LAWS OF MOTION
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.
(iii)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.
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 =
dp / dt ,
F=
m dv/dt,
F=
ma
where, k is a constant of proportionality and its value is one in SI and CGS
system. Examples
(iii) It is easier for a strong adult to push a full shopping cart than it is for a
baby to push the same cart. (This is depending on the net force acting on the
object).
(iv) It is easier for a person to push an empty shopping cart than a full one (This is
depending on the mass of the object).
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 = – F21
Examples
✓ Swimming becomes possible because of third law of motion.
✓ Jumping of a man from a boat onto the bank of a river.
✓ Jerk is produced in a gun when bullet is fired from it.
✓ Pulling of cart by a horse.
Note Newton’s second law of motion is called real law of motion because first and
third laws of motion can be obtained by 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
