L1 - Newton's First Law of Motion, Class 9, Science Video Lecture

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Newton's first law of motion
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in our everyday life we observe that
effort is required to put a stationary
object into motion or to stop a moving
object have you ever wondered if a ball
kicked on the ground would keep moving
forever or after sometime would
naturally stop moving the ball will
continue to move unless an object of
force stops it
here comes Newton's first law which
states a body continues to be in its
state of rest or of uniform motion
unless acted upon by an unbalanced
external force to understand Newton's
first law you need to understand what
unbalanced force is if you have seen a
game of tug-of-war you would have seen
two teams pulling a rope in opposite
directions
if the rope doesn't move the force is
applied by the teams are balanced forces
as they are of equal magnitude in
opposite direction if one of the team
applies greater force it pulls the other
team in the rope towards their side in
this case the forces are unbalanced as
the two forces are of unequal magnitude
in the opposite direction if one of the
team lets go of the rope the opposite
team falls backwards this tendency to
fall backwards
despite letting go of the rope is due to
inertia that is if an object is at rest
it tends to remain at rest and if it is
in motion it tends to stay in motion
this property of an object is called its
inertia inertia can be classified into
three categories inertia of rest inertia
of direction inertia of motion
certain experiences that we have while
travelling in a bus can be explained on
the basis of the law of inertia when a
passenger is standing in a stationary
bus that suddenly moves the movement
pushes the passenger backward why does
this happen this is due to the inertia
of rest experienced by the passenger in
the upper part of his body the body is
at rest when the bus is at rest when the
bus starts all of a sudden the lower
part of the body is in contact with the
vehicle that started moving whereas the
upper part of the body continues to be
at rest because of the inertia of rest
inertia of rest is the tendency of a
body to resist any change in its state
of rest what happens when a fast moving
bus negotiates a right-hand curve on the
road the passengers will tilt to the
left inertia of direction is the
tendency of a body to oppose any change
in the direction of its motion with the
application of sharp brakes the bus
stops all of a sudden and the passengers
tend to fall forward this is due to
inertia of motion experienced by the
upper part of the body when the bus is
moving the whole body of the passenger
is in a state of motion a sudden halt
caused by the lower part of the
passengers body that is in contact with
the bus comes to rest
however the upper portion of the body
remains in the state of motion due to
the inertia of motion causing the
passenger to fall forward when the bus
stops suddenly inertia of motion is a
tendency of a body to resist a change in
its state of uniform motion
thus Newton's first law is also called
the law of inertia
well now this law should make sense do
you understand why a ball on the ground
after being kicked does not keep moving
forever the ball stops after some time
due to the unbalanced external force
applied by the ground in the opposite
direction things to remember Newton's
first law of motion states that a body
continues to be in a state of rest or of
uniform motion unless acted upon by an
unbalanced external force to unequal
oppositely exerted forces are called
unbalanced forces Newton's first law is
also known as the law of inertia the
tendency of an object to continue its
state of rest or of uniform motion in a
straight line is called inertia the
three types of inertia are inertia of
rest inertia of motion and inertia of
direction
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you
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