Law Of Motion
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1. NEWTON'S FIRST LAW OF MOTION (OR GALILEO'S LAW OF INERTIA)
Every body continues its state of rest or uniform motion in a straight line unless compelled by an external force
to change its state.
This law defines the force and states that "force is a factor which can change the state of object."
Definition of force from Newton's first law of motion
"Force is the push or pull which changes or tends to change the state of rest or of uniform motion".
2. FORCE
Any push or pull which either changes or tends to change the state of rest or of uniform motion (constant
velocity) of a body is known as force.
Effects of Resultant force :-
A non zero resultant force may produce the following effects on a body :
(i) It may change the speed of the body.
(ii) It may change the direction of motion.
(iii) It may change both the speed and direction of motion.
(iv) It may change the size or/and the shape of the body
Units for measurement of force :-
Absolute units Other units
(i) N (M.K.S) kg-wt or kg-f (kg-force)
(ii) dyne (C.G.S) g-wt or g-f
INERTIA
Inertia is the property of a body due to which it opposes any change in its state. Mass of a body is the measure
of its inertia of translational motion. It is difficult to change the state of rest or uniform motion of a body of
heavier mass and vice-versa.
l Mass of a body is quantitative or numerical measure of a body's inertia.
l Larger the inertia of a body, more will be its mass.
Inertia of rest : It is the inability of a body to change its state of rest by itself.
Examples :
l When we shake a branch of a mango tree, the mangoes fall down.
l When a bus or train starts suddenly, the passengers sitting inside tends to fall backwards.
l When a horse starts off suddenly, its rider falls backwards.
l A coin is placed on cardboard and this cardboard is placed over a tumbler such that coin is above the
mouth of tumbler. Now if the cardboard is removed with a sudden jerk, then the coin falls into the tumbler.
l The dust particles in a blanket fall off when it is beaten with a stick.
Inertia of motion : It is the inability of a body to change its state of uniform motion by itself.
Examples :
* When a bus or train stops suddenly, the passengers sitting inside lean forward.
* A person who jumps out of a moving train may fall in the forward.
* A bowler runs with the ball before throwing it, so that his speed of running gets added to the speed
of the ball at the time of throw.
* An athlete runs through a certain distance before taking a long jump because the velocity acquired
during the running gets added to the velocity of athlete at the time of jump and hence he can jump
over a longer distance.
* A ball is thrown in the upward direction by a passenger sitting inside a moving train.
The ball will fall :-
* back to the hands of the passenger, if the train is moving with constant velocity.
* ahead of the passenger, if the train is retarding (slowing down)
* behind of the passenger, if the train is accelerating (speeding up)
Inertia of direction : It is the inability of a body to change its direction of motion by itself
Examples :
*When a straight running car turns sharply, the person sitting inside feels a force radially outwards.
* Rotating wheels of vehicle throw out mud, mudguard fitted over the wheels prevent this mud from
spreading.
* When a knife is pressed against a grinding stone, the sparks produced move in the tangential direction
MOMENTUM
The total quantity of motion possessed by a moving body is known as the momentum of the body. It is the
product of the mass and velocity of a body. It is a vector quantity whose direction is along the instantaneous
velocity
NEWTON'S SECOND LAW OF MOTION
According to Newton, the rate of change of momentum of any system is directly proportional to the applied
external force and this change in momentum takes place in the direction of the applied force.
F = MA
Law of Conservation of linear momentum
If net external force on a system is zero then the linear momentum of the system remains constant.
According to Newton's IInd Law.
NEWTON'S THIRD LAW OF MOTION
According to Newton's third law, to every action,
The forces between two
objects A and B are equal and
opposite, whether they are
attractive or repulsive.
there is always an equal (in magnitude) and
opposite (in direction) reaction.
This law is also known as action-reaction law.
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