Download force

Review Question
Define the term “force” in your
own words.
Force is a push or pull
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Vocabulary Words-Force

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Force
Static friction
Inertia
Free fall
Terminal Velocity
Projectile Motion
Friction
Kinetic Friction
Weight
Momentum
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DESCRIBE A SCENARIO
WHEN FRICTION IS
USEFUL.
FRICTION IS ESSENTIAL WHEN YOU
NEED TO STOP OR SLOW DOWN.
WHAT IS THE
RELATIONSHIP BETWEEN
FORCE AND MOTION?
Standard SPS8
Determine the relationship
among force, mass, and
motion.
b. Apply Newton’s three laws to
everyday situations.
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Newton’s Laws of
Motion
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Sir Isaac Newton

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1642-1727
Described
relationship
between motion and
force
Newton’s laws of
motion
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Newton’s 1st Law of Motion
An object at rest will remain at
rest and an object in motion
will remain in motion unless it
experiences a net force.
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Can you describe
some examples of
Newton’s 1st Law of
Motion?
http://www.physics.umd.edu/deptinfo/facilities/le
cdem/services/demos/demosc3/c3-02.mpg
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Inertia

The tendency of an object to
remain at rest or remain in
motion at a constant speed.

Inertia depends on mass.
(more mass = more inertia)
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What is necessary to
change the motion of an
object?

A Net Force must act on an
object to change its motion.
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Julie rides Acrophobia at Six Flags Over
Georgia. As the seats begin to fall 16
stories, she feels as if she is floating. Use
Newton’s 1st Law to describe what is
happening.
Julie was at rest at the top of the ride.
When the ride first starts falling, her body
wants to remain at the top so she feels like
she is floating.
Newton’s 2nd Law of Motion

The acceleration of an object is
directly proportional to the net
force acting on it;
and inversely proportional to its
mass.
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Another way of saying
it….

As mass increases,
acceleration decreases
(inversely proportional)

As force increases,
acceleration increases
(directly proportional)
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Units
F=mxa
kg x m/s2 = Newton
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Practice #1
Zookeepers use a stretcher to lift a 175 kg sedated
lion. If the upward acceleration of the lion and
stretcher is 0.657 m/s2, then what force is needed to
lift the lion?
G
m = 175 kg
a = 0.657 m/s2
UF=?
E
F = ma
S
F = (175 kg)(0.657 m/s2)
S
F = 115 kg x m/s2
115 N
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Practice #2

When a shot-putter exerts a force of 120.0 N on a
shot, the shot accelerates at a rate of 19.0 m/s2.
What is the mass of the shot?
G
E
F = 120.0 N
a = 19.0 m/s2
m=F/a
U
m=?
S
m = 120.0 N /(19.0 m/s2)
S
m = 6.32 kg
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Practice Problems
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Practice Problems
1. What net force is needed to accelerate a 1600 kg
automobile forward at 2.0 m/s2?
G
U
E
S
S
2. A baseball accelerates downward at 9.8 m/s2. If the
gravitational force is the only force acting on the
baseball and is 1.4 N, what is the baseball’s mass?
G
U
E
S
S
3. A sailboat and its crew have a combined mass of 65
kg. If a net force of 895 N is pushing the sailboat
forward, what is the sailboat’s acceleration?
G
U
E
S
S
4. The net forward force on the propeller of a 2.3 kg
model airplane is 7.0 N. What is the acceleration of the
plane?
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G
U
E
S
S
A.
C.
E.
1.
3.
3N
3N
10 N
3200 N
13.8 m/s2
B.
D.
F.
2.
4.
4N
No net force
8N
0.14 kg
3.0 m/s2
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
EQ: What determines
how much an object
speeds up or slows down
as it experiences a net
force?
Mass & Acceleration
SP1. Analyze the
relationship between force,
mass, gravity, and the
motion of objects.
If a Mack truck and a Volkswagen have a head on
collision, which vehicle will experience the
greater impact force?
a)
b)
c)
d)
The Mack truck
The Volkswagen
Both the same
…it depends on other factors
The answer will be
revealed shortly…
Newton’s 3rd Law of Motion
When an object exerts a force on a
second object, the second object exerts
an equal and opposite force back
onto the first object.
Also known as “action-reaction” forces
Action-Reaction Forces
• For each “action” force name the “reaction”
force
Action:
Action: Athlete
Ball hitspushes
bat. barbell upward.
Reaction: Barbell
Bat hits pushes
ball. athlete
downward.
Action: Air
Ballpushes
pushesballoon
pin forward.
surface
outward.
Reaction: Pin pushes ball backward.
Reaction: Balloon surface pushes air
inward.
In other words…
Forces come
in pairs!
Can you identify at least 5 pair of actionreaction forces in the picture?
Now can you answer the question?
If a Mack truck and a Volkswagen have a head on
collision, which vehicle will experience the greater
impact force?
*According
a)
b)
c)
d)
The Mack truck
The Volkswagen
Both the same
…it depends on other factors
to
Newton’s 3rd
Law, forces are
equal and
opposite.
WHAT’S THE BIG IDEA?
• Newton’s First Law
Inertia:
Objects will continue their motion unless
they experience a net force
• Newton’s Second Law
F=ma
Acceleration is directly proportional to force.
Acceleration is inversely proportional to mass.
• Newton’s Third Law
Action-Reaction force pairs
For every force generated, there is another force of
the same size acting in the opposite direction.
Summarize Newton’s Third Law:
Action Reaction
As mass
As force
acceleration _
acceleration _
If you drop a ball, it will fall towards Earth.
A) What force causes this?
B) B) Identify the action-reaction pair for this
scenario.
A) Gravity
B) Earth pulls ball downward. Ball pulls Earth
upward.
Why then do you not see the Earth move upward?
F=ma The earth has more mass, so it does
not accelerate as much as the ball.
Standard 8. Determine the relationships
among force, mass, and motion
c. Relate falling objects to gravitational
force
d. Explain the difference between mass and
weight
Gravity, Weight, & Mass
Mass
• The amount of matter in an object
• Weight depends on gravity, mass does not.
• The mass of an object is always the same,
no matter the location.
Gravity
• A force that pulls objects together
• The larger the objects, the larger the
force. (This is weight.)
• Objects fall towards the center of earth
because of gravity.
• In the absence of air
resistance, all objects
fall with a downward
acceleration of
9.8 m/s2 on Earth.
This is acceleration due
to gravity.
Weight
• The force on an object due to gravity
• To calculate weight:
Fw = mag
F = ma
Practice Problems
1.
Convert your weight in pounds to Newtons
using the conversion factor 9.8 N = 2.2 lbs.
=
Ex: 100 lbs. x 9.8 N
1
2.2 lbs.
980 N
2.2
= 445 N
Practice Problems
2.
Calculate your mass.
Ex: W = 445 N
g = 9.8 m/s2
m=?
m= W
g
= 445 N
9.8 m/s2
= 45.4 kg
Practice Problems
3.
How much would you weigh in Newtons on the
moon where the acceleration due to gravity is
1.6 m/s2?
G
U
E
S
S
m = 45.4 kg
g = 1.6 m/s2
W=?
W = mg
W = (45.4 kg)(1.6 m/s2)
W = 72.6 N
WHAT’S THE BIG IDEA?
• Weight vs Mass
•
•
•
•
•
The same in all locations Mass
Dependent on gravity Weight
Force Weight
Measured in kg Mass
Measured in N Weight
What is the difference
between mass and
weight?
 Mass is the
amount of matter
(kg)
 Weight is a force
(N)
As an astronaut travels from
the earth to space (where
weight
there is no gravity), her _________
mass
will change. Her ________
will
remain the same.
Air Resistance
• The force of air pushing up on a falling
object.
• Opposite of weight
Air Resistance
Weight
Factors Affecting Air Resistance
• Speed of falling
object
• Surface area of
falling object
Terminal Velocity
• As an object falls faster, air resistance
increases
• Eventually air resistance will equal objects
weight
• Object will stop accelerating and continue
to fall at a constant velocity (the terminal
velocity)
Mass vs. Weight Double Bubble
Depends on Gravity
Kilograms
Newtons
Measure of matter
Force
Changes in different locations
Same in all locations
W = mg
EQ: If an elephant
and a paper ball
are dropped from
the roof top at the
same time, which
will hit the ground
first?
They will hit the
ground at the
same time.
Describe the relationship between:
 Force and acceleration:
Directly Proportional
 Mass and acceleration:
Inversely Proportional
 What is the rate of acceleration due
to gravity?
9.8 m/s2
Weight and mass are directly proportional.
• As mass increases, weight also increases.
As mass decreases, weight also decreases.
)
(vf-vi)
a = (vf-vi)
t
a
t
F
F = ma
m a
W
W= mg
m
g
Fold your paper into 6
portions and write the key
concepts for each of the
following topics:
Weight
Mass
Friction
Newton’s 1st
Newton’s 2nd
Newton’s 3rd
What equations have we used so far this semester?

Velocity (using slope)

Velocity

Acceleration

Force

Weight

EQ: What are the appropriate
units for:
slope = (y2-y1) / (x2-x1)
v=d/t
a = (vf – vi) / t
F = ma
W = mg
Equations used so far . . .
d
v
F
t
m a
(vf-vi)
a
t
(y2-y1)
slope
(x2-x1)
W
m
g
What are the appropriate units for:
m/s
2
Acceleration m/s
2
N
Force kg m / s
2
Weight kg m / s
N
 Velocity



or
or
 What’s
next?
ENERGY
 Simple machine
 Compound machine
 Energy
 Potential energy
 Work
 Kinetic energy
 Power
 Mechanical energy
 Mechanical advantage
 Efficiency