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CBSE Physics Subject Notes

CBSE Guess > eBooks > Class IX > Physics by Mr. Sanjeev

Physics (Motion)

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There are many objects around us. Some are stationary like trees which are fixed at a place , house, a school, a factory all are stationary objects. On the other hand, a man, animals, birds, cars, buses, train, aero planes … etc do not remain stationary all time. They can move from one place to another. These are called moving objects and the movement of the body is called motion.

  A common characteristic of all the moving bodies is that they change their position with time  

A body is said to be in motion when its position changes continuously with respect to a stationary object taken as reference point.

 Distance: - The distance travelled by a body is the actual length of the path covered by a moving body irrespective of the direction in which the body travels.
   For example- if a man walks 4 km in east and 3km in north  then actual length covered by the man is 4+ 3= 7 km. so the distance travelled by the man is 7 km.

Displacement:- When the body moves from one position to another, the shortest distance b/w initial position and the final position of the body along with direction is called displacement.

Note:- distance is a scalar quantity having magnitude only but displacement is vector quantity having magnitude and direction both.

Example - In the same case the displacement is 5 km.

Note – the distance traveled by a moving body can’t be zero but final displacement of the moving body can be zero.

Uniform motion:- A body has uniform motion if it travels equal distance in equal interval of time, no matter  how small these time intervals may be.
And the distance time graph for uniform motion is a straight line. 
 
Example:- if a car running at a constant speed of 10 m per second will cover equal distance of 10 m in every second. So its motion will be uniform.

Non- uniform motion:- A body has non uniform motion if it travels unequal distances in equal interval of time.
    

For example- if we drop a ball from the roof of a tall building. We find that it covers unequal distances in equal interval of time.

 Note: - the distance time graph for a body having non uniform motion is a curved line.

Speed:- speed of a  body is the distance travelled  by it per unit time.

Speed = distance traveled / time taken
                      V = s/t
                      Where v = speed, s = distance traveled, t = time taken
             The s.i. unit of speed is m/s. it is a scalar quantity.

Average speed:- The average speed of  a body is the total distance traveled divided by the total time.

Average speed = total distance traveled / total time taken

Uniform speed:-  A body has uniform speed if it travels equal distance in equal interval of time. No matter how small these time intervals may be.
           
Velocity:- Velocity of a body is the distance traveled by it per unit time in a given direction.
  
Velocity = distance traveled in a given direction / time taken
  V= s/t

Where     v = velocity
S = distance traveled in a given direction
T= time taken ( for displacement)

The s.i. unit of velocity is m/s. it is vector quantity

Note:- the direction of the velocity is the same as the direction of displacement of the body.

Uniform velocity:- A body has  a uniform velocity if it travels in a specific direction in a straight line and moves over equal distance in equal interval of time, no matter how small these time intervals may be

   The velocity can be changed in two ways

  1. By changing the speed of the body
  2. By keeping the speed constant but changing the direction.

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