Newton's Laws of Motion
NEWTON’S LAWS OF MOTION
———————————————————————————————————
1. FORCE
A pull or push which changes or tends to change the state of rest or of uniform motion or direction of
motion of any object is called force. Force is the interaction between the object and the source
(providing the pull or push). It is a vector quantity.
Effect of resultant force :
(1) may change only speed
(2) may change only direction of motion.
(3)may change both the speed and direction of motion.
(4) may change size and shape of a body
kg m
Unit of force : Newton and (MKS System)
s2
g cm
dyne and (CGS System)
s2
1 Newton = 105 dyne
Kilogram force (kgf) : The force with which earth attracts a 1kg body towards its centre is called
kilogram force, thus
Forece in newton
kgf =
g
Dimensional Formula of force : [MLT–2 ]
1.1 Fundamental Forces
All the forces observed in nature such as muscular force, tension, reaction, friction, elastic, weight,
electric, magnetic, nuclear, etc., can be explained in terms of only following four basic interactions:
(A) Gravitational Force : The force of interaction which exists between two particles of masses m 1 and
m2, due to their masses is called gravitational force.
m1m2
F G r
r3
= position vector of test particle ‘T’ with respect to source particle ‘S’. and G = universal gravitational constant
= 6.67 × 10–11 Nm2/kg2.
(i) It is the weakest force and is always attractive.
(ii) It is a long range force as it acts between any two particles situated at any distance in the universe.
(iii) It is independent of the nature of medium between the
particles.
An apple is freely falling as shown in figure, When it is at a
height h, force between earth and apple is given by
GMe m
F=
(R e h)2
where Me – mass of earth, Re – radius of earth. It acts towards earth’s centre. Now rearranging above
result,
2
GMe Re
F=m . .
Re h
2
Re
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,Newton's Laws of Motion
2
Re GMe
F = mg g 2
Re h Re
Re
Here h << Re, so 1 F = mg
Re h
This is the force exerted by earth on any particle of mass m near the earth surface. The value
of g = 9.81 m/s2 ~ 10 m/s2 2 m/s2 ~ 32 ft/s2. It is also called acceleration due to gravity near
the surface of earth.
(B) Electromagnetic Force : Force exerted by one particle on the other because of the electric charge
on the particles is called electromagnetic force.
Following are the main characteristics of electromagnetic force
(a) These can be attractive or repulsive.
(b) These are long range forces
(c) These depend on the nature of medium between the charged particles.
(d) All macroscopic forces (except gravitational) which we experience as push or pull or by contact
are electromagnetic, i.e., tension in a rope, the force of friction, normal reaction, muscular force,
and force experienced by a deformed spring are electromagnetic forces. These are
manifestations of the electromagnetic attractions and repulsions between atoms/molecules.
(C) Nuclear Force : It is the strongest force. It keeps nucleons (neutrons and protons) together inside
the nucleus inspite of large electric repulsion between protons. Radioactivity, fission, and fusion,
etc. result because of unbalancing of nuclear forces. It acts within the nucleus that too upto a very
small distance.
(D) Weak Force : It acts between any two elementary particles. Under its action a neutron can change
into a proton emitting an electron and a particle called antineutrino. The range of weak force is very
small, in fact much smaller than the size of a proton or a neutron.
It has been found that for two protons at a distance of 1 Fermi :
FN : FEM : FW : FG : : 1 : 10–2 : 10–7 : 10–38
1.2 Classification of forces on the basis of contact :
(A) Field Force : Force which acts on an object at a distance by the interaction of the object with the
field produced by other object is called field force. Examples
(a) Gravitation force (b) Electromagnetic force
(B) Contact Force : Forces which are transmitted between bodies by short range atomic molecular
interactions are called contact forces. When two objects come in contact they exert contact forces
on each other.
Examples :
(a) Normal force (N) :
It is the component of contact force perpendicular to the surface. It
measures how strongly the surfaces in contact are pressed against
each other. It is the electromagnetic force. A table is placed on
Earth as shown in figure
Here table presses the earth so normal force exerted by four legs of table on earth are as
shown in figure.
Now a boy pushes a block kept on a frictionless surface.
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,Newton's Laws of Motion
Here, force exerted by boy on block is electromagnetic interaction which arises due to similar
charges appearing on finger and contact surface of block, it is normal force.
A block is kept on inclined surface. Component of its weight presses the surface perpendicularly
due to which contact force acts between surface and block.
Normal force exerted by block on the surface of inclined plane is shown in figure.
Force acts perpendicular to the surface
Example 1. Two blocks are kept in contact on a smooth surface as shown in figure. Draw normal force
exerted by A on B.
Solution : In above problem, block A does not push block B, so there is no molecular interaction between
A and B. Hence normal force exerted by A on B is zero.
Note : Normal is a dependent force, it comes in role when one surface presses the other.
———————————————————————————————————
(b) Tension :
Tension in a string is a electromagnetic force. It arises
when a string is pulled. If a massless string is not
pulled, tension in it is zero. A string suspended by rigid
support is pulled by a force ‘F’ as shown in figure, for
calculating the tension at point ‘A’ we draw F.B.D. of
marked portion of the string; Here string is massless.
F.B.D. of marked portion
T
A
F
T=F
String is considered to be made of a number of small
segments which attracts each other due to
electromagnetic nature as shown in figure. The
attraction force between two segments is equal and
opposite due to Newton’s third law.
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, Newton's Laws of Motion
For calculating tension at any segment, we consider two or more than two parts as a system.
Here interaction between segments are considered as internal forces, so they are not shown in
F.B.D.
(C) Frictional force : It is the component of contact force tangential to the surface. It opposes the
relative motion (or attempted relative motion) of the two surfaces in contact.
2. THIRD LAW OF MOTION :
To every action, there is always an equal and opposite reaction. Newton’s law from an 1803 translation
from Latin as Newton wrote
“To every action there is always opposed an equal and opposite reaction : to the mutual actions of two
bodies upon each other are always equal, and directed to contrary parts.”
2.1 Important points about the Third Law
(a) The terms ‘action’ and ‘reaction’ in the Third Law mean nothing else but ‘force’. A simple and clear
way of stating the Third Law is as follows : Forces always occur in pairs. Force on a body A by B is
equal and opposite to the force on the body B by A.
(b) The terms ‘action’ and ‘reaction’ in the Third Law may give a wrong impression that action comes
before reaction i.e. action is the cause and reaction the effect. There is no such cause-effect
relation implied in the Third Law. The force on A by B and the force on B by A act at the same
instant. Any one of them may be called action and the other reaction.
(c) Action and reaction forces act on different bodies, not on the same body. Thus if we are
considering the motion of any one body (A or B), only one of the two forces is relevant. It is an error
to add up the two forces and claim that the net force is zero.
However, if you are considering the system of two bodies as a whole, FAB (force on A due to B) and
FBA (force on B due to A) are internal forces of the system (A + B). They add up to give a null force.
Internal forces in a body or a system of particles thus cancel away in pairs. This is an important fact
that enables the Second Law to be applicable to a body or a system of particles.
3. SYSTEM :
Two or more than two objects which interact with each other form a system.
3.1 Classification of forces on the basis of boundary of system :
(A) Internal Forces : Forces acting each with in a system among its constituents.
(B) External Forces : Forces exerted on the constituents of a system by the outside surroundings are
called as external forces.
(C) Real Force : Force which acts on an object due to other object is called as real force. An isolated
object (far away from all objects) does not experience any real force.
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NEWTON’S LAWS OF MOTION
———————————————————————————————————
1. FORCE
A pull or push which changes or tends to change the state of rest or of uniform motion or direction of
motion of any object is called force. Force is the interaction between the object and the source
(providing the pull or push). It is a vector quantity.
Effect of resultant force :
(1) may change only speed
(2) may change only direction of motion.
(3)may change both the speed and direction of motion.
(4) may change size and shape of a body
kg m
Unit of force : Newton and (MKS System)
s2
g cm
dyne and (CGS System)
s2
1 Newton = 105 dyne
Kilogram force (kgf) : The force with which earth attracts a 1kg body towards its centre is called
kilogram force, thus
Forece in newton
kgf =
g
Dimensional Formula of force : [MLT–2 ]
1.1 Fundamental Forces
All the forces observed in nature such as muscular force, tension, reaction, friction, elastic, weight,
electric, magnetic, nuclear, etc., can be explained in terms of only following four basic interactions:
(A) Gravitational Force : The force of interaction which exists between two particles of masses m 1 and
m2, due to their masses is called gravitational force.
m1m2
F G r
r3
= position vector of test particle ‘T’ with respect to source particle ‘S’. and G = universal gravitational constant
= 6.67 × 10–11 Nm2/kg2.
(i) It is the weakest force and is always attractive.
(ii) It is a long range force as it acts between any two particles situated at any distance in the universe.
(iii) It is independent of the nature of medium between the
particles.
An apple is freely falling as shown in figure, When it is at a
height h, force between earth and apple is given by
GMe m
F=
(R e h)2
where Me – mass of earth, Re – radius of earth. It acts towards earth’s centre. Now rearranging above
result,
2
GMe Re
F=m . .
Re h
2
Re
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,Newton's Laws of Motion
2
Re GMe
F = mg g 2
Re h Re
Re
Here h << Re, so 1 F = mg
Re h
This is the force exerted by earth on any particle of mass m near the earth surface. The value
of g = 9.81 m/s2 ~ 10 m/s2 2 m/s2 ~ 32 ft/s2. It is also called acceleration due to gravity near
the surface of earth.
(B) Electromagnetic Force : Force exerted by one particle on the other because of the electric charge
on the particles is called electromagnetic force.
Following are the main characteristics of electromagnetic force
(a) These can be attractive or repulsive.
(b) These are long range forces
(c) These depend on the nature of medium between the charged particles.
(d) All macroscopic forces (except gravitational) which we experience as push or pull or by contact
are electromagnetic, i.e., tension in a rope, the force of friction, normal reaction, muscular force,
and force experienced by a deformed spring are electromagnetic forces. These are
manifestations of the electromagnetic attractions and repulsions between atoms/molecules.
(C) Nuclear Force : It is the strongest force. It keeps nucleons (neutrons and protons) together inside
the nucleus inspite of large electric repulsion between protons. Radioactivity, fission, and fusion,
etc. result because of unbalancing of nuclear forces. It acts within the nucleus that too upto a very
small distance.
(D) Weak Force : It acts between any two elementary particles. Under its action a neutron can change
into a proton emitting an electron and a particle called antineutrino. The range of weak force is very
small, in fact much smaller than the size of a proton or a neutron.
It has been found that for two protons at a distance of 1 Fermi :
FN : FEM : FW : FG : : 1 : 10–2 : 10–7 : 10–38
1.2 Classification of forces on the basis of contact :
(A) Field Force : Force which acts on an object at a distance by the interaction of the object with the
field produced by other object is called field force. Examples
(a) Gravitation force (b) Electromagnetic force
(B) Contact Force : Forces which are transmitted between bodies by short range atomic molecular
interactions are called contact forces. When two objects come in contact they exert contact forces
on each other.
Examples :
(a) Normal force (N) :
It is the component of contact force perpendicular to the surface. It
measures how strongly the surfaces in contact are pressed against
each other. It is the electromagnetic force. A table is placed on
Earth as shown in figure
Here table presses the earth so normal force exerted by four legs of table on earth are as
shown in figure.
Now a boy pushes a block kept on a frictionless surface.
Reg. & Corp. Office : CG Tower, A-46 & 52, IPIA, Near City Mall, Jhalawar Road, Kota (Raj.)– 324005
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ADVNL - 2
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,Newton's Laws of Motion
Here, force exerted by boy on block is electromagnetic interaction which arises due to similar
charges appearing on finger and contact surface of block, it is normal force.
A block is kept on inclined surface. Component of its weight presses the surface perpendicularly
due to which contact force acts between surface and block.
Normal force exerted by block on the surface of inclined plane is shown in figure.
Force acts perpendicular to the surface
Example 1. Two blocks are kept in contact on a smooth surface as shown in figure. Draw normal force
exerted by A on B.
Solution : In above problem, block A does not push block B, so there is no molecular interaction between
A and B. Hence normal force exerted by A on B is zero.
Note : Normal is a dependent force, it comes in role when one surface presses the other.
———————————————————————————————————
(b) Tension :
Tension in a string is a electromagnetic force. It arises
when a string is pulled. If a massless string is not
pulled, tension in it is zero. A string suspended by rigid
support is pulled by a force ‘F’ as shown in figure, for
calculating the tension at point ‘A’ we draw F.B.D. of
marked portion of the string; Here string is massless.
F.B.D. of marked portion
T
A
F
T=F
String is considered to be made of a number of small
segments which attracts each other due to
electromagnetic nature as shown in figure. The
attraction force between two segments is equal and
opposite due to Newton’s third law.
Reg. & Corp. Office : CG Tower, A-46 & 52, IPIA, Near City Mall, Jhalawar Road, Kota (Raj.)– 324005
Website : www.resonance.ac.in | E-mail :
ADVNL - 3
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, Newton's Laws of Motion
For calculating tension at any segment, we consider two or more than two parts as a system.
Here interaction between segments are considered as internal forces, so they are not shown in
F.B.D.
(C) Frictional force : It is the component of contact force tangential to the surface. It opposes the
relative motion (or attempted relative motion) of the two surfaces in contact.
2. THIRD LAW OF MOTION :
To every action, there is always an equal and opposite reaction. Newton’s law from an 1803 translation
from Latin as Newton wrote
“To every action there is always opposed an equal and opposite reaction : to the mutual actions of two
bodies upon each other are always equal, and directed to contrary parts.”
2.1 Important points about the Third Law
(a) The terms ‘action’ and ‘reaction’ in the Third Law mean nothing else but ‘force’. A simple and clear
way of stating the Third Law is as follows : Forces always occur in pairs. Force on a body A by B is
equal and opposite to the force on the body B by A.
(b) The terms ‘action’ and ‘reaction’ in the Third Law may give a wrong impression that action comes
before reaction i.e. action is the cause and reaction the effect. There is no such cause-effect
relation implied in the Third Law. The force on A by B and the force on B by A act at the same
instant. Any one of them may be called action and the other reaction.
(c) Action and reaction forces act on different bodies, not on the same body. Thus if we are
considering the motion of any one body (A or B), only one of the two forces is relevant. It is an error
to add up the two forces and claim that the net force is zero.
However, if you are considering the system of two bodies as a whole, FAB (force on A due to B) and
FBA (force on B due to A) are internal forces of the system (A + B). They add up to give a null force.
Internal forces in a body or a system of particles thus cancel away in pairs. This is an important fact
that enables the Second Law to be applicable to a body or a system of particles.
3. SYSTEM :
Two or more than two objects which interact with each other form a system.
3.1 Classification of forces on the basis of boundary of system :
(A) Internal Forces : Forces acting each with in a system among its constituents.
(B) External Forces : Forces exerted on the constituents of a system by the outside surroundings are
called as external forces.
(C) Real Force : Force which acts on an object due to other object is called as real force. An isolated
object (far away from all objects) does not experience any real force.
Reg. & Corp. Office : CG Tower, A-46 & 52, IPIA, Near City Mall, Jhalawar Road, Kota (Raj.)– 324005
Website : www.resonance.ac.in | E-mail :
ADVNL - 4
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