Class 9 important question of physics ch1

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                 MOTION

MOTION- A body is said to be in motion when it's position changes with respect to a reference point. 

REST- A body is said to be in a state of rest when it's position does not change with respect to a reference point. 

Types of motion - 

Uniform motion -  When a body travels equal distance in equal interval of time then the motion is said to be in uniform motion.  
Non-uniform motion- When a body travels unequal distance in equal interval of time then the motion is said to be in non-uniform motion.  

Physical quantities -

It is a physical properties of material that can be measured.
1)SCALAR QUANTITY- It is physical quantity having only magnitude not direction. 
Example-Speed , Distance 
2)VECTOR QUANTITY- it is a physical quantity having both magnitude as well as direction. 
Example- Velocity, Acceleration, Displacement 

DISTANCE-

The actual path length travelled by an object is called Distance. 
It is a scalar quantity. 
It is always positive and can't be 0.
It's SI unit is meter (m).

DISPLACEMENT-

The shortest distance between initial position and final position is called displacement .
It is a vector quantity. 
It can be positive, negative and 0.
It's SI unit is meter (m). 

SPEED-

The distance travelled by a body per unit time. 
It is a scalar quantity. 
It is always positive or 0.
It's SI unit is m/s.
Formula = Distance/Time 
Average speed= Total distance/ Total time 

VELOCITY-

The distance travelled by a body per unit in given direction. It is speed with direction.
It is a vector quantity. 
It can be negative, positive or 0.
It's SI unit is m/s.
Formula=Displacement/Time 
Average velocity = Initial velocity+Final velocity /2

ACCELERATION-

The rate of change of velocity of an object with respect to time is called acceleration. 
It may be positive or negative. 
Negative acceleration is also known as retardation or deacceleration.
It's SI unit is m/s².
Positive acceleration - When initial velocity is lower than final velocity. 
a=v-u/t 
Negative acceleration - When initial velocity is higher than final velocity. a=v-u/t

SYMBOLS-

Distance = s
Initial velocity = u
Final velocity = v
Time= t 
Acceleration = a 
Speed = Velocity = v
(v= s/t)

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