Newton’s First Law of Motion – Class 9 Tutorial

Newton’s First Law of Motion – Class 9 Tutorial

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Newton’s First Law Of Motion 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 some
time would it naturally stop moving? Until Newton, most people thought the ball would
eventually stop on its own. However, the ball will continue to move unless
an object or 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. This means that there is a natural tendency
of objects to keep on doing what they’re doing. All objects resist changes in their states
of motion. In the absence of an unbalanced force, an object in motion will maintain this
state of motion. 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 forces applied by the teams are balanced forces, as they are of equal magnitudes in
opposite directions. If one of the teams applies a greater force,
it pulls the other team and the rope towards their side. In this case, the forces are unbalanced
as the two forces are of unequal magnitudes in the opposite directions. 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 is the resistance an object has to a change
in its state of motion. 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 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 right. This is due to the inertia of direction. 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 the lower part of the passenger’s
body, that is in contact with the bus, to come to rest. However, the upper portion of
his 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 the 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. Now 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 its state of rest or of uniform motion unless acted upon by
an unbalanced external force’. Two 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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