8. Motion

(Science-IX)

Speed with Direction: 

Velocity:

The direction of motion along with its speed. The quantity that specifies both these aspects is called velocity.

Definition: Velocity is the speed of an object moving in a definite direction. It is called velocity.

S.I unit of velocity is m/s or m s^-1.

Type of Velocity:

(i) Uniform velocity: When an object covers equal displacement in equal time of interval. It is called uniform velocity. 

(ii) Non-uniform velocity: When a object covers unequal displacement in equal time of interval. It is called non-uniform velocity.  

(iii) Average velocity: Average velocity is given by the arithmetic mean of
initial velocity and final velocity for a given period of time.

Initial velocity : Initial velocity of an object means the velocity when the object starts displacing where time(t) may be zeto or t ≠ 0.

It is denrted by 'u'.

When object starts moving from rest then u = 0. 

When an object is in motion then u ≠ 0. The value will be any integer. 

Final velocity : Final velocity of an object is given by time t. It is the value of final position in velocity of the object. 

• Initial and final velocity of an object can be zero.
• When an object starts from rest its initial velocity is zero.
• If it stops after applying break,its final velocity will be zero. As object is in rest.  

Differences between speed and velocity;

Speed: 

(i) It is a distance covered in unit time.

(ii) It is a scalar quantity. 

(iii) Average speed of moving object can not be zero. 

(iv) Speed of an object can never be zero. 

Velocity: 

(i) It is a displacement covered in unit time with direction. 

(ii) It is a vector quantity. 

(iii) Average velocity of moving object can be zero. 

(iv) Velocity of an object can be positive, negative or zero.  

When the object is moving in a straight line along positive direction and the distance and displacement of an two different objects are equal in same time then the value of speed and velocity will be equal. 

Changing the velocity of an object:

 (i) Change its magnitude. 

(ii) To change its the direction. 

(iii) To change its both magnitude and direction. 

Locomotion:

Motion of animals is known as locomotion.