Balanced and Unbalanced
Forces
Vocabulary
 Vocabulary
Chart
 Vocabulary
Chart with Definitions
Isaac Newton
Sir Isaac Newton was a great scientist
and mathematician.
 Isaac Newton was the first person to
hypothesize that the force that pulls an
apple to the ground also pulls the moon
toward Earth, keeping it in orbit.
 He developed three laws of motion that
we still know to be true today

Newton’s Laws of Motion
 An
object in motion tends to stay in
motion and an object at rest tends
to stay at rest unless acted upon by
an unbalanced force.
 Force equals mass times
acceleration
(F = ma).
 For every action there is an equal
and opposite reaction.
Force
A force is a push or a pull.
 Force gives an object the energy to move,
stop moving, or change direction.
 When you write with a pen you exert a
force. When you peddle your bike, blow
your nose, turn on a faucet, chew your gum,
or swimming in a pool, you are exerting
forces on other objects.
 We would never be able to move without
exerting forces on things.

Newton’s First Law of Motion



An object at rest will remain at rest unless
acted on by an unbalanced (unequal) force.
An object in motion continues in motion with
the same speed and in the same direction
unless acted upon by an unbalanced (unequal)
force.
This law is often called "the law of inertia".
What is meant by unbalanced
force?
If the forces on an object are equal and opposite, they are
said to be balanced, and the object experiences no change
in motion. If they are not equal and opposite, then the
forces are unbalanced and the motion of the object changes.
Some Examples from Real Life
A soccer ball is sitting at rest. It
takes an unbalanced force of a kick
to change its motion.
Two teams are playing tug of war. They are both
exerting equal force on the rope in opposite
directions. This balanced force results in no
change of motion.
Law of Inertia
 Inertia-
the tendency of an object
to resist a change in motion.
 This law is the same reason why
you should always wear your
seatbelt.
Inertia
 Newton’s
First Law says that objects do
not accelerate spontaneously, they
need an outside force to move.
 This property of matter, which causes
objects to resist acceleration, has been
named “inertia”.
 Newton’s First Law is often called the
Law of Inertia.
Inertia at Work
 Car
crash with and without seatbelts
and air bags
Examples of the Law of Inertia

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
index card/penny example
magician's trick of pulling a tablecloth out from
under dishes on a table.
when riding a horse, the horse suddenly stops and
you fly over its head
car turns left and you appear to slide to the right
football player running with the ball
Inertia in action – video clip
More Examples from Real Life
A powerful locomotive begins to pull a
long line of boxcars that were sitting at
rest. Since the boxcars are so massive,
they have a great deal of inertia and it
takes a large force to change their
motion. Once they are moving, it takes
a large force to stop them.
On your way to school, a bug
flies into your windshield. Since
the bug is so small, it has very
little inertia and exerts a very
small force on your car (so small
that you don’t even feel it).
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
Friction

Friction is a force that opposes motion.
Friction acts in a direction opposite to the
objects direction in motion.
 Without friction, the object would continue to
move at a constant speed forever.
 Example: sliding friction. This is when two
surfaces slide one over the other. A
snowboarder slides over the snow covered
slopes using sliding friction every day.
Friction
 The
amount of friction depends on:
 The
condition (smoothness/roughness) of
the surfaces
 The normal force between the surfaces
Gravity
Newton was the first person to seriously
study gravity
 Gravity is a force that attracts all objects
toward each other.
 The force of gravity is measured in units
called Newtons (N).

•
What is gravity?
Gravity
 The
strength of gravity between two
objects depends on two factors:
 1. masses of the objects (If mass
increases, force also increases)
 2. distance between the objects (If
distance increases, force decreases)
Gravity
Gravity
The greater the mass, the greater the
force
 The greater the distance, the less the
force
 Acceleration due to gravity = 9.8 m/s/s
or 9.8 m/s2

Mass vs. Weight
 Mass-
is the amount of matter in an
object
 Weight- is the force of gravity on an
object
 The greater the mass the greater the
force (weight)
 Weightlessness – free from the
effects of gravity
 Mass vs. Weight
Newton’s
st
1
Law activities
 http://science360.gov/obj/video/70fadaa
8-c3d4-4132-ba1fc98be5caeb14/science-nfl-footballnewtons-first-law-motion
 Wacky washers and penny lab
How does Friction work?
 In
today’s experiment, we will see just
how friction effect our movements in
everyday life.
 Teacher
Notes
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.
Newton’s Second Law
• Force = Mass x Acceleration
• Force is measured in Newtons
ACCELERATION of GRAVITY(Earth) = 9.8 m/s2
• Weight (force) = mass x gravity (Earth)
Moon’s gravity is 1/6 of the Earth’s
If you weigh 420 Newtons on earth,
what will you weigh on the Moon?
70 Newtons
If your mass is 41.5Kg on Earth
what is your mass on the Moon?
Newton’s Second Law
• WEIGHT is a measure of the
gravity on the
force of ________
mass of an object
Newtons
• measured in __________
Newton’s Second Law
One rock weighs 5 Newtons.
The other rock weighs 0.5
Newtons. How much more
force will be required to
accelerate the first rock
at the same rate as the
second rock?
Ten times as much
What does F = ma say?
F = ma basically means that the force of an
object comes from its mass and its
acceleration.
Something very massive (high
mass) that’s changing speed very
slowly (low acceleration), like a
glacier, can still have great force.
Something very small (low mass)
that’s changing speed very quickly
(high acceleration), like a bullet, can
still have a great force. Something
very small changing speed very
slowly will have a very weak force.
Working with Newton’s 2nd
the graphing Newton’s
second law activity sheet
 Complete
Newton’s Third Law
For every action there is an equal and
opposite reaction.
Newton’s
rd
3
Law
 For
every action there is an equal and
opposite reaction.
Book to
earth
Table to
book
What does this mean?
For every force acting on an object, there is an equal
force acting in the opposite direction. Right now,
gravity is pulling you down in your seat, but
Newton’s Third Law says your seat is pushing up
against you with equal force. This is why you are
not moving. There is a balanced force acting on
you– gravity pulling down, your seat pushing up.
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push
against a wall? You slide in the opposite
direction (away from the wall), because
you pushed on the wall but the wall
pushed back on you with equal and
opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on
a rock, the rock exerts an equal force back
on your toe. The harder you hit your toe
against it, the more force the rock exerts
back on your toe (and the more your toe
hurts).
Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth
Reaction: road pushes on tire
Action: tire pushes on road
Reaction: gases push on rocket
Action: rocket pushes on gases
Consider hitting a baseball with a bat. If
we call the force applied to the ball by the
bat the action force, identify the reaction
force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
rd
3
Law of Motion Wrap Up
 Video
clip
Net Force
 How
can you tell if something is equal
and opposite?
 Calculate
it’s net force!
Fettuccine Lab

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The height shall be at least 5 cm, supporting the book 5 cm
above the table. All parts of the book should be at least 5 cm
above the table.
Use all or part of the 20 pieces of fettuccine and all or part of the
1 m of masking tape. No additional tape or pasta may be used.
Tape must be used to secure the support structure. The
structure must be one unit that can be picked up and moved
The structure must have a minimum of three points of support.
Test the pasta structure by placing a piece of paper on the
structure first. Then a lightweight book is placed on the
structure. The lightweight book is removed, and a medium
weight book is placed on the structure. Last, a medium weight
book is removed and a textbook is placed on the structure. To
be successful, the structure must hold the item for ten seconds.
Tug of War
 Today
we will compete against the other
science classes in a demonstration of
Newton’s third law of motion.
 Summarize
War
your experience during Tug of
Brief History of magnets
 Magnets
have been known for
centuries.
 The
Chinese and Greeks knew about
the “magical” properties of magnets.
 They
used the mineral “magnetite” to
help in navigation, because it always
pointed in the same direction.
Magnetic Forces
 Magnetic
forces are produced by
magnetic poles.
 Every magnet has both a North and
South pole.
 Like poles repel, unlike poles attract.
Magnetic Fields

Magnetic fields transmit magnetic forces.
 Direction of the field is from N to S.
 Field is stronger where field lines are closer.
 Unit of magnetic field strength is the Tesla.
The older unit Gauss is sometimes used.

Earth’s magnetic field strength is about 10-4 Tesla
or about 1 Gauss
How Magnets Attract
A
magnet near an unmagnetized piece
of iron causes Attractive magnetic force
on the iron
 This causes the iron to become
temporarily magnetized
How big is the earths
magnetic field?
Earth’s magnetic field extends far into space.
We call this the “magnetosphere.”
 When magnetic particles (mainly from the
sun), called “solar wind”, strike the
magnetosphere, we can see a phenomenon
called the…


Aurora Borealis in the Northern Hemisphere.
 Aurora Australis in the Southern Hemisphere.
How does a compass work?
We use the Earth’s magnetic field to find
direction.
 The needle of the compass always points
toward the magnetic south pole.


YES! That is correct. We call this direction
“North”. Remember!! Opposites Attract!
Electric Currents & Magnetism

Since moving charges create magnetic fields,
an electric current creates a magnetic field.
 A coil of wire can concentrate the magnetic
field and create an electromagnet.
Motors
 An
electric motor uses a magnet to
exert a force on a current-carrying coil
of wire.
Review
 Magnet
 Truth
and Iron Fillings Demonstration
About Magnets p.1

 Truth
About Magnets p.2
 Answers
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
1stlaw: Homer is large and has much
mass, therefore he has much inertia.
Friction and gravity oppose his motion.
2nd law: Homer’s mass x 9.8 m/s/s
equals his weight, which is a force.
3rd law: Homer pushes against the
ground and it pushes back.
Bill Nye
 Force
and Motion Video
Jeopardy
 We
will review for the test by playing a
game of Jeopardy!
 Force
and Motion Jeopardy
 Forces
Jeopardy