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&
Isaac Newton
(1642–1727)
proposed that
the tendency of
an object was to
maintain in its
current state of
motion.
“Law of Inertia”
An object at rest remains at rest,
and an object in motion continues in motion
at a constant speed in a straight line,
unless acted on by a nonzero net force
(unbalanced force).
* OR *
The velocity of an object remains constant
unless acted on by an unbalanced force.
Objects tend to stay at rest OR
object tend to stay in motion at a
constant speed.
Zero Net Force
the condition of an object
that is not accelerating
If an object is at rest
or moving at constant speed,
there will be no net force acting on
the object. All forces will be balanced.
INERTIA
the tendency of an object
to resist any change in its motion
Inertia is a property of matter and does not
depend on the position or location of the object.
MASS (kg)
a quantitative measure of inertia
The greater the mass 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
f) car turns left and you appear to slide
to the right
To sum it up…
Newton’s 1st Law- Inertia
• In the absence of “a net force,” an object moves
with constant velocity.
• In the absence of “the thing that will cause a
“change in velocity,” an object moves with
constant velocity.
• The cause of constant velocity is NOTHING that
will change the velocity.
• The cause of motion is NOTHING!
• Motion Happens!
“Law of Acceleration”
A net force causes an object to accelerate
in the direction of the net force.
The acceleration is
directly proportional to the net force
and inversely proportional
to the object’s mass.
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
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)
WEIGHT
a measure of the gravitational force
that a massive object,
such as a star or planet,
puts on another mass
F = ma
weight = mass x acceleration of gravity
Fg = mag
An object’s weight on planet Earth in newtons
is equal to its mass in kilograms times 9.8 m/s2.
Fnet
a=
m
Fnet = m a
The SI unit of force is the newton, named
in honor of Isaac Newton.
One newton of force
is the amount of force
needed to cause a one
kilogram mass to accelerate
at a rate of 1 m/s2.
The net force acting on an object is the
vector sum of all the forces acting on it.
Examples:
8N
8N
4N
7N
12 N
9N
5N
6N
3N
7N
4N
If an object is remaining at rest, it
is incorrect to assume that there
are no forces acting on the object.
We can only conclude that the
net force on the object is zero.
8N
8N
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
The second law states that
unbalanced forces cause
objects to accelerate with an
acceleration which is directly
proportional to the net force
and inversely proportional to
the mass. This one is telling
us that big heavy objects
don’t move as fast or as
easily as smaller lighter
objects. It takes more to
slow down a charging bull
then to slow down a
charging mouse.
What is a Force?
FORCE = Any push or pull which causes
something to change its speed or direction
What is a Force?
Forces can be BALANCED or UNBALANCED
Balanced forces are equal in size (magnitude)
and opposite in direction
UNbalanced forces are not equal in size
(magnitude) and/or opposite in direction. If
the forces on an object are UNBALANCED, we
say a NET force results.
What is a Force?
Can you think of examples of forces?
Balanced Forces?
Unbalanced Forces?
Identifying a force:
• Source, agent exerting the force
• Type: “contact” or “long-range”
– “Contact”- the 2 objects HAVE to be touching
• EX. air resistance, applied (push or pull), friction, normal,
tension, thrust, etc.
– “long-range”- the 2 objects DO NOT have to be
touching
• EX. gravitational, electric, magnetic
What is Gravity?
GRAVITY: An attraction force between all
masses
The greater the mass, the greater the force
Acceleration due to gravity = 9.8 m/s/s or 9.8
m/s2
Weight (F ) is a measure of the
g
gravitational force between two objects
The greater the mass the greater the force
(weight)
Measured in units called newtons (N)
Weightlessness – free from the effects of
gravity, a myth - true no where
Air resistance:
The force of air exerted
on a falling object
 The air pushes up as gravity pulls down
 Dependent upon the shape and surface area of the
object and its speed
 When the air resistance magnitude equals the
force of gravity magnitude, terminal speed is
reached
 Terminal speed is the highest speed that an object
will reach as it falls freely in air
The force of friction
• The push things give to each other when they rub
together. This changes motion into heat.
• Friction is the force that acts in the opposite
direction of motion of the object
• In Newton’s world, if you could eliminate friction,
then you would see more things keep on moving
in a straight line.
• Newton’s first law only makes sense when all
forces, including friction, are accounted for.
As a baseball is being caught it’s velocity goes
from
30.0 m/s to 0.0 m/s in about 0.0050 s. The mass of
the
baseball is 0.145kg.
•
•
•
What is the baseball’s acceleration?
a = ΔV / Δt
What is the magnitude and direction of the force
acting on the ball?
What is the magnitude and direction of the force
acting on the player who caught the ball?
“Law of Interaction”
Short Version
“For every action force there is
an equal and opposite reactive force.”
Longer Version
When one object exerts a force on a
second object, the second exerts a force
on the first that is equal in magnitude,
but opposite in direction.
third law: For every
action force, there
is an equal and
opposite reaction
force. (Forces are
always paired)
Examples of
rd
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
Examples
of Newton’s
rd
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.
Examples of Newton’s 3rd Law
Example
• When you bump into something, the size of
the bruise is determined by Newton's 3rd
Law. When you hit the object it hits you
back with the same force. You only get
back what you put into it.
Newton’s 3rd Law, the Law of Interaction
If (object 1) exerts a (type) force in the
(direction) direction on (object 2), then
(object 2) is exerting a (same type)
force directed (opposite direction) on
(object 1) simultaneously.
Consider the interaction depicted below
between foot A, ball B, and foot C. The three
objects interact simultaneously (at the same
time). Identify the two pairs of action-reaction
forces. Use the notation "foot A", "foot C", and
"ball B" in your statements.
http://www.physicsclassroom.com/Class/newtl
aws/u2l4b.cfm
Identify at least six pairs of action-reaction force
pairs in the following diagram.
http://www.physicsclassroom.com/Class/newt
laws/u2l4b.cfm
a = ? m/s2
m = 65 kg
•
•
•
a = 3.0 m/s2
m = 45 kg
A 65 kg boy and a 45 kg girl use an elastic rope
while playing tug – of – war. The two are on an
icy frictionless surface. If the acceleration of the
girl is 3.0 m/s2, then find the magnitude of and
direction of the acceleration of the boy toward the
girl.
FBD
F b on g = -F g on b
Newton’s 3rd Law
F g on b = -F b on g
How do Newton’s Laws of motion
apply to these situations?
•
•
•
•
•
•
•
an object rests in your hand
a ball is tossed upward
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