Essential Question
• How can I explain the
causes of motion on an
object?
• Learning Objectives: Laws
of Motion, Friction, Force
Background
Sir Isaac Newton (1643-1727) an English
scientist and mathematician famous for his
discovery of the law of gravity also
discovered the three laws of motion. He
published them in his book Philosophiae
Naturalis Principia Mathematica
(mathematic principles of natural
philosophy) in 1687. Today these laws are
known as Newton’s Laws of Motion and
describe the motion of all objects on the
scale we experience in our everyday lives.
Newton’s Laws of Motion
• 1st Law – An object at rest will stay at
rest, and an object in motion will stay in
motion at constant velocity, unless acted
upon by an unbalanced force.
• 2nd Law – Force equals mass times
acceleration.
• 3rd Law – For every action there is an
equal and opposite reaction.
First Law: An object at rest stays at rest or
an object in motion, stays in motion (in
the same direction/at the same speed)
unless acted upon by an unbalanced
force
Also called the law of inertia
Inertia
A property of matter
The tendency of an object to resist any
change in its motion
The greater the mass the greater the inertia
The greater the speed the greater the inertia
Examples of Newton’s 1st Law
a) car suddenly stops and you strain against the seat belt
b) when riding a horse, the horse suddenly stops and you fly
over its head
c) the magician pulls the tablecloth out from under a table
full of dishes
d) the difficulty of pushing a dead car
e) lawn bowling on a cut and rolled lawn verses an uncut
lawn appear
f) car turns left and you
to slide to the right
Examples of Newton’s 1st Law
1st Law
• Once airborne,
unless acted
on by an
unbalanced
force (gravity
and air – fluid
friction), it
would never
stop!
1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
Newton’s First Law
• Newton’s First Law of Motion
– An object at rest will remain at rest and
an object in motion will continue moving
at a constant velocity unless acted upon
by a net force.
Newton’s First Law
• Newton’s First Law of Motion
– “Law of Inertia”
• Inertia
– tendency of an object to resist any change in its
motion
– increases as mass increases
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.
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.
Why then, do we observe
every day objects in motion
slowing down and becoming
motionless seemingly without
an outside force?
It’s a force we sometimes cannot see –
friction.
What is this unbalanced force that acts on an object in motion?
• There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water
resistance
– Static friction: initial friction when moving an
object
Newton’s First Law
An object at rest tends to stay at rest
and an object in motion tends to stay
in motion unless acted upon by an
unbalanced force.
What does this mean?
Review…
Basically, an object will “keep doing what it
was doing” unless acted on by an
unbalanced force.
If the object was sitting still, it will remain
stationary. If it was moving at a constant
velocity, it will keep moving.
It takes force to change the motion of an
object.
Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.
2nd Law
The net force of an object is
equal to the product of its
mass and acceleration, or
F=ma.
Second law: The greater the
force applied to an object,
the more the object will
accelerate. It takes more
force to accelerate an
object with a lot of mass
than to accelerate
something with very little
mass.
The player in black had more
acceleration thus he hit with a
greater amount of force
2nd Law
What is Force?
on an object!!
It is any push or pull
Second law:
The greater the force, the greater the acceleration
The greater the mass, the greater the force needed
for the same acceleration
Calculated by: F = ma
 (F = force, m = mass, a = acceleration)
Examples of Newton’s 2nd Law
a) hitting a baseball, the harder the hit, the faster the ball
goes
b) accelerating or decelerating a car
c) The positioning of football players - massive players on
the line with lighter (faster to accelerate) players in the
backfield
d) a loaded versus an
unloaded truck
Examples of Newton’s 2nd Law
Newton’s Second Law
• Newton’s Second Law of Motion
– The acceleration of an object is directly proportional
to the net force acting on it and inversely
proportional to its mass.
F = ma
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.
2nd Law
• When mass is in kilograms and
acceleration is in m/s/s, the unit of force is
in newtons (N).
• One newton is equal to the force required
to accelerate one kilogram of mass at one
meter/second/second.
2nd Law (F = m x a)
• How much force is needed to accelerate a
1400 kilogram car 2 meters per
second/per second?
• Write the formula
• F=mxa
• Fill in given numbers and units
• F = 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800
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 = ma
F = ma
98 N = 10 kg x 9.8 m/s/s
9.8 N = 1 kg x 9.8 m/s/s
ConceptTest
• Is the following statement true or false?
– An astronaut has less mass on the
moon since the moon exerts a weaker
gravitational force.
– False! Mass does not depend on gravity,
weight does. The astronaut has less
weight on the moon.
Check Your Understanding
• 1. What acceleration will result when a 12 N net force applied to a 3
kg object? A 6 kg object?
• 2. A net force of 16 N causes a mass to accelerate at a rate of 5
m/s2. Determine the mass.
• 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 9.8
m/sec/sec?
third law: For every
action force, there is
an equal and opposite
reaction force. (Forces
are always paired)
3rd Law
According to Newton,
whenever objects A
and B interact with
each other, they exert
forces upon each
other. When you sit in
your chair, your body
exerts a downward
force on the chair and
the chair exerts an
upward force on your
body.
Newton’s 3rd Law in Nature
• Consider the propulsion
of a fish through the
water. A fish uses its fins
to push water backwards.
In turn, the water reacts
by pushing the fish
forwards, propelling the
fish through the water.
• The size of the force on
the water equals the size
of the force on the fish;
the direction of the force
on the water (backwards)
is opposite the direction
of the force on the fish
(forwards).
3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.
rd
Examples of Newton’s 3 Law
a) rockets leaving earth
b) guns being fired
c) two cars hit head on
d) astronauts in space
e) pool or billiards
f) jumping out of a boat onto
the dock
g) sprinklers rotating
rd
Examples of Newton’s 3 Law
Newton’s third law: "For every action, there is
an equal and opposite reaction." When you fire
a gun you feel the recoil. Some of the funniest
things in cartoons follow physics that have
been exaggerated or just plain ignored. Wyle
Coyote hangs suspended in space over that
canyon for a lot longer than an object would
in reality, but it is the anticipation of the drop
and Wyle's facial recognition of the upcoming
pain that is so classically cartooney. So some
laws are stretched for comical effect.
Other examples of Newton’s Third
Law
• The baseball forces
the bat to the left (an
action); the bat forces
the ball to the right
(the reaction).
3rd Law
• Consider the motion
of a car on the way to
school. A car is
equipped with wheels
which spin
backwards. As the
wheels spin
backwards, they grip
the road and push the
road backwards.
Examples of Newton’s 3rd Law
Newton’s Third Law
• Newton’s Third Law of Motion
– When one object exerts a force on a
second object, the second object exerts
an equal but opposite force on the first.
Newton’s Third Law
• Problem:
– How can a horse
pull a cart if the cart
is pulling back on
the horse with an equal but opposite force?
– Aren’t these “balanced forces” resulting in no
acceleration?
NO!!!
Newton’s Third Law
• Explanation:
– forces are equal and opposite but act on
different objects
– they are not “balanced forces”
– the movement of the horse depends on the
forces acting on the horse
Newton’s Third Law
• Action-Reaction Pairs
• The hammer exerts a
force on the nail to the
right.
• The nail exerts an equal
but opposite force on
the hammer to the left.
Newton’s Third Law
• Action-Reaction Pairs
• The rocket exerts a
downward force on the
exhaust gases.
• The gases exert an equal
but opposite upward force
on the rocket.
FG
FR
Newton’s Third Law
For every action there is an equal and
opposite reaction.
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).
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.
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.
Which of Newton’s Laws of Motion
apply to these situations?
•
•
•
•
•
•
•
•
An object rests in your hand
A ball is tossed into the air
A car windshield hits a bug
A person sits on a table
A person jumps up from the floor
A baseball bat hits a baseball
A truck and car hit head on
Any others?
http://smartweed.olemiss.edu/nmgk8/curriculum/middlesch
ool/eighth/discovering_newtons_laws/Discovering%20Newt
ons%20Laws.ppt
http://www3.science.tamu.edu/CMSE/powerpoint/Newton's
Laws.pptv