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Physical Science
Newton’s Laws
And Forces
GALILEO:
Since he
experimented to get
EVIDENCE for his
conclusions,
he is considered to
be the
father of
experimental
physics.
He also
figured
out that…
 Things
in motion tend to
STAY IN MOTION; they
do NOT tend to SLOW
DOWN OR STOP ON
THEIR OWN.
 In
fact, Galileo figured
out that it TAKES a force
to make something slow
down or stop. The usual
force is (he discovered):
FRICTION.
 So
in fact, GALILEO figured out
the principle behind what we call
Newton’s 1st Law
NEWTON’S 1ST LAW
 An
object at rest
tends to stay at
rest, and an object
in motion tends to
stay in motion in a
straight line at
constant velocity
unless acted on by
an outside net
force.
INERTIA refers to Newton’s 1st Law
 Inertia
is the TENDENCY of ANY MASS to
KEEP DOING what it is ALREADY DOING
(resting, or going in a straight line at a
constant speed).
A
FORCE is a push or a pull, and
INERTIA IS
 IT
NOT A FORCE!!!
IS A TENDENCEY OR
CHARACTERISTIC!
…INERTIA…




Inertia is a measure of an object’s
RESISTANCE to CHANGE in state of motion.
(Things—like people—don’t want to change!)
The word inertia comes from the Italian word
for “laziness.”
Inertia is DIRECTLY PROPORTIONAL TO MASS
ONLY. (More mass means more inertia.)
Inertia has nothing to do with size,
speed, or anything else.
What is the difference between
mass and weight?
MASS is the
amount of
MATTER in an
object.
Mass does NOT
change,
regardless of
location.
Mass is measured
in kg.
WEIGHT is the
pull due to
gravity.
–Weight
CHANGES due
to location!
–Weight is a
FORCE!
inertia…
 If
you are floating
around in space outside
the space shuttle, and
go up to it and give it a
kick, what will be the
results?
a)The space shuttle will go flying off, of course,
since it has no weight!
b) You will go flying off in the opposite direction
with a hurt foot, of course, since the space
shuttle has the same amount of mass (and
inertia) in space as it does here on earth!
 WHAT
IS VOLUME?
– The amount of space something takes up
 WHAT
IS DENSITY?
– The amount of mass per unit of volume
D=m/V
 WHAT
HAS MORE VOLUME: A
kilogram of lead, or a kilogram of
bread?
– THE BREAD TAKES UP MORE SPACE,
OF COURSE!
WHAT HAS A HIGHER DENSITY: A
kilogram of lead, or a kilogram of
bread?
– THE LEAD DOES—IT IS MORE
COMPACT—it has more matter packed
into a smaller space!
What is force?
A
push or a pull.
A
NET force is the leftover (or
unbalanced) amount of force after you
add up all the forces acting in a certain
direction.
What is ACCELERATION?
 A CHANGE in motion!

– That is ANY change in SPEED AND/OR
DIRECTION!!!
 Only
NET forces cause
accelerations!
 No
net force means NO CHANGE in
speed or direction!
Newton’s 2nd Law:
The BIGGER the amount of the force,
the BIGGER the acceleration!

The BIGGER the mass,
the LESS the acceleration!

In fancy terms
The acceleration of a body is directly proportional
to the net force on it, and inversely proportional
to the mass.

FORCE UNITS
 Think
of Newton’s 2nd law:
F=m a
 N = kg m/s2
 Since
mass is in kg, and a is in
m/s/s, or m/s2, F or Forces are
measured in kg m/s2.
 That
is the SAME as a Newton, or N.
F = ma
100N = 100kg X 1.00m/s2
a = F/m
1.00m/s2 = 100N / 100kg
…Newton’s 2nd Law…
 This
makes sense…the
more massive something
is, the harder it is to
accelerate. And the
more force there is, the
easier something will
accelerate.
 Zero
net force
means zero
acceleration.
F = ma
A
2 kg mass accelerating at 3 m/s2
has how much net force on it?
SOLUTION
A
2 kg mass accelerating at 3 m/s2
has how much net force on it?
F
= ma = 2kg X 3 m/s2 = 6 N
F = ma
 If
W = mg
you weigh 150
pounds, your mass is
about 68 kg. (1 kg =
2.2 lbs.) What is your
WEIGHT in Newtons
here on Earth?
SOLUTION
 If
you weigh 150 pounds, your mass is
about 68 kg. (1 kg = 2.2 lbs.) What is
your WEIGHT in Newtons here on
Earth?
W
= mg = 68kg X 10m/s2 = 680 N
 F = ma
F = ma
A
a = F/m
force of 16 N is pulling on a 2 kg
wagon. If there is no friction, how fast
is the wagon accelerating?
SOLUTION
A
force of 16 N is pulling on a 2 kg
wagon. If there is no friction, how
fast is the wagon accelerating?
a
= F/m = 16N / 2kg = 8 m/s2
F = ma
 If
a = F/m
the wagon above is going at a
CONSTANT VELOCITY with the 16N
pull, how much friction is there?
SOLUTION
 If
the wagon above is going at a
CONSTANT VELOCITY with the 16N
pull, how much friction is there?
 16
N ! If going constant velocity,
acceleration is 0, & net force is 0 so
all forces are BALANCED (equal)
in any 1 direction.
Newton’s 3rd Law:
 Every
action has an
equal and opposite
reaction.
 There are NO single
forces. They are
always an
action/reaction pair.”
 For
example, you touch
the table with 1N of
Force, it touches you
back with 1N of Force
SIMULTANEOUSLY (not a
little later!).
 Newton’s
3rd Law says that an action
reaction pair would be hands pushing
on barbell, barbell pushing on hands.
 Then HOW can the barbell be
ACCELERATED upward?
 What OTHER action reaction pairs are
involved?
More on Newton’s 3rd…
 If
you smash the wall with your
fist with 80 N of force, the wall
will hit you right back with 80 N.
 But
what if you hold a piece of
paper in the air and try to hit it
with 80 N?
If the paper …
 cannot
support or hit you back with
80 N, can YOU really hit IT with 80
N?
 If
the paper can only hit you back
with 5 N, what happens to the other
75 N from your punch??!
HINT:
How do they accelerate?
Force of friction
Opposes
Occurs
motion
between
surfaces in
contact
Gravitational force…
 Is
the
WEAKEST of all
the forces, but
acts over the
GREATEST
distances!
You are pulling and pulled by EVERY
ATOM IN THE UNIVERSE!!!
Electromagnetic
forces…
 Are
the forces that act due to
electron configuration in atoms
 These
forces are responsible
for the PROPERTIES or
characteristics of elements,
compounds, and materials
 This
force
makes paint
stick to a wall,
makes
diamonds
extremely hard,
makes gold
malleable,
rubber stretchy,
water a bipolar
molecule, etc.