CLASS-9TH SCIENCE CHAPTER -7 MOTION THEORY NOTES

CLASS-9TH SCIENCE CHAPTER -7 MOTION THEORY NOTES

Rest

 

A body is said to be at rest, if it does not change its position with respect to time / to the observer

Example – A Chair lying on floor, A book kept on the table,etc.

Motion

A body is said to be in motion if it changes its position with respect to time / observer

Example – A car moving on the road, Kite flying in air etc.

Types of motion

1. Uniform motion
2. Non uniform motion

Uniform motion

When a body covers equal distances in equal intervals of time is called uniform motion

Non-Uniform motion

When a body covers unequal distances in equal intervals of time is called non-uniform motion

Scalar quantity

Such physical quantity which has only magnitude is called scalar quantity

Example – Distance, Volume, Mass, Work, etc.

Vector quantity

Such physical quantity which has both magnitude and direction is called vector quantity

Example – Displacement, Velocity, Acceleration,  etc.

Some important physical quantity

1. Distance
2. Displacement
3. Speed
4. Velocity
5. Acceleration

Distance

• It is the total path length covered by a body
• It is a scalar quantity
• Distance travelled by a body is always positive
•  It cannot be zero for a moving body

Displacement

• It is the shortest distance between initial and final position of the body.
• It is vector quantity.
• Displacement of body may be positive or negative or zero .

Speed

• The distance covered by a body per unit time is called speed .
• Speed is a scalar quantity.
• Speed = distance travelled / Time
• v = s/t
• S.I unit = meter / second
• C.G.S unit = centimeter / second

Uniform Speed

When a body covered equal distance in equal intervals of time, howsoever small the time interval may be, then the body is said possess uniform speed.

Non-uniform speed

When a body covered unequal distance in equal time interval, then it is said to possess non- uniform speed.

Average speed

Average speed is the total distance travelled by the body divided by the total time taken.

Average speed = Total distance travelled / Total time taken

Velocity

The distance travelled by a body per unit time in a specified direction is called velocity.

Velocity is a vector quantity.

Velocity = Displacement / time taken

(v) = s/t

Uniform Velocity

A body is said to be moving with uniform velocity if ii moves along a fixed direction and covers equal distances in equal intervals of time, however small these intervals of time may be.

Non-uniform Velocity/Variable velocity

A body is said to be moving with variable velocity if:

1. its speed change but direction does not change.
2. its speed remains the same, but direction of motion change, or
3. both its speed and direction of motion change.

Examples of non-uniform velocity:

1. A stone thrown vertically upwards.
2. A stone dropped from a height.
3. A car moving towards east on a crowded road.

Average velocity

Average velocity is that velocity with which a body would cover the same distance along a particular direction (same displacement) at the same time.

Average velocity(V) = Displacement/Total time taken = S/T

Acceleration:

The rate of change of velocity of a body is called acceleration.

Acceleration= Change in velocity/Time taken for change

Acceleration= V-U/T

The S.I. Unit of acceleration is MS^-2.

Retardation:

The negative acceleration is also called retardation.

Equation of motion:

The equation which correlates initial velocity (u), final velocity(v), acceleration(a), time taken(t) and displacement(s) of a body for an activity in which a body has uniform acceleration are called equations of motion.

(1) First equation of motion: We know that acceleration is change in velocity per unit time.

a = v-u/t ∴ at = v-u

∴ V = u + at          . . . (

(2) Second equation of motion: We know that Average velocity = Initial velocity + Final velocity / 2

V _{a v = u + v / 2}

_{Then, distance covered (s) = Average velocity * Time} 

∴ s = u + v/ 2 *t

But v = u + at

[ First equation of motion]

∴ s = [ u+( u + at)] * t/2 = 2ut + at²   /2

or S = u t + 1/2 at²       . . . (II)

(3) Third equation of motion: We know that Average velocity = v + u/2

Distance covered (s) = Average velocity * Time

∴ s = (v + u/2)

But v = u + at

∴   t = v-u/a           . . . (III)

∴ s = (v-u/2) * (v-u/a)

^{or V2} – U^{2 =} 2as          . . . (IV)

 

CLASS-9TH SCIENCE CHAPTER -7 MOTION THEORY NOTES

WRITTEN BY – SHEELU YADAV & POOJA SHARMA = 9871217997

TYPIST – SHIVAM TIWARI

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