Important Points: Motion | Science Class 9 PDF Download

Rest 

A body is said to be in a state of rest when its position does not change with respect to a reference point.

Object at Rest

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.

Object in Motion

• Circulatory Motion / Circular Motion – In a circular path.
• Linear Motion – In a straight line path.
• Oscillatory / Vibratory Motion – To and fro path with respect to origin.

Physical Quantity

There are seven basic physical quantity. Every quantity in written in two parts : first write the magnitude of the physical quantity and then write the unit of the quantity.
i.e. magnitude 4 Unit.
Physical Quantities can be grouped into two

Scalar Quantities

• Those Physical quantities that has only magnitude but no direction.
• i.e. :- speed, distance, mass, volume, time, temperature, work, electric current.

Vector quantities 

• Those physical quantities that has both magnitude as well as direction.
• i.e. velocity, force, momentum, displays etc.

Distance 

The actual path or length travelled by an object during its journey from its initial position to its final position is called the distance. 

• It is denoted by 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 

• 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.
• Distance and displacement are denoted by ‘S’.

Difference between Distance and Displacement 

Uniform Motion

• When a body travels equal distance in equal interval of time, then the motion is said to be uniform motion.
• eg :- movements of hands of a clock rotation and revolution of the earth.

Non – uniform Motion

• In this type of motion, the body will travel unequal distances in equal intervals of time. 
• Example: motion of a car on busy road.

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Types of Non – uniform

Non – uniform motion is of two types 

Accelerated Motion: When motion of a body increases with unequal time.
De – accelerated Motion or Non uniform Retardation: When motion of a body decreases with unequal interval of time.

Speed

• The measurement of distance travelled by a body per unit time is called speed. It is denoted by v.
• Speed = Distance travelled / Time taken
• V = 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

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. It is also denoted by v 
• For non – uniform motion in a given line, average velocity will be calculated 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
• velocity: Initial velocity + Final velocity Avg / 2
• V(avg) = u+v/2
• where, u = initial velocity, v = final velocity  
• SI unit of velocity = ms^-1
• Velocity = Displacement/Time

Acceleration 

Acceleration is seen in uniform motion and it can be defined as the rate of change of velocity with time.

• Acceleration: Change in velocity/Time
  Acceleration: Final Velocity Initial/Velocity Time
• a = v-u/t
• where, v = final velocity, u = initial velocity 
• If v > u, then ‘a’ will be positive (+ve).

Retardation/Deacceleration

• Deacceleration is seen in uniform motion during decrease in velocity with time. It has same definition as acceleration.
• => Change in velocity/time
• A=V-u/t
• Here v < u, ‘a’ = negative (-ve).
• Unit of Acceleration and deacceleration is m/s² or ms^-2

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Uniform Circular Motion

• If a body is moving in a circular path with uniform speed, It is motion is called 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 due to continuous change in direction. Thus, uniform circular motion is an accelerated motion. 
• so, velocity of an object in a circular motion is
• V=2πr/t