Important Points and Formulas: Motion | Science Class 9 PDF Download

 Page 1


 
  
 
1 
 
MOTION 
 
 
 Motion 
• Motion means movement. 
• The motion of an object is perceived when its position changes continuously with 
respect to some stationary object. 
• In certain situations, motion is inferred through indirect evidence. 
• The states of rest and motion are relative. An object may appear to be moving with 
respect to one person, and the same object may appear to be at rest with respect to another 
person. 
• To locate the position of an object, we have to choose some suitable reference point, 
called the origin. 
 Distance and Displacement 
• The distance travelled by an object is the length of the actual path traversed by the 
object during motion. 
• The displacement of an object in motion is the shortest distance between the initial 
position and final position of the object. 
• Distance is a scalar quantity having magnitude only. Displacement is a vector quantity 
having both the magnitude as well as direction. 
• The distance travelled by an object in motion can never be zero or negative. The 
displacement can be positive, zero or negative. 
• When final position of an object in motion coincides with its initial position, the 
displacement is zero, but the distance travelled is not zero. 
• Between two given positions, distance travelled can never be less than the 
displacement. 
 
 Speed and Velocity 
Speed of a body is a measure of rate of motion of the body. It is equal to distance travelled 
by the body in unit time. 
 
 It is measured in m/s or cm/s or km/h. 
Velocity of a body is defined as the distance travelled by the body in a given direction in unit 
time. Thus, 
          
 
 Acceleration 
Acceleration of a body is defined as the rate of change of velocity of the body with time. 
 
 Unit of acceleration =  
) ( taken time
) ( travelled distance
) ( speed
t
s
v =
taken time
direction given a in travelled distance
velocity =
) ( time
) ( nt displaceme
t
s
v
!
!
=
t
u v
a
-
= =
taken time
velocity in change
) ( on Accelerati
2 2 2
km/h , cm/s , m/s
Page 2


 
  
 
1 
 
MOTION 
 
 
 Motion 
• Motion means movement. 
• The motion of an object is perceived when its position changes continuously with 
respect to some stationary object. 
• In certain situations, motion is inferred through indirect evidence. 
• The states of rest and motion are relative. An object may appear to be moving with 
respect to one person, and the same object may appear to be at rest with respect to another 
person. 
• To locate the position of an object, we have to choose some suitable reference point, 
called the origin. 
 Distance and Displacement 
• The distance travelled by an object is the length of the actual path traversed by the 
object during motion. 
• The displacement of an object in motion is the shortest distance between the initial 
position and final position of the object. 
• Distance is a scalar quantity having magnitude only. Displacement is a vector quantity 
having both the magnitude as well as direction. 
• The distance travelled by an object in motion can never be zero or negative. The 
displacement can be positive, zero or negative. 
• When final position of an object in motion coincides with its initial position, the 
displacement is zero, but the distance travelled is not zero. 
• Between two given positions, distance travelled can never be less than the 
displacement. 
 
 Speed and Velocity 
Speed of a body is a measure of rate of motion of the body. It is equal to distance travelled 
by the body in unit time. 
 
 It is measured in m/s or cm/s or km/h. 
Velocity of a body is defined as the distance travelled by the body in a given direction in unit 
time. Thus, 
          
 
 Acceleration 
Acceleration of a body is defined as the rate of change of velocity of the body with time. 
 
 Unit of acceleration =  
) ( taken time
) ( travelled distance
) ( speed
t
s
v =
taken time
direction given a in travelled distance
velocity =
) ( time
) ( nt displaceme
t
s
v
!
!
=
t
u v
a
-
= =
taken time
velocity in change
) ( on Accelerati
2 2 2
km/h , cm/s , m/s
    
2 
 
  
  
 Three equations of motion 
When a body is moving along a straight line with uniform acceleration, the equations, which 
govern this motion, are: 
 (i) v = u + at (velocity-time relation). 
 (ii) s = ut +  (position-time relation). 
 (iii)   (position-velocity relation). 
 In all the three equations, u represents initial velocity of the body, a is uniform 
acceleration of the body; v is final velocity of the body after t seconds and s is the distance 
travelled by the body in this time. 
Circular Motion 
In uniform circular motion, speed of the body along a circular path is constant. But the direction 
of motion is changing continuously, being along the tangent to the circular path at every 
instant. Therefore, velocity of the body changes in uniform circular motion. Hence uniform 
circular motion is an accelerated motion. 
An external force is required to move a body uniformly in a circle. This force is called 
centripetal force. 
If a body takes t seconds to go once round a circle of radius r, then uniform speed of the body, 
. 
 
2
2
1
at
as u v 2
2 2
= -
t
r
v
p
=
2