Inertia
The property of an object by virtue of which it cannot change its state of rest or of uniform
motion along a straight line its own, is called inertia.
Inertia is a measure of mass of a body. Greater the mass of a body greater will be its inertia or
vice-versa.
Inertia is of three types:
(i) Inertia of Rest When a bus or train starts to move suddenly, the passengers sitting in it falls
backward due to inertia of rest.
(ii) Inertia of Motion When a moving bus or train stops suddenly, the passengers sitting in it
jerks in forward direction due to inertia of motion.
(iii) Inertia of Direction We can protect yourself from rain by an umbrella because rain drops
can not change its direction its own due to inertia of direction.
Force
Force is a push or pull which changes or tries to change the state of rest, the state of uniform
motion, size or shape of a body.
Its SI unit is newton (N) and its dimensional formula is [MLT
-2
].
Forces can be categorized into two types:
(i) Contact Forces Frictional force, tensional force, spring force, normal force, etc are the
contact forces.
(ii) Action at a Distance Forces Electrostatic force, gravitational force, magnetic force, etc are
action at a distance forces.
Impulsive Force
A force which acts on body for a short interval of time, and produces a large change in
momentum is called an impulsive force
Linear Momentum -The total amount of motion present in a body is called its momentum. Linear momentum of a
body is equal to the product of its mass and velocity. It is denoted by p.
Linear momentum p = mu.
Its S1 unit is kg-m/s and dimensional formula is [MLT
-1
].
It is a vector quantity and its direction is in the direction of velocity of the body.
Impulse
The product of impulsive force and time for which it acts is called impulse.
Impulse = Force * Time = Change in momentum
Its S1 unit is newton-second or kg-m/s and its dimension is [MLT
-1
].
It is a vector quantity and its direction is in the direction of force.
Newton’s Laws of Motion
1. Newton’s First Law of Motion
A body continues to be in its state of rest or in uniform motion along a straight line unless an
external force is applied on it.
This law is also called law of inertia.
Examples
(i) When a carpet or a blanket is beaten with a stick then the dust particles separate out from it.
(ii) If a moving vehicle suddenly stops then the passengers inside the vehicle bend outward.
2. Newton’s Second Law of Motion
The rate of change of linear momentum is proportional to the applied force and change in
momentum takes place in the direction of applied force.
Mathematically
F = k (d / dt) (mv)
where, k is a constant of proportionality and its value is one in SI and CGS system.
F= mdv / dt = ma
3. Newton’s Third Law of Motion
For every action there is an equal and opposite reaction and both acts on two different bodies
Mathematically F12
= – F
21
e.g.
(i) Swimming becomes possible because of third law of motion.
(ii) Jumping of a man from a boat onto the bank of a river.
(iii) Jerk is produced in a gun when bullet is fired from it.
The modern version of these laws is
(i) A body continues in its initial state of rest or motion with uniform velocity unless acted on
by an unbalanced external force.
(ii) Forces always occur in pairs. If body A exerts a force on body B, an equal but opposite
force is exerted by body B on body A.
Law of Conservation of Linear Momentum
If no external force acts on a system, then its total linear momentum remains conserved.
Linear momentum depends on frame of reference but law of conservation of linear momentum
is independent of frame of reference.
Newton’s laws of motion are valid only in inertial frame of reference.
Weight (w)
It is a field force, the force with which a body is pulled towards the centre of the earth due to
gravity. It has the magnitude mg, where m is the mass of the body and g is the acceleration due
to gravity.
w = mg
unit-Newton
The property of an object by virtue of which it cannot change its state of rest or of uniform
motion along a straight line its own, is called inertia.
Inertia is a measure of mass of a body. Greater the mass of a body greater will be its inertia or
vice-versa.
Inertia is of three types:
(i) Inertia of Rest When a bus or train starts to move suddenly, the passengers sitting in it falls
backward due to inertia of rest.
(ii) Inertia of Motion When a moving bus or train stops suddenly, the passengers sitting in it
jerks in forward direction due to inertia of motion.
(iii) Inertia of Direction We can protect yourself from rain by an umbrella because rain drops
can not change its direction its own due to inertia of direction.
Force
Force is a push or pull which changes or tries to change the state of rest, the state of uniform
motion, size or shape of a body.
Its SI unit is newton (N) and its dimensional formula is [MLT
-2
].
Forces can be categorized into two types:
(i) Contact Forces Frictional force, tensional force, spring force, normal force, etc are the
contact forces.
(ii) Action at a Distance Forces Electrostatic force, gravitational force, magnetic force, etc are
action at a distance forces.
Impulsive Force
A force which acts on body for a short interval of time, and produces a large change in
momentum is called an impulsive force
Linear Momentum -The total amount of motion present in a body is called its momentum. Linear momentum of a
body is equal to the product of its mass and velocity. It is denoted by p.
Linear momentum p = mu.
Its S1 unit is kg-m/s and dimensional formula is [MLT
-1
].
It is a vector quantity and its direction is in the direction of velocity of the body.
Impulse
The product of impulsive force and time for which it acts is called impulse.
Impulse = Force * Time = Change in momentum
Its S1 unit is newton-second or kg-m/s and its dimension is [MLT
-1
].
It is a vector quantity and its direction is in the direction of force.
Newton’s Laws of Motion
1. Newton’s First Law of Motion
A body continues to be in its state of rest or in uniform motion along a straight line unless an
external force is applied on it.
This law is also called law of inertia.
Examples
(i) When a carpet or a blanket is beaten with a stick then the dust particles separate out from it.
(ii) If a moving vehicle suddenly stops then the passengers inside the vehicle bend outward.
2. Newton’s Second Law of Motion
The rate of change of linear momentum is proportional to the applied force and change in
momentum takes place in the direction of applied force.
Mathematically
F = k (d / dt) (mv)
where, k is a constant of proportionality and its value is one in SI and CGS system.
F= mdv / dt = ma
3. Newton’s Third Law of Motion
For every action there is an equal and opposite reaction and both acts on two different bodies
Mathematically F12
= – F
21
e.g.
(i) Swimming becomes possible because of third law of motion.
(ii) Jumping of a man from a boat onto the bank of a river.
(iii) Jerk is produced in a gun when bullet is fired from it.
The modern version of these laws is
(i) A body continues in its initial state of rest or motion with uniform velocity unless acted on
by an unbalanced external force.
(ii) Forces always occur in pairs. If body A exerts a force on body B, an equal but opposite
force is exerted by body B on body A.
Law of Conservation of Linear Momentum
If no external force acts on a system, then its total linear momentum remains conserved.
Linear momentum depends on frame of reference but law of conservation of linear momentum
is independent of frame of reference.
Newton’s laws of motion are valid only in inertial frame of reference.
Weight (w)
It is a field force, the force with which a body is pulled towards the centre of the earth due to
gravity. It has the magnitude mg, where m is the mass of the body and g is the acceleration due
to gravity.
w = mg
unit-Newton
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