Motion (Class-IX)

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What is motion?

State of Rest: An object is said to be in the state of rest if it does not change its position with respect to time and its surroundings.

State of Motion: An object is said to be in the state of motion if it changes its position with respect to time and its surroundings.

Note: Rest & Motion are relative terms.

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.

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  (ii) Linear motion – In a straight line path.

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  (iii) Oscillatory/Vibratory motion – To and fro path with respect to origin.

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Scalar quantity : Physical quantities having magnitude but no direction are called scalars. e.g., distance, speed.

Vector quantity : Physical quantities having both magnitude and direction are called vectors. e.g., displacement, velocity.

Distance (or Path length) & Displacement

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Path length (or distance) : The actual path or length travelled by a object during its journey from its initial position to its final position is called the distance.

Note:

(i) Distance is a scalar quantity which requires only magnitude but no direction.

     Example, Manju travelled 65 km. 

(ii) Distance is measured by odometer in vehicles.

Displacement: It is the shortest distance between the initial and final position of a object. 

Note:

(i) The shortest distance between two point is a straight line.

(ii) Displacement is a vector quantity which requires both magnitude and direction for its explanation.

   Example, Nitin travelled 65 km south-west from Clock Tower.

(iii) Displacement can be zero (when initial point and final point of motion are same) Example, circular motion.

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Difference between Distance & Displacement

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Solved Examples:

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Solution:

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Given,

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ee1

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Example 2. A body travels 4 km towards North then he turn to his right and travels another 4 km before coming to rest. Calculate (i) total distance travelled, (ii) total displacement.

Solution:

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Speed and Velocity

Speed : The measurement of distance travelled by a body per unit time is called speed.

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SI Unit: meter/second (m/s)

Velocity: It is the speed of a body in given direction.

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NOTE:

(1) velocity is a vector quantity. Its value changes when either its magnitude or direction changes.

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

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.

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Non-uniform Motion :
In this type of motion, the body will travel unequal distances in equal intervals of time.

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Non-uniform motion is of two types :

(i) Accelerated Motion : When motion of a body increases with time.

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

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NOTE:

  • If a body is executing uniform motion, then there will be a constant speed.
  • 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.

For non-uniform motion, average speed will describe one single value of speed throughout the motion.

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NOTE:

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

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(II) For uniformly changing velocity, the average velocity can be calculated as follows :

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where, u = initial velocity, v = final velocity

Conversion factor:

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Solved Examples:

Example : What will be the speed of body in m/s and km/hr if it travels 40 km in 5 hrs ?

Solution :

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

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where, v = final velocity, u = initial velocity
If v > u, then ‘a’ will be positive (+ve).

Retardation/Deceleration : Deceleration is seen in non-uniform motion during decrease in velocity with time. It has same definition as acceleration.

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Here v < u, ‘a’ = negative (-ve).


Solved Examples:

Example 1 : A car speed increases from 40 km/hr to 60 km/hr in 5 sec. Calculate the acceleration of car.

Solution :

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Example 2. A car travelling with a speed of 20 km/hr comes into rest in 0.5 hrs. What will be the value of its retardation ?

Solution :

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Equation of Motion (For Uniformly Accelerated Motion)

(i) 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’.

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For such a body there will be an acceleration.

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(ii) Second Equation:

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Distance travelled by object

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(iii) Third Equation:

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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.

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