Laws of Motion Class 11 Notes Physics Chapter 4

 Page 1


Physics Class XI
58
3.1 Inertia
(1) Inherent property of all the bodies by virtue of which they cannot 
change their state of rest or uniform motion along a straight line by 
their own is called inertia.
(2) Two bodies of equal mass possess same inertia because it is a factor 
of mass only.
3.2 Linear Momentum
(1) Linear momentum of a body is the quantity of motion contained in 
the body.
(2) It is measured as the product of the mass of the body and its velocity 
i.e., Momentum = mass × velocity.
If a body of mass m is moving with velocity  then its linear 
momentum  is given by 
(3) It is a vector quantity and it’s direction is the same as the direction 
of velocity of the body.
(4) Units : kg-m/sec [S. I.], g cm/sec [C.G. S.]
(5) Dimension : [MLT
–1
]
3.3 Newton’s First Law
A body continue to be in its state of rest or of uniform motion along a  
straight line, unless it is acted upon by some external force to change 
the state.
(1) If no net force acts on a body, then the velocity of the body cannot 
change i.e., the body cannot accelerate.
Page 2


Physics Class XI
58
3.1 Inertia
(1) Inherent property of all the bodies by virtue of which they cannot 
change their state of rest or uniform motion along a straight line by 
their own is called inertia.
(2) Two bodies of equal mass possess same inertia because it is a factor 
of mass only.
3.2 Linear Momentum
(1) Linear momentum of a body is the quantity of motion contained in 
the body.
(2) It is measured as the product of the mass of the body and its velocity 
i.e., Momentum = mass × velocity.
If a body of mass m is moving with velocity  then its linear 
momentum  is given by 
(3) It is a vector quantity and it’s direction is the same as the direction 
of velocity of the body.
(4) Units : kg-m/sec [S. I.], g cm/sec [C.G. S.]
(5) Dimension : [MLT
–1
]
3.3 Newton’s First Law
A body continue to be in its state of rest or of uniform motion along a  
straight line, unless it is acted upon by some external force to change 
the state.
(1) If no net force acts on a body, then the velocity of the body cannot 
change i.e., the body cannot accelerate.
(2)	 Newton’ s 	 first 	 law 	 defines 	 inertia 	 and 	 is 	 rightly 	 called 	 the	 law 	 of	
inertia. Inertia are of three types :
Inertia of rest, Inertia of motion, Inertia of direction.
3.4 Newton’s Second Law
(1) 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.
(2) If a body of mass m, moves with velocity then its linear momentum 
can be given by and  if force  is applied on a body, then 
 or (K = 1 in C. G. S. and S. I. units)
 (Force = mass × acceleration)
3.5 Force
(1) Force is an external effect in the form of a push or pulls which :
(i) Produces or tries to produce motion in a body at rest.
(ii) Stops or tries to stop a moving body.
(iii) Changes or tries to change the direction of motion of the body.
(2) Dimension : Force = mass × acceleration
[F] = [M][LT
–2
] = [MLT
–2
]
(3) Units : Absolute units : (i) Newton (S.I.) (ii) Dyne (C.G.S.)
Gravitational units : (i) Kilogram-force (M.K.S.) (ii) Gram-froce  
(C.G.S.)
(4)  formula is valid only if force is changing the state of rest  
or 	 motion 	 and 	 the 	 mass 	 of 	 the 	 body	 is 	 constant 	 and 	 finite.
(5) if m is not constant 
(6) No force is required to move a body uniformaly along a straight line.
= ma, ? = 0 (As a = 0)
(7) When force is written without direction then positive force means 
repulsive while negative force means attractive.
Page 3


Physics Class XI
58
3.1 Inertia
(1) Inherent property of all the bodies by virtue of which they cannot 
change their state of rest or uniform motion along a straight line by 
their own is called inertia.
(2) Two bodies of equal mass possess same inertia because it is a factor 
of mass only.
3.2 Linear Momentum
(1) Linear momentum of a body is the quantity of motion contained in 
the body.
(2) It is measured as the product of the mass of the body and its velocity 
i.e., Momentum = mass × velocity.
If a body of mass m is moving with velocity  then its linear 
momentum  is given by 
(3) It is a vector quantity and it’s direction is the same as the direction 
of velocity of the body.
(4) Units : kg-m/sec [S. I.], g cm/sec [C.G. S.]
(5) Dimension : [MLT
–1
]
3.3 Newton’s First Law
A body continue to be in its state of rest or of uniform motion along a  
straight line, unless it is acted upon by some external force to change 
the state.
(1) If no net force acts on a body, then the velocity of the body cannot 
change i.e., the body cannot accelerate.
(2)	 Newton’ s 	 first 	 law 	 defines 	 inertia 	 and 	 is 	 rightly 	 called 	 the	 law 	 of	
inertia. Inertia are of three types :
Inertia of rest, Inertia of motion, Inertia of direction.
3.4 Newton’s Second Law
(1) 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.
(2) If a body of mass m, moves with velocity then its linear momentum 
can be given by and  if force  is applied on a body, then 
 or (K = 1 in C. G. S. and S. I. units)
 (Force = mass × acceleration)
3.5 Force
(1) Force is an external effect in the form of a push or pulls which :
(i) Produces or tries to produce motion in a body at rest.
(ii) Stops or tries to stop a moving body.
(iii) Changes or tries to change the direction of motion of the body.
(2) Dimension : Force = mass × acceleration
[F] = [M][LT
–2
] = [MLT
–2
]
(3) Units : Absolute units : (i) Newton (S.I.) (ii) Dyne (C.G.S.)
Gravitational units : (i) Kilogram-force (M.K.S.) (ii) Gram-froce  
(C.G.S.)
(4)  formula is valid only if force is changing the state of rest  
or 	 motion 	 and 	 the 	 mass 	 of 	 the 	 body	 is 	 constant 	 and 	 finite.
(5) if m is not constant 
(6) No force is required to move a body uniformaly along a straight line.
= ma, ? = 0 (As a = 0)
(7) When force is written without direction then positive force means 
repulsive while negative force means attractive.
(8) Out of so many natural forces nuclear force is strongest while 
gravitational force weakest.
(9)  Central force : If a position dependent force is always directed 
towards 	 or 	 away	 from 	 a 	 fixed	 point 	 it 	 is 	 said 	 to 	 be 	 central	 otherwise 	
non-central.
(10)  Conservative or non-conservative force : If under the action of 
a force the work done in a round trip is zero or the work is path 
independent, the force is said to be conservative otherwise non- 
conservative.
Example : Conservative force : Gravitational force, electric force, elastic  
force. 
Non-conservative force : Frictional force, viscous force.
(11) Common forces in mechanics :
(i) Weight : Weight of an object is the force with which earth 
attracts it. (W = mg)
(ii) Reaction or Normal force : When a body is placed on a rigid 
surface, the body experience a force which is perpendicular to 
the surfaces in contact. Then force is called ‘Normal force’ or 
‘Reaction’.
(iii) Tension : The force exerted by the end of taut string, rope or 
chain against pulling  (applied) force is called the tension. The 
direction of tension is so as to pull the body.
(iv) Spring force : Every spring resists any attempt to change its 
length. This resistive force increases with changes in length. 
Spring force is given by F = – Kx; where x is the change in 
length and K is  the spring constant (unit N/m).
3. 6 Equilibrium of Concurrent Force
(1) If all the force working on a body are acting on the same point, then 
they are said to be concurrent.
(2) A body, under the action of concurrent forces, is said to be in 
equilibrium, when there is no change in the state of rest or of uniform 
motion along a straight line.
(3) The condition for the equilibrium of a body is that the vector sum 
of all the forces acting on the body must be zero.
Page 4


Physics Class XI
58
3.1 Inertia
(1) Inherent property of all the bodies by virtue of which they cannot 
change their state of rest or uniform motion along a straight line by 
their own is called inertia.
(2) Two bodies of equal mass possess same inertia because it is a factor 
of mass only.
3.2 Linear Momentum
(1) Linear momentum of a body is the quantity of motion contained in 
the body.
(2) It is measured as the product of the mass of the body and its velocity 
i.e., Momentum = mass × velocity.
If a body of mass m is moving with velocity  then its linear 
momentum  is given by 
(3) It is a vector quantity and it’s direction is the same as the direction 
of velocity of the body.
(4) Units : kg-m/sec [S. I.], g cm/sec [C.G. S.]
(5) Dimension : [MLT
–1
]
3.3 Newton’s First Law
A body continue to be in its state of rest or of uniform motion along a  
straight line, unless it is acted upon by some external force to change 
the state.
(1) If no net force acts on a body, then the velocity of the body cannot 
change i.e., the body cannot accelerate.
(2)	 Newton’ s 	 first 	 law 	 defines 	 inertia 	 and 	 is 	 rightly 	 called 	 the	 law 	 of	
inertia. Inertia are of three types :
Inertia of rest, Inertia of motion, Inertia of direction.
3.4 Newton’s Second Law
(1) 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.
(2) If a body of mass m, moves with velocity then its linear momentum 
can be given by and  if force  is applied on a body, then 
 or (K = 1 in C. G. S. and S. I. units)
 (Force = mass × acceleration)
3.5 Force
(1) Force is an external effect in the form of a push or pulls which :
(i) Produces or tries to produce motion in a body at rest.
(ii) Stops or tries to stop a moving body.
(iii) Changes or tries to change the direction of motion of the body.
(2) Dimension : Force = mass × acceleration
[F] = [M][LT
–2
] = [MLT
–2
]
(3) Units : Absolute units : (i) Newton (S.I.) (ii) Dyne (C.G.S.)
Gravitational units : (i) Kilogram-force (M.K.S.) (ii) Gram-froce  
(C.G.S.)
(4)  formula is valid only if force is changing the state of rest  
or 	 motion 	 and 	 the 	 mass 	 of 	 the 	 body	 is 	 constant 	 and 	 finite.
(5) if m is not constant 
(6) No force is required to move a body uniformaly along a straight line.
= ma, ? = 0 (As a = 0)
(7) When force is written without direction then positive force means 
repulsive while negative force means attractive.
(8) Out of so many natural forces nuclear force is strongest while 
gravitational force weakest.
(9)  Central force : If a position dependent force is always directed 
towards 	 or 	 away	 from 	 a 	 fixed	 point 	 it 	 is 	 said 	 to 	 be 	 central	 otherwise 	
non-central.
(10)  Conservative or non-conservative force : If under the action of 
a force the work done in a round trip is zero or the work is path 
independent, the force is said to be conservative otherwise non- 
conservative.
Example : Conservative force : Gravitational force, electric force, elastic  
force. 
Non-conservative force : Frictional force, viscous force.
(11) Common forces in mechanics :
(i) Weight : Weight of an object is the force with which earth 
attracts it. (W = mg)
(ii) Reaction or Normal force : When a body is placed on a rigid 
surface, the body experience a force which is perpendicular to 
the surfaces in contact. Then force is called ‘Normal force’ or 
‘Reaction’.
(iii) Tension : The force exerted by the end of taut string, rope or 
chain against pulling  (applied) force is called the tension. The 
direction of tension is so as to pull the body.
(iv) Spring force : Every spring resists any attempt to change its 
length. This resistive force increases with changes in length. 
Spring force is given by F = – Kx; where x is the change in 
length and K is  the spring constant (unit N/m).
3. 6 Equilibrium of Concurrent Force
(1) If all the force working on a body are acting on the same point, then 
they are said to be concurrent.
(2) A body, under the action of concurrent forces, is said to be in 
equilibrium, when there is no change in the state of rest or of uniform 
motion along a straight line.
(3) The condition for the equilibrium of a body is that the vector sum 
of all the forces acting on the body must be zero.
61
3.7 Newton’s Third Law
To every action, there is always an equal (in magnitude) and opposite (in  
direction) reaction.
 If = force exerted on body A by body B (Action) and = force 
exerted on body B by body A (Reaction)
Then according to Newton’s third law of motion  =  –
Example :
(i) A book lying on a table exerts a force on the table which is equal to 
the weight of the book. This is the force of action.
(ii) Swimming is possible due to third law of motion.
(iii)	 W h e n 	 a 	 g un 	 i s 	 fire d , 	 t h e 	 b ul l e t 	 m o ves 	 fo rwa r d	 (a c ti o n ) . 	 T h e 	 gu n	
recoils backward (reaction).
3.8 Frame of Reference
(1) A frame in which an observer is situated and makes his observations 
is known as his ‘Frame of reference’.
It is associated with a co-ordinate system.
(2) Frame of reference are of two types : (i) Inertial frame of reference 
(ii) Non-inertial frame of reference. 
(i) Inertial frame of reference :
(a) A frame of reference which is at rest or which is moving with 
a uniform velocity along a straight line is called an inertial 
frame of reference.
(b) In inertial frame of reference Newton’s laws of motion holds 
good.
(c) Ideally no inertial frame exist  in universe. For  practical 
purpose a frame of reference may be considered as inertial it 
it’s acceleration is negligible with respect to the acceleration 
of the object to be observed.
Example : The lift at rest, lift moving (up or down) with constant velocity.
(ii) Non inertial frame of reference :
(a) Accelerated frame of references are called non-inertial frame 
of reference.
(b) Newton’s laws of motion are not applicable in non-inertial 
frame of reference.
Page 5


Physics Class XI
58
3.1 Inertia
(1) Inherent property of all the bodies by virtue of which they cannot 
change their state of rest or uniform motion along a straight line by 
their own is called inertia.
(2) Two bodies of equal mass possess same inertia because it is a factor 
of mass only.
3.2 Linear Momentum
(1) Linear momentum of a body is the quantity of motion contained in 
the body.
(2) It is measured as the product of the mass of the body and its velocity 
i.e., Momentum = mass × velocity.
If a body of mass m is moving with velocity  then its linear 
momentum  is given by 
(3) It is a vector quantity and it’s direction is the same as the direction 
of velocity of the body.
(4) Units : kg-m/sec [S. I.], g cm/sec [C.G. S.]
(5) Dimension : [MLT
–1
]
3.3 Newton’s First Law
A body continue to be in its state of rest or of uniform motion along a  
straight line, unless it is acted upon by some external force to change 
the state.
(1) If no net force acts on a body, then the velocity of the body cannot 
change i.e., the body cannot accelerate.
(2)	 Newton’ s 	 first 	 law 	 defines 	 inertia 	 and 	 is 	 rightly 	 called 	 the	 law 	 of	
inertia. Inertia are of three types :
Inertia of rest, Inertia of motion, Inertia of direction.
3.4 Newton’s Second Law
(1) 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.
(2) If a body of mass m, moves with velocity then its linear momentum 
can be given by and  if force  is applied on a body, then 
 or (K = 1 in C. G. S. and S. I. units)
 (Force = mass × acceleration)
3.5 Force
(1) Force is an external effect in the form of a push or pulls which :
(i) Produces or tries to produce motion in a body at rest.
(ii) Stops or tries to stop a moving body.
(iii) Changes or tries to change the direction of motion of the body.
(2) Dimension : Force = mass × acceleration
[F] = [M][LT
–2
] = [MLT
–2
]
(3) Units : Absolute units : (i) Newton (S.I.) (ii) Dyne (C.G.S.)
Gravitational units : (i) Kilogram-force (M.K.S.) (ii) Gram-froce  
(C.G.S.)
(4)  formula is valid only if force is changing the state of rest  
or 	 motion 	 and 	 the 	 mass 	 of 	 the 	 body	 is 	 constant 	 and 	 finite.
(5) if m is not constant 
(6) No force is required to move a body uniformaly along a straight line.
= ma, ? = 0 (As a = 0)
(7) When force is written without direction then positive force means 
repulsive while negative force means attractive.
(8) Out of so many natural forces nuclear force is strongest while 
gravitational force weakest.
(9)  Central force : If a position dependent force is always directed 
towards 	 or 	 away	 from 	 a 	 fixed	 point 	 it 	 is 	 said 	 to 	 be 	 central	 otherwise 	
non-central.
(10)  Conservative or non-conservative force : If under the action of 
a force the work done in a round trip is zero or the work is path 
independent, the force is said to be conservative otherwise non- 
conservative.
Example : Conservative force : Gravitational force, electric force, elastic  
force. 
Non-conservative force : Frictional force, viscous force.
(11) Common forces in mechanics :
(i) Weight : Weight of an object is the force with which earth 
attracts it. (W = mg)
(ii) Reaction or Normal force : When a body is placed on a rigid 
surface, the body experience a force which is perpendicular to 
the surfaces in contact. Then force is called ‘Normal force’ or 
‘Reaction’.
(iii) Tension : The force exerted by the end of taut string, rope or 
chain against pulling  (applied) force is called the tension. The 
direction of tension is so as to pull the body.
(iv) Spring force : Every spring resists any attempt to change its 
length. This resistive force increases with changes in length. 
Spring force is given by F = – Kx; where x is the change in 
length and K is  the spring constant (unit N/m).
3. 6 Equilibrium of Concurrent Force
(1) If all the force working on a body are acting on the same point, then 
they are said to be concurrent.
(2) A body, under the action of concurrent forces, is said to be in 
equilibrium, when there is no change in the state of rest or of uniform 
motion along a straight line.
(3) The condition for the equilibrium of a body is that the vector sum 
of all the forces acting on the body must be zero.
61
3.7 Newton’s Third Law
To every action, there is always an equal (in magnitude) and opposite (in  
direction) reaction.
 If = force exerted on body A by body B (Action) and = force 
exerted on body B by body A (Reaction)
Then according to Newton’s third law of motion  =  –
Example :
(i) A book lying on a table exerts a force on the table which is equal to 
the weight of the book. This is the force of action.
(ii) Swimming is possible due to third law of motion.
(iii)	 W h e n 	 a 	 g un 	 i s 	 fire d , 	 t h e 	 b ul l e t 	 m o ves 	 fo rwa r d	 (a c ti o n ) . 	 T h e 	 gu n	
recoils backward (reaction).
3.8 Frame of Reference
(1) A frame in which an observer is situated and makes his observations 
is known as his ‘Frame of reference’.
It is associated with a co-ordinate system.
(2) Frame of reference are of two types : (i) Inertial frame of reference 
(ii) Non-inertial frame of reference. 
(i) Inertial frame of reference :
(a) A frame of reference which is at rest or which is moving with 
a uniform velocity along a straight line is called an inertial 
frame of reference.
(b) In inertial frame of reference Newton’s laws of motion holds 
good.
(c) Ideally no inertial frame exist  in universe. For  practical 
purpose a frame of reference may be considered as inertial it 
it’s acceleration is negligible with respect to the acceleration 
of the object to be observed.
Example : The lift at rest, lift moving (up or down) with constant velocity.
(ii) Non inertial frame of reference :
(a) Accelerated frame of references are called non-inertial frame 
of reference.
(b) Newton’s laws of motion are not applicable in non-inertial 
frame of reference.
62
Example : Car moving in uniform circular motion, lift which 
is moving upward or downward with some acceleration, plane 
which is taking off.
3.9 Impulse
(1) When a large force works on a body for very small time interval, it 
is called impulsive force.
An 	 impulsive 	 force 	 does 	 not 	 remain 	 constant, 	 but 	 changes 	 first 	 from 	
zero to maximum and then from maximum to zero. In such case we 
measure the total effect of force.
(2) Impulse of a force is a measure of total effect of force.
(3) 
(4) Impulse is a vector quantity and its direction is same as that of force.
(5) Dimension : [MLT
–1
]
(6) Units : Newton second or Kg-ms
–1
 (S. I.) and Dyne second or gm cm  
s
–1
 (C.G. S.)
(7) Force-time graph- Impulse is equal to the area under F-t curve.
 I = Area between curve and time axis = Ft
(8) If F
av
 is the average magnitude of the force, then
I = = = F
av
?t
(9) From Newton’s second law, = 
 or = ? = = 
i.e., The impulse of a force is equal to the change in momentum.