Transcript PPT File

Using the
FORCE
Using the
FORCE
Inertia
“The vis insita, or innate force of matter is a power of
resisting, by which every body, as much as in it lies, endeavors
to preserve in its present state, whether it be of rest, or of
moving uniformly forward in a straight line.”
In English, please!
Inertia is the resistance of an object to a
change in its state of motion.
More inertia
A bowling ball has more inertia
than a soccer ball.
Don’t believe me?
Kick the bowling ball, then kick
the soccer ball.
More mass = More inertia
Less inertia
Force?
A force is a push or pull
upon an object resulting
from the object's
interaction with another
object.
Forces only exist as a
result of an interaction.
Force has both
direction and
magnitude
Balanced vs. Unbalanced forces
A soccer ball sits
alone on the field,
quiet and still…
Ground pushing up
Forces are
balanced so
ball does
not move
Gravity pulling down
Balanced vs. Unbalanced forces
Suddenly….
An unbalanced
force is exerted on
the ball in the
direction of the
arrow, causing it
to move in that
direction until…
Balanced vs. Unbalanced forces
…its motion is
stopped by an
interaction with
another force!
Once again, the
forces are
balanced and all
is well in the
universe.
Balanced Forces
If the forces
acting on an
object are
balanced, the
object is at rest.
We say the
“Net Force” = 0
Net force = sum of all forces acting on an object
Unbalanced Forces
If the forces acting
on an object become
unbalanced,
Force applied in this direction
the object moves in
the direction of the
greatest net force.
In this case, the “Net Force” = 5 Newtons to the left
Mmmm,
Oh,
sorry,
wrong
Newtons
Newtons!
The newton is a unit of force that is defined as the amount
of force required to accelerate a mass of one kilogram at
a rate of one meter per second per second.
Algebraically: 1 N = 1
kg ● m
s2
Diagramming Forces
If you were paying attention, you would have
noticed that forces were illustrated using arrows.
The size and direction of the arrow represents the
relative strength and direction of the force.
Balance forces - arrows equal in length
Unbalanced forces - arrows not equal,
Movement is to the right
Calculating Net Forces
To calculate net forces,
add all forces exerted
on the object.
In this example, the net
force up the ramp is
greater than gravity,
and the piano is moved
forward and up the
ramp into the truck.
For this example
Net force = (force 1 + force 2) – force 3
3
Calculating Net Forces
1
For this example
Net force = force 1 - force 2
The force due to gravity in this example is negligible
2
What other
forces can you
identify in this
picture?
Calculating Net Forces
1
2
The dogs exert a
force on the toy
as they bite
down on it
Feeling the Force
Which ram seems to
have the advantage?
Why?
Newton’s First Law of Motion
“Every body perseveres in its state of being at rest or of moving
uniformly straight forward, except insofar as it is compelled to change
its state by force impressed.”
“An object at rest will remain at rest unless acted on by
an unbalanced force. An object in motion continues in
motion with the same speed and in the same direction
unless acted upon by an unbalanced force.”
AKA - The Law of Inertia
Newton’s First Law of Motion
Said another way, Newton’s First Law of Motion states
that a moving object moves in a straight line with
constant speed unless a force acts on it.
 An object will not start moving unless a force acts on it
 An object will not stop moving unless a force acts on it
 An object will not change speed unless a force acts on it
 An object will not change direction unless a force acts on it
Friction
A force that opposes motion between two
surfaces that are touching
Even surfaces that seem to be extremely smooth
have microscopic hills and valleys, and when two
surfaces are in contact, the hills and valleys of
one stick to the hills and valleys of the other,
causing friction to resist the force of motion
Friction
Static friction
The friction that exists between two objects in contact
Friction
Sliding friction
When force is applied that is strong enough to break
the bonds of static friction and movement starts,
sliding friction acts to slow that object down
Friction
Rolling friction
The resistance
that occurs when
a round object
such as a ball or
tire rolls on a flat
surface
Friction
Fluid friction
Fluid friction
occurs when a
solid object
travels through
a liquid or gas.
Wake turbulence and wingtip
vortices from jet airliner
passing through a layer of
clouds, showing the fluid
nature of air
Friction
Increasing Force
Force required to
overcome friction
Static friction
Movement starts
Sliding friction
Rolling friction
Fluid friction
Friction always acts
in the opposite
direction of
movement, and
always acts to slow
object down.
Force and Newton’s Laws note-taking sheet
Section 1
A. Force
1. net
2. balanced
3. unbalanced
B. first law
C. Friction
1. slows down
2. Static
3. Sliding
4. Rolling
Newton’s Second Law of Motion
Force equals mass times acceleration (F = ma)
The net force on an object is equal to the mass (m) of
the object multiplied by its acceleration (a)
Units of Force
Mass = kilograms (kg)
Acceleration = (m/s2)
Therefore….
F = kg ● m/s2
Force is measured in
(Newtons, that is)
What are the forces acting on this
bicycle and rider, coasting along at 25
km/h on this flat, wet, Alaskan road?
A = force of gravity
B = force of the road
C = rolling frictional force
D = force of momentum*
E = fluid frictional force (air or wind resistance)
A
E
D
C
B
*Momentum = mass (kg) ● velocity (m/s)
A = force of gravity
B = force of the road
C = rolling frictional force
D = force of momentum*
E = fluid frictional force (air or wind resistance)
Is the net force
balanced?
A
E
D
Write out the
formula…
C
B
Net force = (A-B) + D - (E+C)
Gravity
Gravity is a force that always attracts or pulls objects toward each other
without direct contact or impact.
Gravitational attraction depends on the mass of the two objects and the
distance they are apart.
Objects on Earth are pulled toward the center of Earth.
The force of gravity, like all other forces, can cause changes in the speed
of objects. As an object falls, its speed will continually increase as Earth’s
gravity continually pulls it downward. When air resistance is ignored, all
objects will speed up at the same rate as they fall.
Gravity can also cause an object that is thrown into the air to change its
upward motion, slow down, and fall back toward Earth’s surface.
The pull of Earth’s gravity keeps the Moon in orbit; the moon is constantly
changing direction because of gravity.
The acceleration due to gravity is 9.8 m/sec2
When air resistance is ignored, all objects will
speed up at the same rate as they fall.
When the ball is dropped off the cliff, the ball
will accelerate by 9.8 meters/second each
second.
At the end of 1 second, the ball is traveling at
9.8 m/sec
At the end of 2 seconds, the ball’s velocity is
19.6 m/sec (9.8 m/sec X 2)
Etc…..
Gravity
Factoring in air resistance (fluid friction) will
cause an object to reach a limit to its
acceleration. This concept is known as terminal
velocity. For an average skydiver, terminal
velocity is approximately 195 km/sec (55 m/sec).
Terminal velocity is reached when the net
force between gravity ( Fg ) and fluid friction,
or drag ( Fd ) reaches zero.
Self-check questions – pg. 484
1) Think about our happy cyclist from earlier in the
discussion
2) Your weight will decrease as you get farther from Earth
because the gravitational pull of Earth is decreasing
3) Greater the speed, greater the air resistance, up to the
point of terminal velocity
4) Net force will push diagonally at some angle on the car
and to the right
5) The motion of the box will not change until the forces
become unbalanced
Self-check questions
6)
F = ma
F = 1500 kg X 2.0 m/s2 = 3,000 N
7)
F = ma
m = F/a = 300 N / 1500 m/s2 = 0.2 kg
or (300 kg ● m/s2) / 1500 m/s2 = 0.2 kg
Force and Newton’s Laws note-taking sheet
Section 2
A. second law
B. Gravity; weight
C. calculate
D. centripetal
E. terminal velocity
F. unbalanced
Newton’s Third Law of Motion
Lex III: Actioni contrariam semper et æqualem esse reactionem: sive
corporum duorum actiones in se mutuo semper esse æquales et in
partes contrarias dirigi.
Whenever a particle (A) exerts a force on another particle (B),
(B) simultaneously exerts a force on (A) with the same
magnitude in the opposite direction.
This law is often simplified into the sentence,
"To every action
there is an equal and opposite
reaction."
Third Law in Action
Third Law in
Action?
Think about
Sabine’s actions
in this photo.
Do they
represent
Newton’s Third
Law of Motion?
If yes, explain.
Sabine Lisinki,
2009 Family Circle Cup
Champion
If no, which
law(s) are
illustrated?
Quick Review
A paratrooper has a mass of 100 kg. He jumps
from his C-17 at an altitude of 10,000 feet and
accelerates toward the ground at 9.8 m/sec2.
What is the force on the paratrooper? Explain
how you got your answer, and which of
Newton’s laws of motion is involved.
A roller coaster reaches the top of the big hill traveling
at a speed of 15 m/sec. When it reaches the bottom
of the hill 3 seconds later it is traveling at 27 m/sec.
Calculate the average acceleration. Explain how you
got your answer.
Quick Review
A paratrooper has a mass of 100 kg. He jumps from his C-17
at an altitude of 10,000 feet and accelerates toward the
ground at 9.8 m/sec2. What is the force on the paratrooper?
Explain how you got your answer, and which of Newton’s
laws of motion is involved.
F = ma
F = 100 kg ● 9.8 m/sec2
F = 980 kg ● m/sec2
(1 Newton = 1 kg ● m/sec2)
Answer = 980 Newtons (or 980 N)
This is Newton’s Second Law of Motion
Quick Review
A roller coaster reaches the top of the big hill traveling at a
speed of 15 m/sec. When it reaches the bottom of the hill
3 seconds later it is traveling at 27 m/sec. Calculate the average
acceleration. Explain how you got your answer.
Average acceleration = (Final velocity – Initial velocity) / Time
= (27 m/sec – 15 m/sec) / 3 seconds
= (12 m/sec) / 3 seconds
= 4 m/sec2
2
1
Look carefully at
these illustrations.
Decide which of
Newton’s laws is
illustrated in each
example. Explain how
the situation
illustrates the law you
chose.
3
5
6
4
2
1
First Law – the football remains
at rest b/c net force = 0.
Third Law – the Boat moves back
when the boy moves forward.
3
4
First Law – the car stops but passengers
Inertia keeps them moving forward.
5
Second Law– same force on large ball
produces less acceleration than small ball.
Second Law – the golf ball moves with
more force with larger acceleration.
6
First Law – bike continues to coast at
constant speed w/o unbalanced force.
Sir Isaac Newton
Newton was an English physicist,
mathematician, astronomer, natural
philosopher, alchemist, and
theologian and one of the most
influential men in human history.
Among many other things, he is
credited with describing the
Universal Law of Gravitation and
the Three Laws of Motion.
A 2005 survey of the British Royal
Society ranked Newton ahead of
even Einstein as having a greater
influence on the history of science.
January 4th, 1643 – March 31st, 1727
Philosophiæ Naturalis Principia Mathematica
Latin for "mathematical principles of natural philosophy"
A three-volume work by Isaac Newton
published on 5 July 1687 that contains the
statement of Newton's laws of motion forming
the foundation of classical mechanics, as well
as his law of universal gravitation and a
derivation of Kepler's laws for the motion of
the planets (which were first obtained
empirically).
The Principia is widely regarded
as one of the most important
scientific works ever written.
Newton's law of universal gravitation
The physical law describing the gravitational attraction
between bodies with mass
It states the following:
Every point mass attracts every other point mass by a force pointing along the
line intersecting both points. The force is proportional to the product of the two
masses and inversely proportional to the square of the distance between the
point masses:
m1m 2
F1 = F2 = G
r2
where:
• F is the magnitude of the gravitational force
between the two point masses,
• G is the gravitational constant,
• m1 is the mass of the first point mass,
• m2 is the mass of the second point mass,
• r is the distance between the two point masses.