9th Physics Motion

 Class 9th Science -Physics

ICSE/ CBSE Notes-By Er. Amit Singh

For more Notes click this link https://cbsenotesallsub.blogspot.com/

Class 9th Physics -Motion

MOTION 

a-State of rest

An object is said to be at rest if the position of the object dose not change with respect to fix a point.

That fix point is said to be reference point 

Reference point - A fix point  w.r.t. the position of an object is specified  is known  as reference point or origin

b-State of motion

  The motion is the change in position of an object with respect to its surroundings in a given interval of time. 

 motion  can be described by terms -

• 1-distance
• 2-displacement
• 3-speed
• 4-velocity
• 5-time
• 6-acceleration

 

Uniform motion and non-uniform motion

Uniform motion

•  When an object covers equal distances in equal intervals of time it is in uniform motion.
• The distance time graph for uniform motion is a straight line.

Uniform motion

Non-uniform motion

• When an object covers unequal distances in equal intervals of time it is said to be in non-uniform motion.
• The distance -time graph  for  a body having non uniform motion is a curved line

Non Uniform motion

Vectors and Scalar Quantities:

1- Those quantities which have both magnitude and direction are called 

vector quantities

eg- displacement ,acceleration,momentum ,torque  etc

2- Those quantities which have only magnitude have no direction are called scalar quantities 

eg- distance , time ,mass, energy etc

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 Vectors  v/s  Scalar Quantities

Vectors  Quantities	Scalar Quantities
It can be specified by both magnitude and direction	It can be specified by only magnitude
These physical quantities follow the rule of vector algebra	These physical quantities  not follow the rule of vector algebra
Eg  velocity ,force  weight  , momentum  etc	Eg temperature ,mass, energy  ,density etc

Distance  and  Displacement 

Distance- 

• It is  defined as the length of the actual path  traveled by the body .

• It  is a scalar  quantity 

• It can never be negative

• It is always greater or equal to the displacement in any direction

Displacement-

• It is defined  as the shortest distance between the initial and final position of a moving object
• Displacement always a straight line  joining  both points

• It is a vector quantity

• It can be positive , negative and zero

• It is always less than or equal to the distance

SPEED

• Speed of the body is distance travelled by it per unit time.
• speed of a body gives us an idea of how slow or fast that body is moving.
• SI unity of speed is  m/s .
• Speed also express in Kmph means kilometres per hours .
• speed is scalar quantity.
• speed of running car is measured by Speedometer.
• Distance travelled by any object is measured by Odometer.

Average Speed

• The average speed of the body is the total  distance travelled divided by total time taken.
• Average speed = Total distance Travelled / Total time taken.

                      V = 60 km/h

Velocity

• Velocity of a body is  distance travelled  by it per unit time in a given direction.
• Velocity  = Displacement / Time taken   (V= d/t)
• SI Unit of velocity is m/s .
• Velocity is a vector quantity.

Uniform Velocity- 

• A body is said to be moving with uniform velocity if it covers equal distances in equal intervals of time in a specified direction.
• For Example a scooter moving with a constant speed along a straight line.

Non -Uniform Velocity

• A body is said to be moving non  uniform velocity if it covers unequal distances in equal intervals of time 
• For Example a cycle moving on a straight road with variable  speed

Average Velocity

• Average velocity is the ratio of total displacement of a body to time taken.
• it is also written as  -     Average velocity = Total Displacement / Total time taken
• Average velocity  = (initial  velocity  +  Final velocity)/ 2

 

Acceleration (Rate of change of velocity)

• Acceleration of a body is defined as the rate of change of its velocity with time.
• The SI unit of acceleration is  m/s square .
• Acceleration = Change in velocity / time taken for change
• change in velocity = Final velocity - initial velocity 
•  acceleration       a  =  (v-u)/t
• Acceleration is a vector quantity  and can be positive ,negative and zero.

Uniform  Acceleration- If velocity increases by equal amount in equal interval of time ,called  uniform acceleration.

Eg - Motion of free falling body .

Non uniform Acceleration- If velocity increases by unequal  amount in equal interval of time called Non uniform acceleration. 

Eg- Moment of a car in a crowded road.

NOTE-

• If the velocity of an object  increases , then the object said to be moving with positive acceleration. In this case  ( v > u)  ,  a is positive

• If the velocity of an object decreases , then the object said to be moving with negative acceleration . In this case (v <  u ), a is negative

• If the change in velocity is Zero , ie Either the object remains at rest or moving with uniform  velocity . In this case  (v =u ) , a is zero

Retardation 

• It is also called Deceleration or Negative Acceleration.
• it is acceleration with the negative sign.
• if  final velocity  less than initial velocity then acceleration is negative.
• its unit is same as the acceleration.

Equations of Uniformly  Accelerated Motion

There are three Equation of motion given as below-

1. First Equation of motion    
2. Second Equation of motion  
3. Third Equation of motion