Transcript Forces

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Forces
Dynamics
FORCEMAN
Construct an explanation based on
Goals experimental evidence to support the
claims presented in Newton’s three
laws of motion.
Explain
Relate falling objects to gravitational
the
difference force
Analyze and interpret data to identify
in mass
the relationship between mass and
and
gravitational force for falling objects.
weight.
Apply Newton’s three laws to everyday
situations by explaining the following:
-Inertia
-Relationship between force, mass and
acceleration
-Equal and opposite forces
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What causes things to move?
• What is a force?
– A push or a pull
that one body
exerts on another.
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•Balanced
No change in motion
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Question: Can there be balanced forces on a object and
the object still have motion?
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Unbalanced
• If the forces acting on a body are
"unbalanced" this means that there is a "net"
or "resultant" force.
• The object will do one of the following:
1.Speed Up
2.Slow Down
3.Change the Direction of its Motion
• In other words….
• The Body will Accelerate
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Questions: How do we measure force?
Newtons
• SI unit of force is the Newton
(N).
• Defined as the amount of force
that when acting on a 1 kg object
produces an acceleration of
1m/s2.
• Therefore, 1N = 1 kg • m
s2
What will happen?
1. Abruptly stopping a cart with a suitcase on top….
2. You set your coffee cup on the dash and then
make a sharp left turn…..
3. You throw a tennis ball in outer space.
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Newton’s First Law
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Newton’s First Law
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Newton’s First Law
• An object at rest stays at rest, and
an object in motion stays in motion
unless acted upon by a net external
force.
• ?What do you mean by “Net Force”?
• Net means: Final Sum, so net external
force is the sum of all the forces
acting on the object.
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Newton’s First Law
•Newton’s First Law is also called
the Law of Inertia.
•
Inertia is the tendency of
an object to resist any
change in its motion.
Question: Does an object at rest still have inertia?
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Why does the ladder
continue to move?
Inertia!
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Why does the person continue to move?
Inertia!
Clip
Lab Activity
Question: How will the different balls affect the distance the
block moves? Be able to explain your answer…
14 Newton’s
Second Law
• The acceleration of an object is
directly proportional to the net
external force acting on the object
and inversely proportional to the
object’s mass.
•F=ma
• Clip
Newton's 2nd Law of Motion
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Newton's 2nd Law of Motion:
•Force = mass x acceleration
F = m x a



Mass is in kg
2
Acceleration is in m/s
2
Force is in kg x m/ s (or N)
Let’s do a sample problem…next slide 
Force Problems:
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What force is needed to give an
object with a mass of 25.0 kg an
acceleration of 15.0 m/s2 ?
Given:
m = 25.0 kg
Formula
F = ma
Unknown:
F=?
a = 15.0 m/sec²
F = (25 kg)(15 m/sec²)
F = 375 kg•m/sec² = 375 N
34 Newton’s Third Law of Motion
…to
every action force
there is an equal and
opposite reaction force
NewtoN’s
Third Law
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ActionReaction
forces act on
different
objects
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Hockey Video Clip
Newton’s 3 laws
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Forces
that
oppose
motion
Friction
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Forces that
oppose motion
Friction
Song 1
Friction
Song 2
Forces that
oppose motion
-Air resistance is the force
air exerts on a moving object
•
•
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Friction
What Causes Friction?
Microscopic, electrostatic interactions
between contacting surfaces
3 types of Friction
1. Static Friction
2. Sliding Friction
3. Rolling Friction
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Types of Friction
 Types of friction:
1. Static – force required
to overcome inertia of a
stationary object
Static frictionoccurs between
the surfaces of
2 objects that
touch but do
not move
against each
other. Static
friction must be
overcome for
one of the
objects to
move.
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Static Friction (FS)
• Not enough static friction
23Types
of Friction (cont’.)
2. Sliding – force required to keep an object
sliding at a constant speed
3. Rolling – force
required to keep an
object
rolling
at
a
To decrease friction, surfaces can be lubricated
with a liquid suchconstant
as oil.
speed
Friction Song
FRICTION
LAB
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CLIP
The amount of force
required to overcome
static friction is always
greater than the force
required to overcome
sliding or rolling
friction.
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GRAVITY!!
• The force of gravity is an attractive force
between objects.
• The amount of GRAVITATIONAL FORCE
between objects depends upon their:
• 1- Masses (greater mass means more gravity)
• 2- Distance apart (greater distance means
less gravity)
*All objects fall toward the earth at the same rate
of acceleration, regardless of their masses!
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Calculating the Gravitational Force
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Gravitation
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Weightlessness
Gravity in
Space
Moon
Moon Hammer & Feather
29Free fall and Terminal Velocity
• The opposing force of air resistance
increases with speed.
• Objects accelerate towards the Earth
until the force of gravity is cancelled or
balanced by this air resistance.
• This is the highest speed an object can
reach.
• Different objects have different terminal
velocities.
• Practice Problems
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Weight and Gravity
W =m x g
BECAUSE IT’S A FORCE!
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Weight
• When the mass of an
object and the
acceleration due to
gravity are known, the
weight of an object can
be calculated.
Ex. Mass = 100kg
Acc.= 9.81 m/s2
Weight = 981 N
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Example Problem
• What is your weight if your mass 4.52
kilograms?
Given:
Unknown:
m = 4.52 kg
W=?
g = a = 9.8 m/sec²
Formula: W = mg
W = (4.52kg)(9.8 m/sec²)
W = 44.296 kg•m/sec² = 44.296 N
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Projectile Motion
Projectiles follow a curved path
because
of the Earth’s gravitational
pull.
They have 2 types of motion that are
independent of each other:
1. Horizontal motion
2. Vertical motion
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Projectile Motion
Which will hit the ground first?
A ball that is dropped straight
down or a ball that is “shot”
out horizontally?
* An object launched horizontally
will land on the ground at the
same time as an object simply
dropped from the same height!!!!
Another
Example
of
Projectile
Motion
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40 Circular Motion
•
Roller Coaster Clip
An object moving in a
circular motion at the same
speed is accelerating toward
the center because its
direction is constantly
changing.
Centripetal acceleration is
acceleration toward the
center of a curved path.
'Centripetal' comes from the Latin word for 'center seeking'
Centrifugal 'force' is really a function of the
inertia of the object being pushed into a circle.
It is not really a force at all, it is simply the
tendency of an object to go in a straight line.
only
centripetal is
the real
force
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Circular Motion cont.’
• Centripetal
force is the
force that
causes a
moving object
to move in a
curved path
Without a
centripetal
force, an object
in motion
continues along
a straight-line
path.
With a
centripetal
force, an object
in motion will
be accelerated
and change its
direction.
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How do rollercoasters work!?!
Amusement Park Forces
Review of Newton’s 3 laws
• http://www.teachertube.com/video/threelaws-of-motion-539