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

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1 
 
FORCE AND LAWS OF MOTION 
 
 
 Force 
The cause of motion is the force. No one has seen force or tasted force. However, we see or 
feel the effects of a force. Hence, concept of force is explained by describing what happens 
when some force is applied on an object. Force is an external effort in the form of pushing, 
pulling, stretching, compressing etc., 
The forces are of two types: Balanced forces and unbalanced forces. 
 Balanced and Unbalanced Forces 
If the resultant of all the forces acting on a body is zero, the forces are called balanced 
forces.The balanced forces cannot change the state of rest or state of uniform motion or the 
direction of motion of the body. They can however change the shape and size of the body. 
If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced 
forces. 
 
Newton’s First Law of Motion 
According to Newton’s first law of motion, a body continues to be in a state of rest or in a state 
of uniform motion along a straight line, unless an external force is applied on the body to 
change the state. 
Newton’s first law of motion gives us qualitative definition of force. Further, this law 
means that a body on its own, cannot change its state of rest or state of uniform motion along 
a straight line. This property is called inertia. Therefore, Newton’s first law of motion is also 
called the law of inertia. 
Inertia 
Inertia of a body is of three types: (i) Inertia of rest (ii) Inertia of motion and (iii) Inertia of 
direction. 
 (i)  Inertia of Rest 
 (a)  When a bus suddenly starts moving forward, the passengers in the bus fall 
backward. 
 (b)  The carpet is beaten with a stick to remove the dust particles. 
 (c)  Place a fifty paisa coin on a piece of a card-board covering the glass. Coin falls 
into the glass when the card board is hit. 
 (d)   When a tree is vigorously shaken, some of the fruits fall from the tree. 
 (ii)  Inertia of motion 
 (a)  The passengers fall forward when a fast moving bus stops suddenly. 
 (b)   A person falls forward while getting down from a moving bus or train. 
 (c)   A luggage is usually tied with a rope on the roof of buses. 
 (iii)  Inertia of Direction 
 (a)  When a fast moving bus negotiates a curve on the road, passengers fall towards 
the centre of the curved road. 
 (b)  The sparks produced during sharpening of a knife against a grinding wheel leave 
the rim of the wheel tangentially. This is because of the inertia of direction. 
Page 2


 
  
 
1 
 
FORCE AND LAWS OF MOTION 
 
 
 Force 
The cause of motion is the force. No one has seen force or tasted force. However, we see or 
feel the effects of a force. Hence, concept of force is explained by describing what happens 
when some force is applied on an object. Force is an external effort in the form of pushing, 
pulling, stretching, compressing etc., 
The forces are of two types: Balanced forces and unbalanced forces. 
 Balanced and Unbalanced Forces 
If the resultant of all the forces acting on a body is zero, the forces are called balanced 
forces.The balanced forces cannot change the state of rest or state of uniform motion or the 
direction of motion of the body. They can however change the shape and size of the body. 
If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced 
forces. 
 
Newton’s First Law of Motion 
According to Newton’s first law of motion, a body continues to be in a state of rest or in a state 
of uniform motion along a straight line, unless an external force is applied on the body to 
change the state. 
Newton’s first law of motion gives us qualitative definition of force. Further, this law 
means that a body on its own, cannot change its state of rest or state of uniform motion along 
a straight line. This property is called inertia. Therefore, Newton’s first law of motion is also 
called the law of inertia. 
Inertia 
Inertia of a body is of three types: (i) Inertia of rest (ii) Inertia of motion and (iii) Inertia of 
direction. 
 (i)  Inertia of Rest 
 (a)  When a bus suddenly starts moving forward, the passengers in the bus fall 
backward. 
 (b)  The carpet is beaten with a stick to remove the dust particles. 
 (c)  Place a fifty paisa coin on a piece of a card-board covering the glass. Coin falls 
into the glass when the card board is hit. 
 (d)   When a tree is vigorously shaken, some of the fruits fall from the tree. 
 (ii)  Inertia of motion 
 (a)  The passengers fall forward when a fast moving bus stops suddenly. 
 (b)   A person falls forward while getting down from a moving bus or train. 
 (c)   A luggage is usually tied with a rope on the roof of buses. 
 (iii)  Inertia of Direction 
 (a)  When a fast moving bus negotiates a curve on the road, passengers fall towards 
the centre of the curved road. 
 (b)  The sparks produced during sharpening of a knife against a grinding wheel leave 
the rim of the wheel tangentially. This is because of the inertia of direction. 
2 
 
 (c)  A stone tied to a string is whirling in a horizontal circle. If the string breaks, the 
stone flies away tangentially. 
 (d)  An umbrella protects us from rain. 
 Momentum and Newton’s Second Law of Motion 
Linear momentum (p) of a body is the product of mass of the body (m) and velocity of 
the body (v), i.e.  
According to Newton’s second law of motion, the rate of change of linear momentum of a 
body is directly proportional to the external force applied on the body, and this change 
takes place always in the direction of the applied force. 
The SI unit of force is newton (N). One newton is that much force which produces an 
acceleration of 1 m/s
2
 in a body of mass one kilogram. 
 i.e.,  1 N = 1 kg × 1 m/s
2
 
     = 1 kg m/s
2
 
Newton’s Third Law of Motion 
According to Newton’s third law of motion, “To every action, there is always an equal and 
opposite reaction”. 
Law of Conservation of Momentum 
According to the law of conservation of momentum, when two or more bodies interact 
with one another, the vector sum of their linear momenta remains constant (i.e., 
conserved), and is not affected due to their mutual action and reaction. The only condition is 
that no external unbalanced forces should be acting on the system of bodies. This law is 
deduced from Newton’s third law of motion. 
  
 
  
 
v m p
! !
=
 
U
A
 
A 
B 
m
B
 
U
B
 
Before Collision 
(a) 
A 
B 
F AB 
During Collision 
(b) 
F AB 
v
A
 
A 
B 
m
B
 
v
B
 
After Collision 
(c) 
m
A
 
m
A