CHAPTER – 4 Newton’s First Law
4.1 Aristotle on Motion
Aristotle was a fourth century b.c. Greek
scientist that stated motion is divided into
two types:
1. natural motion
2. violent motion
Natural Motion =
Either straight up or straight down motion.
Objects will seek their natural resting
place.
Boulders on the ground compared to
smoke, clouds or fog in the air.
Heavy things will fall and light things will
rise.
Circular motion was natural for “the
heavens” (all celestial bodies,
planets, moons, stars, … etc) since
there was no beginning nor end.
Since these motions were considered
natural they were not caused by
forces.
Violent Motion
Motion is a result of forces acting on
an object by either pushing or pulling
the object. Therefore, the motion has
an external cause.
Example:
A cart being pulled by a horse, a ship
being pushed by the wind.
For over 2000 years the mindset was that
if an object was moving “against its
nature” then some force was acting on it.
The proper state of an object was at rest,
unless being pushed or pulled.
It was not considered possible that there
was enough force to move the Earth, so
the Earth must not move.
4.2 Copernicus and the Moving
Earth
Nicolaus Copernicus (14731543) theorized that the
Earth and other planets
orbited a stationary sun.
Copernicus did not publish
his theory for fear of being
put in jail or even possibly
receiving the death
sentence.
Q: WHY ?
A: During this particular time period “The
Church” deemed that the Earth was the
center of the universe, and everything
circled (orbited) the Earth. Any other
theory would be considered blasphemy,
with the result being jailed or put to death.
Q: Did Copernicus ever publish his work?
A: YES, Copernicus finally published his
works/theories on May 24, 1543. Ironically,
this was also the day he died.
4.3 Galileo on Motion
Galileo Galilei (1564-1642)
Galileo supported the theories of
Copernicus. As a result, he was put
on house arrest. He was permitted to
carry out experiments but not publish
any of his findings.
Galileo’s greatest contribution to
physics was his idea that a force is
not required to keep an object
moving.
Force =
Any push or pull
Friction =
The force that acts between materials
that touch as they move past each other.
Even surfaces that are very smooth have
some amount of friction between them.
Nothing is completely frictionless.
If friction did not exist, a moving object
would never stop moving and our ever day
lives would be completely different.
A ball rolling down an incline will increase
speed.
SKETCH
Q: Why?
A: Because gravity is constantly pulling the
ball downward, towards the center of the
Earth.
A ball rolling up an incline will decrease
speed and eventually stop.
SKETCH
Q: Why?
A: Because gravity is constantly pulling the
ball downward, towards the center of the
Earth.
A ball rolling on a very smooth surface
(bowling ball/alley) will roll nearly at a
constant speed for a very long time.
SKETCH
Q: Why will the ball eventually slow down?
A: Gravity is pulling only in a downward
direction,  some other force (FRICTION)
is acting between the two surfaces and
friction in the form of air resistance is
causing the ball to slow down and
eventually stop.
Therefore, if the ball were set in
motion on a perfectly flat surface and
there were NO forces of friction,
including air resistance, the ball would
continue to roll forever. NO FORCE
would be required to keep the ball
rolling.
SKETCH, dual incline
If the ball is released at point A it will roll
down the ramp and up the other ramp to
position B which is at an equal height to
point A, were the ball was originally
released, NEGLECTING FRICTION.
With friction, the smoother the surfaces
are the closer the ball will roll to the height
at which it was released.
SKETCH Incline 3 & 4
Incline 4 is at a lesser angle than 3 but is a
longer ramp.
Q: What will happen when the ball is
released at point A, neglecting friction?
A: The ball will reach the same height at
which it was released on the opposite
ramp.
With friction, the smoother the surface the
closer the ball will reach point B.
Galileo stated it was the “natural”
tendency for objects that are moving
to continue to move, resisting a
change in the object’s motion.
Inertia=
The resistance to a change in an
object’s motion.
Galileo’s findings about motion and
his concept of inertia, discredited
Aristotle’s theory of motion.
4.4 Newton’s Law of Inertia
Sir Isaac Newton
(1642-1727) was
born on
Christmas day
the same year
Galileo died.
Newton developed the Three Laws of
Motion that still hold true today. His three
laws completely changed the beliefs of
science for the previous 2000 years.
Newton’s First Law (aka: Law of Inertia)
states that an object at rest will remain at
rest and an object in motion will remain in
motion, with the same speed and
direction, until some net force acts on the
object. In other words, the object wants to
keep on doing what it is already doing.
SKETCH
If a ball is thrown horizontally with a velocity of
40m/s it would travel in a straight line for ever at
40m/s until some outside/net force acted on the
ball.
Q: What force(s) are responsible for the ball to
travel on its downward curved path and
eventually hit the ground?
A: Gravity will cause the ball the ball to fall in a
vertical direction only. The frictional force of air
resistance would cause the ball to slow down in
both the horizontal and vertical direction.
Q: If there were no gravity, what would
happen to the ball when it was thrown?
A: The ball would travel in a perfectly
straight line, however, air resistance
would eventually cause the ball to slow
down and stop.
If you slid a
hockey puck on
the sidewalk it
would slow down
and stop rather
quickly. If you
slid a hockey
puck across
smooth ice it
would slide a lot
farther.
Q: WHY?
A: Because there is a difference in the
force of friction between the two different
surfaces.
Friction =
The force that acts to resist (oppose) the
relative motion, or attempted motion, of
objects, materials, or surfaces that are in
contact with each other.
Q: What would happen if you threw a
baseball from the space station in “outer
space” ?
A: The ball would travel in a perfectly
straight line at the same speed forever.
Because of the ball’s inertia and lack of
any net force acting on the ball.
4.5 Mass-A Measure of Inertia
Inertia =
The property of an object to resist a
change in its motion.
The amount of inertia an object has
depends on the amount of mass it has,
more mass = more inertia.
Mass =
The amount of matter an object has.
Mass is measured in kilograms (kg).
Volume =
The measurement of the amount of space
an object takes up. Measured in liters (L).
Weight =
The measure of the force of gravity on an
object’s mass.
Q: If you were to shake a bowling ball
back and forth on Earth, on the moon,
or in outer space would you notice
any difference between any of the
locations?
EXPLAIN, why/why not.
An object’s mass can never change
no matter were it is, on Earth, the
moon or in outer space (zero
gravity/weightlessness). The mass
would be the same every where.
Therefore the object would have the
same amount of inertia whether the
object is on Earth, the moon or in
outer space.
Therefore, if you were to shake a
bowling ball on Earth, it would
take the same amount of force to
shake the bowling ball on the
moon or in outer space because
the bowling ball has the same
amount of mass/inertia in all three
locations.
An object’s weight
depends on were the
object is located. A
100 lb cheerleader
would weigh only
16.67 lbs on the
moon and nothing in
space.
Mass and weight are proportional to each
other, the more mass an object has the
more weight it will have provided there is a
gravitational force acting on it.
Q: Does a 20kg bowling ball have twice
the inertia, twice the mass, twice the
weight and twice the volume as a 10kg
bowling ball in the exact same location?
A: inertia = yes
weight = yes
mass = yes
volume = yes
Q: Does a 40kg bag of potatoes have
twice the inertia, twice the mass, twice the
weight and twice the volume as a 20kg
bowling ball when weighed in the same
location?
A: inertia = Yes
mass = Yes
weight = No
volume = No
USA standard of mass =
Slug ( = 32.2lbs)
The rest of world standard unit of mass =
kilogram (kg)
USA standard unit of weight =
Pound (lb)
The rest of world standard unit of weight =
Newton (N)
Common Conversions Used
1kg = 2.21lbs = 9.8N
1lb = 0.45kg = 4.45N
Example - 1
7kg of wood = _____ lbs
7kg
1
2.21lbs
X
1kg
= 15.47lbs
Example - 2
What is the mass (metric) of 23lbs of
apples?
23lbs
1
X
1kg
2.21lbs = 10.4kg
Example - 3
What is the weight (metric) of 17kg of
blueberries?
17kg
9.8N
1
X 1kg
= 166.6N
4.6 Net Force
Net Force =
The combination (sum) of all the forces
acting on an object.
If the net force acting on an object is
zero, the object will keep on doing what it
is already doing. If it is at rest, it will
remain at rest, if it is in motion, it will
remain in motion at the same speed and
direction until some net force acts on it.
If an object does have a net
force acting on it, the object will
accelerate in the direction of the
net force.
Net Force = 0
Net Force = some value (x)
Various examples of Net force = 0
and net force = some value (x)
4.7 Equilibrium
Draw a book laying on a desk.
Q: What forces are acting on a book laying
on a desk?
The support force or force of the table
is called the normal force. The normal
force (FN) is the upward force that
opposes gravity. The normal force is
drawn perpendicular to the surface of
the object. Therefore, on a horizontal
surface the FN would be = to the force
of gravity acting on the object.
FBD =
Free Body Diagram
How is an FBD drawn?
An object is represented by a dot with all
the forces on it. The vector forces are
drawn with their tails touching the dot and
their tips radiating from the dot in the
proper directions.
EXAMPLES:
Equilibrium =
When the net force acting on an object = 0
When equilibrium is reached there
can be NO CHANGE in an object’s
motion because there are NO net
forces acting on it.
FBD
EXAMPLES
A rock thrown in space.
A rock thrown in this room.
A penny resting on a desk.
A penny being pushed at a constant
velocity across a desk.
A penny being pushed at a constant
acceleration a cross a desk.
A penny that was pushed on a desk the
instant it leaves the top of the desk.
Text page 52 Figure 4.12, example
of a girl hanging on a trapeze bar.
Draw sketch
The tension in each arm/rope will be equal
to exactly ½ the weight of the girl.
What is the force in the girl’s arm if the girl
holds on with one arm?
Examples with spring scales & masses.
Use single and double scales.
PRE SECTION 4.8
DEMONSTRATION:
with possible bonus points
4.8 Vector Addition of Forces
Refer to transparencies and/or sketches
on board with regard to changing forces in
ropes/chains as the angles between them
change.