# Class 9 Motion Notes

Notes Important Questions NCERT Solutions MCQ Quiz – 1 MCQ Quiz – 2

**Motion Notes**

## Motion:-

When a body changes its position with respect to time then the body is said to be in motion.

## Rest:-

A body is said to be in rest if it does not change its position with respect to time.

## Rectilinear Motion:-

Motion in a straight line is called rectilinear motion.

## Scalar Quantity:-

The physical quantity which depends only on magnitude is called scalar quantity. **Ex:-** distance, mass, speed etc.

## Vector Quantity:-

The physical quantity which depends on both magnitude and direction is called vector quantity. **Ex:-** displacement, velocity, weight, acceleration etc.

## Distance:-

The length of the actual path covered by an object is called distance.

- S.I. unit:-
**meter**(**m**) - It is a
**scalar**quantity.

## Displacement:-

The shortest distance between initial and final point is called displacement.

- S.I. unit:-
**meter**(**m**) - It is a
**vector**quantity.

## Uniform Motion:-

The motion of an object is said to be uniform if it covers equal distances in equal intervals of time.

## Non-uniform Motion:-

The motion of an object is said to be non-uniform if it does not cover equal distances in equal intervals of time.

## Speed:-

The distance covered by an object per unit time is called speed.

**Speed = Distance/Time**- S.I. unit:-
**meter/second** - Quantity:-
**Scalar** **Average speed = Total distance covered/Total time taken**

## Velocity:-

The displacement of an object per unit time is called velocity.

**Velocity = Displacement/Time**- S.I. unit:-
**meter/second** - Quantity:-
**Vector** **Average velocity = (Initial velocity + Final velocity)/2 = (u + v)/2**

## Acceleration(a):-

The change in velocity of an object per unit time is known as acceleration.

**a = (v – u)/t**- S.I. unit:-
**meter/second**Or,^{2}**m/s**^{2} - Quantity:-
**Vector**

**Odometer**measures the distance covered by vehicles.**Speedometer**measures the speed of the vehicles.

## Distance – Time Graphs:-

.

- The slope of distance – time graph gives the speed of the object.

## Velocity – Time Graphs:-

- The area under the velocity-time graph gives the distance (magnitude of displacement).
- The slope of velocity – time graph gives the acceleration of the object.

## Equation For Velocity – Time Relation:-

Consider an object moving with initial velocity ‘u’ and acceleration ‘a’. Let after time ‘t’, its final velocity is ‘v’.

From graph, OD = u, OC = v and OE = t

acceleration = Change in velocity/Time

or, a = DC/OE

or, a = (OC – OD)/OE

or, a =(v – u)/t

or, at = v – u

or, **v = u + at**

## Equation For Position – Time Relation:-

Consider an object moving with initial velocity ‘u’ and acceleration ‘a’. Let after time ‘t’, its final velocity is ‘v’.

From graph, OD = u, OC = BE = v and OE = t

Distance traveled = area of trapezium ODBE

or, S = 1/2(sum of parallel sides) x distance between them

or, S = 1/2(OD + BE) x OE

or, S = 1/2(u + v) x t

or, S = 1/2(u + u + at) x t **[since, v = u + at]**

or, S = 1/2(2u + at) x t

or, **S = ut + (1/2)at ^{2}**

## Equation For Position – Velocity Relation:-

Consider an object moving with initial velocity ‘u’ and acceleration ‘a’. Let after time ‘t’, its final velocity is ‘v’.

From graph, OD = u, OC = BE = v and OE = t

Distance traveled = area of trapezium ODBE

or, S = 1/2(sum of parallel sides) x distance between them

or, S = 1/2(OD + BE) x OE

or, S = 1/2(u + v) x t

or, S = 1/2(u + v)(v – u)/a **[since, v = u + at So, t = (v – u)/a]**

or, S = (v^{2} – u^{2})/2a

or, **2aS = v ^{2} – u^{2}**

## Uniform circular motion:-

When a body moves in a circular path with uniform speed, its motion is called uniform circular motion.

**Note:-** Circular motion is always an accelerated motion because the object changes its direction at every point of its motion.

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