Newton's Laws of Motion

1. Newton’sNewton’s
Laws ofLaws of
MotionMotionI.I. Law of InertiaLaw of Inertia
II.II. F=maF=ma
III.III. Action-ReactionAction-Reaction

2. While most people knowWhile most people know
what Newton's laws say,what Newton's laws say,
many people do not knowmany people do not know
what they mean (or simply dowhat they mean (or simply do
not believe what they mean).not believe what they mean).

3. Newton’s Laws of MotionNewton’s Laws of Motion
 11stst
LawLaw – An object at rest will stay at rest,– An object at rest will stay at rest,
and an object in motion will stay inand an object in motion will stay in
motion at constant velocity, unless actedmotion at constant velocity, unless acted
upon by an unbalanced force.upon by an unbalanced force.
 22ndnd
LawLaw –– Force equals mass timesForce equals mass times
acceleration.acceleration.
 33rdrd
LawLaw –– For every action there is anFor every action there is an
equal and opposite reaction.equal and opposite reaction.

4. 11stst
Law of MotionLaw of Motion
(Law of Inertia)(Law of Inertia)
An object at rest will stay atAn object at rest will stay at
rest, and an object in motionrest, and an object in motion
will stay in motion atwill stay in motion at
constant velocity, unless actedconstant velocity, unless acted
upon by an unbalanced force.upon by an unbalanced force.

5. 11stst
LawLaw
 Inertia is theInertia is the
tendency of antendency of an
object to resistobject to resist
changes in itschanges in its
velocity:velocity:
whether inwhether in
motion ormotion or
motionless.motionless.
These pumpkins will not move unless acted on
by an unbalanced force.

6. 11stst
LawLaw
 Once airborne,Once airborne,
unless acted onunless acted on
by anby an
unbalanced forceunbalanced force
(gravity and air(gravity and air
– fluid friction),– fluid friction),
it would neverit would never
stop!stop!

7. 11stst
LawLaw
 Unless actedUnless acted
upon by anupon by an
unbalancedunbalanced
force, this golfforce, this golf
ball would sit onball would sit on
the tee forever.the tee forever.

8. Why then, do we observe everyWhy then, do we observe every
day objects in motion slowingday objects in motion slowing
down and becoming motionlessdown and becoming motionless
seemingly without an outsideseemingly without an outside
force?force?
It’s a force we sometimes cannot see –It’s a force we sometimes cannot see –
friction.friction.

9. Objects on earth, unlike theObjects on earth, unlike the
frictionless space the moonfrictionless space the moon
travels through, are under thetravels through, are under the
influence of friction.influence of friction.

10.  There are four main types of friction:There are four main types of friction:
 Sliding friction:Sliding friction: ice skating
 Rolling friction:Rolling friction: bowling
 Fluid friction (air or liquid):Fluid friction (air or liquid): air or water resistance
 Static friction:Static friction: initial friction when moving an
object
What is this unbalanced force that acts on an object in motion?What is this unbalanced force that acts on an object in motion?

11. Slide a bookSlide a book
across a table andacross a table and
watch it slide to a restwatch it slide to a rest
position. The bookposition. The book
comes to a restcomes to a rest
because of thebecause of the
presencepresence of a force -of a force -
that force being thethat force being the
force of friction -force of friction -
which brings the bookwhich brings the book
to a rest position.to a rest position.

12.  In the absence of a force of friction, the bookIn the absence of a force of friction, the book
would continue in motion with the same speedwould continue in motion with the same speed
and direction - forever! (Or at least to the endand direction - forever! (Or at least to the end
of the table top.)of the table top.)

13. Newtons’s 1Newtons’s 1stst
Law and YouLaw and You
Don’t let this be you. Wear seat belts.Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changesBecause of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stoppedin their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.by the brick wall, your body keeps moving at 80 m/hour.

14. 22ndnd
LawLaw

15. 22ndnd
LawLaw
The net force of an object isThe net force of an object is
equal to the product of its massequal to the product of its mass
and acceleration, or F=ma.and acceleration, or F=ma.

16. 22ndnd
LawLaw
 When mass is in kilograms and acceleration isWhen mass is in kilograms and acceleration is
in m/s/s, the unit of force is in newtons (N).in m/s/s, the unit of force is in newtons (N).
 One newton is equal to the force required toOne newton is equal to the force required to
accelerate one kilogram of mass at oneaccelerate one kilogram of mass at one
meter/second/second.meter/second/second.

17. 22ndnd
Law (F = m x a)Law (F = m x a)
 How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
 Write the formulaWrite the formula
 F = m x a
 Fill in given numbers and unitsFill in given numbers and units
 F = 1400 kg x 2 meters per second/second
 Solve for the unknownSolve for the unknown

2800 kg-meters/second/second or 2800 N

18. If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.

19. Newton’s 2nd
Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd
Law we know
that they don’t hit the
ground with the same
force.
F = maF = ma
98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s
F = maF = ma
9.8 N = 1 kg x 9.89.8 N = 1 kg x 9.8
m/s/sm/s/s

21. Check Your UnderstandingCheck Your Understanding
 1. What acceleration will result when a 12 N net force applied to a 3 kg1. What acceleration will result when a 12 N net force applied to a 3 kg
object? A 6 kg object?object? A 6 kg object?
 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s22
..
Determine the mass.Determine the mass.
 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
 4. What is the force on a 1000 kg elevator that is falling freely at 9.84. What is the force on a 1000 kg elevator that is falling freely at 9.8
m/sec/sec?m/sec/sec?

22. Check Your UnderstandingCheck Your Understanding
 1. What acceleration will result when a 12 N net force applied to a 3 kg object?1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s12 N = 3 kg x 4 m/s/s
 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s22
. Determine the. Determine the
mass.mass.
16 N = 3.2 kg x 5 m/s/s16 N = 3.2 kg x 5 m/s/s
 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N66 kg-m/sec/sec or 66 N
 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
 9800 kg-m/sec/sec or 9800 N9800 kg-m/sec/sec or 9800 N

24. 33rdrd
LawLaw
 For every action, there is anFor every action, there is an
equal and opposite reaction.equal and opposite reaction.

25. 33rdrd
LawLaw
According to Newton,According to Newton,
whenever objects A andwhenever objects A and
B interact with eachB interact with each
other, they exert forcesother, they exert forces
upon each other. Whenupon each other. When
you sit in your chair,you sit in your chair,
your body exerts ayour body exerts a
downward force on thedownward force on the
chair and the chairchair and the chair
exerts an upward forceexerts an upward force
on your body.on your body.

26. 33rdrd
LawLaw
There are two forcesThere are two forces
resulting from thisresulting from this
interaction - a force oninteraction - a force on
the chair and a force onthe chair and a force on
your body. These twoyour body. These two
forces are calledforces are called actionaction
andand reactionreaction forces.forces.

27. Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature
 Consider the propulsion of aConsider the propulsion of a
fish through the water. Afish through the water. A
fish uses its fins to pushfish uses its fins to push
water backwards. In turn,water backwards. In turn,
the waterthe water reactsreacts by pushingby pushing
the fish forwards, propellingthe fish forwards, propelling
the fish through the water.the fish through the water.
 The size of the force on theThe size of the force on the
water equals the size of thewater equals the size of the
force on the fish; theforce on the fish; the
direction of the force on thedirection of the force on the
water (backwards) iswater (backwards) is
opposite the direction of theopposite the direction of the
force on the fish (forwards).force on the fish (forwards).

28. 33rdrd
LawLaw
Flying gracefullyFlying gracefully
through the air, birdsthrough the air, birds
depend on Newton’sdepend on Newton’s
third law of motion. Asthird law of motion. As
the birds push down onthe birds push down on
the air with their wings,the air with their wings,
the air pushes theirthe air pushes their
wings up and giveswings up and gives
them lift.them lift.

29.  Consider the flying motion of birds. A bird flies byConsider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push airuse of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the birddownwards. In turn, the air reacts by pushing the bird
upwards.upwards.
 The size of the force on the air equals the size of theThe size of the force on the air equals the size of the
force on the bird; the direction of the force on the airforce on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on(downwards) is opposite the direction of the force on
the bird (upwards).the bird (upwards).
 Action-reaction force pairs make it possible for birdsAction-reaction force pairs make it possible for birds
to fly.to fly.

31. Other examples of Newton’sOther examples of Newton’s
Third LawThird Law
 The baseball forces theThe baseball forces the
bat to the left (anbat to the left (an
action); the bat forcesaction); the bat forces
the ball to the right (thethe ball to the right (the
reaction).reaction).

32. 33rdrd
LawLaw
 Consider the motion ofConsider the motion of
a car on the way toa car on the way to
school. A car isschool. A car is
equipped with wheelsequipped with wheels
which spin backwards.which spin backwards.
As the wheels spinAs the wheels spin
backwards, they grip thebackwards, they grip the
road and push the roadroad and push the road
backwards.backwards.

33. 33rdrd
LawLaw
The reaction of a rocket isThe reaction of a rocket is
an application of the thirdan application of the third
law of motion. Variouslaw of motion. Various
fuels are burned in thefuels are burned in the
engine, producing hotengine, producing hot
gases.gases.
The hot gases push againstThe hot gases push against
the inside tube of the rocketthe inside tube of the rocket
and escape out the bottomand escape out the bottom
of the tube. As the gasesof the tube. As the gases
move downward, the rocketmove downward, the rocket
moves in the oppositemoves in the opposite
direction.direction.