA person is in motion if the distance between that person and ourself is varying.

We say that person is moving away from us if the distance between the person and us is

increasing and we say that the person is moving towards us if the distance between the

person and us is decreasing.

"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 thetrain itself is in motion. Therefore, we may say that "A person seated in a moving train is at rest".

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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 personseated 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 platformA 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 withrespect 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 inthe 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

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(2) The Wheel of a moving vehicle

(3) The hands of a clock

A bicycle in total executes translatory motionA 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 ofrotation and periodically returns to its original position.

We use the word "ideal" rotation since many a times we come across objects that undergo bothtranslation 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. Thevehicle 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.

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Circular MotionA motion in which the body traverses a circular path is called circular motion. This is a

kind of translatory motion where the body reaches the initial position each time itcompletes traversing the circle.

Eg: (1)(2) Each particle of a body executing rotatory motion executes circular motion.

Periodic MotionAny Motion that repeats itself at regular intervals of time is called periodic motion.

Eg: (1) A bouncing ball

(2) A Vibrating spring

(3) Every body executing circular motion can be said to be executing periodic motion

(4) Every body executing oscillatory motion can be said to be executing periodic

motion

Random MotionIrregular Motion of bodies changing the nature of motion frequently is called Random

Motion.

Eg: (1) Motion of Football player on the ground.

(2) Motion of house files.

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

classification:

ant arrow bullet compact disc drop forge electron fan blade honeybee housefly

library book ocean current pencil satellite sewage stapler typewriter key washing machine

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weather balloon

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