Short Notes - Motion

# Motion Class 9 Notes Science Chapter 7

 Table of contents Introduction Distance and Displacement Uniform and Non-uniform Motions Speed Velocity Acceleration Retardation/Deaceleration Graphical Representation of Equation Equation of Motion (For Uniformly Accelerated Motion) Uniform Circular Motion

## Introduction

• Rest: A body is said to be in a state of rest when its position does not change with respect to a reference point.
• Motion: A body is said to be in a state of motion when its position change continuously with reference to a point.

Motion can be of different types depending upon the type of path by which the object is going through.
(i) Circulatory motion/Circular motion – In a circular path.
(ii) Linear motion – In a straight line path.
(iii) Oscillatory/Vibratory motion – To and fro path with respect to origin.

• Scalar quantity: It is the physical quantity having own magnitude but no direction.
• Example: distance, speed.
• Vector quantity: It is the physical quantity that requires both magnitude and direction.
• Example: displacement, velocity.

## Distance and Displacement

• The actual path or length travelled by a object during its journey from its initial position to its final position is called the distance.
• Distance is a scalar quantity which requires only magnitude but no direction to explain it.
• Example: Ramesh travelled 65 km. (Distance is measured by odometer in vehicles.)
• Displacement is a vector quantity requiring both magnitude and direction for its explanation.
• Example: Ramesh travelled 65 km south-west from Clock Tower.
• Displacement can be zero (when initial point and final point of motion are same)
• Example: circular motion.

Difference between Distance and Displacement

## Uniform and Non-uniform Motions

Uniform Motion
When a body travels equal distance in equal interval of time, then the motion is said to be uniform motion.

Non-uniform Motion

In this type of motion, the body will travel unequal distances in equal intervals of time.

Two types of non-uniform-motion
(i) Accelerated Motion: When motion of a body increases with time.

(ii) De-accelerated Motion: When motion of a body decreases with time.

## Speed

• The measurement of distance travelled by a body per unit time is called speed.
• Speed (v) = Distance Travelled/Time Taken = s/t
• SI unit = m/s (meter/second)
• If a body is executing uniform motion, then there will be a constant speed or uniform motion.
• If a body is travelling with non-uniform motion, then the speed will not remain uniform but have different values throughout the motion of such body.
• For non-uniform motion, average speed will describe one single value of speed throughout the motion of the body.
• Average speed = Total distance travelled/Total time taken

### Conversion Factor

• Change from km/hr to m/s = 1000m/(60×60)s = 5/18 m/s

## Velocity

It is the speed of a body in given direction.

• Velocity = Displacement/Time
• Velocity is a vector quantity. Its value changes when either its magnitude or direction changes.

For non-uniform motion in a given line, average velocity will be alculated in the same way as done in average speed.

• Average velocity = Total displacement/Total time
• For uniformly changing velocity, the average velocity can be calculated as follows :
• Avg. Velocity (vavg) = (Initial velocity + Final velocity)/2 = (u+v)/2
• where,  u = initial velocity, v = final velocity
• SI unit of velocity = ms-1
• Velocity = Displacement/Time

It can be positive (+ve), negative (-ve) or zero.

## Acceleration

• Acceleration is seen in non-uniform motion and it can be defined as the rate of change of velocity with time.
• Acceleration (a) = Change in velocity/Time = (v-u)/t
where, v = final velocity, u = initial velocity
• If v > u, then ‘a’ will be positive (+ve).

## Retardation/Deaceleration

→ Deaceleration is seen in non-uniform motion during decrease in velocity with time. It has same definition as acceleration.

• Deaceleration (a') = Change in velocity/Time = (v-u)/t

Here, v < u, ‘a’ = negative (-ve).

## Graphical Representation of Equation

Distance-Time Graph (s/t graph)
(i) s/t graph for uniform motion:

(ii) s/t graph for non-uniform motion:

(iii)  s/t graph for a body at rest:

v = (s2 - s1)/(t2 - t1)
But, s2 - s1
∴ v = 0/(t2 - t1) or v=0

### Velocity-Time Graph (v/t graph)

(i) v/t graph for uniform motion:

a = (v2 - v1)/(t2 - t1)
But, v2 - v1
∴ a = 0/(t2 - t1) or a = 0

(ii) v/t graph for uniformly accelerated motion:

In uniformly accelerated motion, there will be equal increase in velocity in equal interval of time throughout the motion of body.

(iii) v/t graph for non-uniformly accelerated motion:

a2 ≠ a1
(iv) v/t graph for uniformly decelerated motion:

or, a1' = a2'

(v) v/t graph for non-uniformly decelerated motion:

a1' ≠ a2'

Note: The area enclosed between any two time intervals is ‘t2 - t1’ in v/t graph will represent the total displacement by that body.

Total distance travelled by body between t2 and t1, time intervals
= Area of ∆ABC + Area of rectangle ACDB
= ½ × (v2 – v1)×(t2 - t1) + v1× (t2 - t1)

## Equation of Motion (For Uniformly Accelerated Motion)

First Equation: v = u + at
Final velocity = Initial velocity + Acceleration × Time

Graphical Derivation
Suppose a body has initial velocity ‘u’ (i.e., velocity at time t = 0 sec.) at point ‘A’ and this velocity changes to ‘v’ at point ‘B’ in ‘t’ secs. i.e., final velocity will be ‘v’.

For such a body there will be an acceleration.
a = Change in velocity/Change in Time
⇒ a = (OB - OA)/(OC-0) = (v-u)/(t-0)
⇒ a = (v-u)/t
⇒ v = u + at

Second Equation: s = ut + ½ at2
Distance travelled by object = Area of OABC (trapezium)
= Area of OADC (rectangle) + Area of ∆ABD
= u × t + ½ × t × (v – u)
= ut + ½ × t × at
⇒ s = ut + ½ at2  (∵a = (v-u)/t)

Third Equation: v2 = u2 + 2as
s = Area of trapezium OABC

## Uniform Circular Motion

• If a body is moving in a circular path with uniform speed, then it is said to be executing uniform circular motion.
• In such a motion the speed may be same throughout the motion but its velocity (which is tangential) is different at each and every point of its motion. Thus, uniform circular motion is an accelerated motion.

The document Motion Class 9 Notes Science Chapter 7 is a part of the Class 9 Course Science Class 9.
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## FAQs on Motion Class 9 Notes Science Chapter 7

 1. What is motion?
Ans. Motion is the change in position of an object with respect to its surroundings. It can be described in terms of displacement, velocity, and acceleration.
 2. How is motion measured?
Ans. Motion can be measured using various parameters such as distance, time, speed, and acceleration. Distance is the total path covered by an object, while time is the duration taken to cover that distance. Speed is the rate at which an object covers a certain distance in a given time, and acceleration is the change in velocity over time.
 3. What are the different types of motion?
Ans. There are several types of motion, including linear motion, circular motion, rotational motion, and oscillatory motion. Linear motion refers to motion in a straight line, while circular motion involves objects moving along a circular path. Rotational motion is the motion of an object spinning around an axis, and oscillatory motion is the back and forth motion of an object.
 4. What is the difference between speed and velocity?
Ans. Speed and velocity are both measures of motion, but they have different meanings. Speed is a scalar quantity that refers to the rate at which an object covers a certain distance in a given time. Velocity, on the other hand, is a vector quantity that includes both speed and direction. It indicates the rate of change of displacement with respect to time.
 5. What is the relationship between distance, time, and speed in motion?
Ans. The relationship between distance, time, and speed in motion can be described by the formula: speed = distance/time. This formula shows that speed is directly proportional to distance and inversely proportional to time. In other words, if the distance covered by an object increases, its speed will also increase, while if the time taken to cover the distance increases, the speed will decrease.

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