Newton's first law of motion states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. It describes how objects resist changes in their motion due to their inertia. Greater the mass of an object, greater is its inertia and more force is needed to change its state of motion. Examples include passengers falling in a suddenly moving bus due to inertia of rest and being thrown forward in a suddenly stopping bus due to inertia of motion.

1. Newton’sNewton’s
First law ofFirst law of
Motion!Motion!
ByBy
Anshuman Saboo – DPS BangaloreAnshuman Saboo – DPS Bangalore

2. Newton’s First LawNewton’s First Law
of Motionof Motion
(also known as the law of inertia)(also known as the law of inertia)
An object at rest will stay at rest, and an object inAn object at rest will stay at rest, and an object in
motion will stay in motion at constant velocity, unlessmotion will stay in motion at constant velocity, unless
acted upon by an unbalanced force.acted upon by an unbalanced force.
Newton’s first law describes the behaviour of such bodies which are in aNewton’s first law describes the behaviour of such bodies which are in a
state of rest or of uniform motion in a straight linestate of rest or of uniform motion in a straight line
The first part of law deals with the state of rest of a body.According to this,The first part of law deals with the state of rest of a body.According to this,
every body preserves its state of rest unless some external force compels itevery body preserves its state of rest unless some external force compels it
to change its state of rest.to change its state of rest.
The second part of the law deals with the state of motion. According to this,
everybody continues in its state of uniform motion in a straight line unless some
external force compels it to change that state

3. The first law can be stated also asThe first law can be stated also as
 An object that is at rest will stay at rest unlessAn object that is at rest will stay at rest unless
an external force acts upon it.an external force acts upon it.
 An object that is in motion will not change itsAn object that is in motion will not change its
velocity unless an external force acts upon it.velocity unless an external force acts upon it.

4. InertiaInertia
 The tendency of a body to remain at rest(stationary)The tendency of a body to remain at rest(stationary)
or, if moving, to continue moving in a straight line, isor, if moving, to continue moving in a straight line, is
called inertia.called inertia.
 Newton’s first law recognizes that every body hasNewton’s first law recognizes that every body has
some inertia.some inertia.
 Greater the inertia of a body, greater will be the forceGreater the inertia of a body, greater will be the force
required to bring a change in its state of rest or ofrequired to bring a change in its state of rest or of
uniform motionuniform motion
 Inertia also of a body depends on its mass .Inertia also of a body depends on its mass .
 If a body has more mass , it has more inertiaIf a body has more mass , it has more inertia

5. Inertia depends on massInertia depends on mass
 Here the toy truck has a larger mass than the to car, so itHere the toy truck has a larger mass than the to car, so it
has a larger inertia , and hence its a little more difficult tohas a larger inertia , and hence its a little more difficult to
push the toy truckpush the toy truck
 Here the toy car has a small mass, so it has small inertia, andHere the toy car has a small mass, so it has small inertia, and
hence can be moved easily by pushinghence can be moved easily by pushing

6. To overcome inertia and make a body moveTo overcome inertia and make a body move
from rest we must apply an external forcefrom rest we must apply an external force
 From the above examples it is clear that objects continue toFrom the above examples it is clear that objects continue to
remain in their state of rest or of uniform motion until anremain in their state of rest or of uniform motion until an
external force is appliedexternal force is applied
 Place a cardboard on an empty tumbler and a coin on thePlace a cardboard on an empty tumbler and a coin on the
cardboard as shown in the figure.cardboard as shown in the figure.
 Flick the cardboard with the finger. What do you observe? TheFlick the cardboard with the finger. What do you observe? The
coin drops into the tumbler. When we flick the cardboard thecoin drops into the tumbler. When we flick the cardboard the
cardboard moves fast whereas the coin continues in its state ofcardboard moves fast whereas the coin continues in its state of
rest and hence drops into the tumbler.rest and hence drops into the tumbler.


7. Examples of Newton’s first lawExamples of Newton’s first law
 When a bus suddenly starts, the passengersWhen a bus suddenly starts, the passengers
sitting or standing in the bus tend to fallsitting or standing in the bus tend to fall
backward. This is due to inertia of rest and canbackward. This is due to inertia of rest and can
be explained as follows: when the bus suddenlybe explained as follows: when the bus suddenly
starts, the lower part of the body of the passengerstarts, the lower part of the body of the passenger
which is in contact with the bus moves alongwhich is in contact with the bus moves along
with the bus while the upper part of the bodywith the bus while the upper part of the body
tends to retain its state of rest due to inertia. As atends to retain its state of rest due to inertia. As a
result, the passenger falls backward.result, the passenger falls backward.
 When moving bus suddenly stops, theWhen moving bus suddenly stops, the
passengers sitting or standing in the bus arepassengers sitting or standing in the bus are
thrown forward. This is due to inertia of motionthrown forward. This is due to inertia of motion
and can be explained as follows: when theand can be explained as follows: when the
moving bus suddenly stops, the lower part of themoving bus suddenly stops, the lower part of the
body of the passenger contact with the busbody of the passenger contact with the bus
suddenly comes to rest while the upper part ofsuddenly comes to rest while the upper part of
the body tends to retain its state of motion due tothe body tends to retain its state of motion due to
inertia. As a result, the passenger is throwninertia. As a result, the passenger is thrown

8. More examplesMore examples
 A rider on a running horse is thrown forward whenA rider on a running horse is thrown forward when
the horse stops suddenly due to inertia of motion.the horse stops suddenly due to inertia of motion.
 . When a branch of a tree is vigorously shaken the. When a branch of a tree is vigorously shaken the
fruits and seeds in It fall down to inertia of rest.fruits and seeds in It fall down to inertia of rest.
 When a blanket is given a sudden jerk, the dustWhen a blanket is given a sudden jerk, the dust
particles in it fail off due to inertia of restparticles in it fail off due to inertia of rest..
 When a person jumps out of a moving vehicle,When a person jumps out of a moving vehicle,
he falls forward due to inertia of motion.he falls forward due to inertia of motion.

9. CreditsCredits
Thank youThank you
If you like it, you canIf you like it, you can
email me atemail me at
superanshuman01@gsuperanshuman01@g
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10. CreditsCredits
Thank youThank you
If you like it, you canIf you like it, you can
email me atemail me at
superanshuman01@gsuperanshuman01@g
mail.commail.com