Concepts

Class 9
Chapter 8 Class 9 - Motion

## What is Acceleration

It is the rate of change in velocity over time

Formula for Acceleration

Acceleration = Change in Velocity/Time Taken

Acceleration = (Final Velocity - Initial Velocity)/Time Taken

a = (v - u)/t

where

v = Final Velocity

u = Initial Velocity

t = time taken

What is Unit of Measurement of Acceleration
it is m/s 2

(We know that formula for velocity is m/s.because we again divide it by s,Measure becomes m/s 2

#### Example 1

A car is moving at a speed of 6 meters/Second

In 5 Seconds, he increases its speed to 16 meters /second

What is Acceleration

Initial Velocity = u = 6

Final Velocity = v = 16

Time Taken = t = 5

Acceleration=Increase in Velocity/Time Taken

a = (Final Velocity - Old Velocity)/Time Taken

a = (v - u)/t

a = (16 - 6)/5

a = (10)/5

a = 2 Meters/Second 2

#### Example 2

A car Is stationary

In 5 Seconds,it reaches its speed to 16 meters /second

What is Acceleration

Because,Car is Stationary,Its initial velocity = 0

So u = 0

V = 16

Acceleration = Increase in Velocity/Time Taken

a = (Final Velocity - Old Velocity)/Time Taken

a = (v - u)/t

a = (16 - 0)/5

a = (16)/5

a = 3.2 Meters/Second 2

#### Negative Acceleration

Sometimes,acceleration of a body decreases

Example

Speed of car decreases

This is called negative acceleration or Retardation

#### Example 3

A car is running at a speed of 30 meters/second

It applies brake and reaches speed of 22 meters per second in 2 Seconds

What is acceleration

Initial Velocity = u = 30

Final Velocity = v = 22

Time Taken = t = 2

Acceleration=Increase in Velocity/Time Taken

a = (Final Velocity - Old Velocity)/Time Taken

a = (v - u)/t

a = (22 - 30)/2

a = -8/2

a = -4 Meters/Second 2

So in this case,there is Acceleration of -4 Meters/Second 2

We can say that there is Negative Acceleration of 4 Meters/Second 2

Note
Negative Acceleration or Retardation of 4 Meters/Second 2

Note: Negative Acceleration is also called retardation

## Questions

Q 1 Page 103 - When will you say a body is in
(i) uniform acceleration? (ii) nonuniform acceleration?

Q 2 Page 103 - A bus decreases its speed from 80 km h–1 to 60 km h–1 in 5 s. Find the acceleration of the bus.

Q 3 Page 103 - A train starting from a railway station and moving with uniform acceleration attains a speed 40 km h–1 in 10 minutes. Find its acceleration

NCERT Question 9 - State which of the following situations are possible and give an example for each of these:

Example 8.4 - Starting from a stationary position, Rahul paddles his bicycle to attain a velocity of 6 m s–1 in 30 s. Then he applies brakes such that the velocity of the bicycle comes down to 4 m s-1 in the next 5 s. Calculate the acceleration of the bicycle in both the cases.

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