9/10/2012
Remember IFSMU
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Force and Motion
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What is a force?
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A force is a push or a pull
I: Identify Variables
F: Formula
S: Substitute variables (#’s) into formula
M: Do the math
U: Add the correct units.
IF SMU (If you) do your math problems
this way you will get points
IF SMU don’t … you won’t
Example of net force
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Types of Forces
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Electrical forces (balloon
with hair)
You and your brother are working
together to move a box. You are both
moving in the same direction. You are
pulling at 30 N to the left and your
brother is pushing at 20 N to the left.
What is the net force?
Net Force answer
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Magnetic forces (magnet
pulling a paperclip)
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Force of Gravity (apple
falling from a tree and hits
the ground due to gravity)
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I: Force=30 N left, 20 N left
F: Add
S: Net Force= 30 N left + 20 N left
M: 50
U: 50 N to the left
Net force (the total amount
of push or pull acting on an
object) measured in
Newtons (N)
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9/10/2012
Net Force Example
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Newton’s Law of Gravity
You and your brother are fighting over
the TV remote. You are pulling at 10 N
to the left and he is pulling at 8 N to
the right.
Who wins?
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Main points on Gravity
TV Remote Net Force
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I: Force=10 N left, 8 N right
F: Subtract
S: 10 N left – 8 N right
M: 2
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U: 2 N left You Win!
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Main Points
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A force is a push or a pull
A force always acts in a certain
direction
Forces in nature include: electrical,
magnetic, and gravity.
Weight is a force.
Gravity
Force of attraction between all objects
in the universe
An object with a LARGER MASS will
have a LARGER FORCE of GRAVITY.
So…a smaller mass will have a smaller
force of gravity
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Isaac Newton discovered the law of gravity
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All objects in the universe move toward each
other because of the force of gravity between
them.
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The amount of gravitational force between two
objects depends on their mass.
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The force of gravity between two objects
decreases as the distance between them
increases.
Friction, Speed, and Velocity
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What is friction?
Friction is a force that opposes the motion
of an object
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9/10/2012
Types of Friction
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Sliding friction (book moving across a
desk)
Air resistance (object falling in air is
pushed by air resistance)
Rolling friction (wheel of bicycle and the
road)
Is friction helpful or not
helpful?
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Depends!
Helpful because on earth friction between the road
and bicycle wheel, your feet and the floor, cars,
and trains all keep in motion from friction…without
friction you would not be able to move the bike,
car, train, or walk on the road.
Speed
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What is speed?
Speed is the distance traveled per unit of
time
Speed has two parts: distance and time
Speed= distance/time
Motion is a change in position
Types of speed
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Instantaneous speed is
the speed at any given
instance or point in
time
Average speed =
total distance/total time
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Not helpful metal parts touching in a machine can
cause too much friction = heat and energy and
parts will break or overheat.
Main points of friction
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To stop moving object, a force must act
in the opposite direction
Friction is a force that opposes the
motion of an object
There are different types of friction
Friction makes motion possible
Friction can sometimes be a problem
Velocity
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What is velocity?
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Velocity describes the speed (distance and
time) and the direction of a moving object
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Moving from place to place involves
distance, time, and direction so it is
measured in velocity
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9/10/2012
Main points of speed and
velocity
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Motion is a change in position
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Average speed=
total distance/total time for the trip
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Speed at any instant is called instantaneous
speed
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Velocity describes the speed (distance and
time) and direction of a moving object
Newton’s Laws of Motion
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Newton’s three laws
of motion can be
used to explain the
movement of all
objects in the
universe.
What is Newton’s First Law of
Motion?
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Inertia: tendency of an object to stay at
rest or in motion
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An object will stay at rest unless acted
on by an unbalanced force. An object
in motion will stay in motion at the
same speed and in the same direction
unless acted on by an unbalanced force
Newton’s First Law of Motion
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If your car is traveling at 45 km/h then
you are traveling at 45 km/h
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If your car suddenly stops because of a
wall…you are still traveling at 45 km/h
until you are stopped by another force
(your seatbelt or possibly the ground).
What is Newton’s Second Law
of Motion?
Force= mass X Acceleration
The force (push or pull) of an object is
equal to its mass (amount of matter)
times its acceleration (change in
object’s velocity over time).
Formulas:
F=ma, a=f/m, m= f/a
Newton’s 2nd Law of Motion
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If you apply the same force on a small
stone as you do a boulder…the small
stone will accelerate much faster than the
boulder.
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9/10/2012
Math Problem:
f=ma, a=f/m, m=f/a
Units to Remember
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Force(F) measured in Newtons (N)
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Mass (m) measured in kilograms (kg)
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Acceleration (a) measured in
meters/seconds2 (m/s2)
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A bowling ball has a mass of 7.0 kg and
is dropped from a building at a force of
68.6 N. What is the acceleration of the
bowling ball?
Math Problem:
f=ma, a=f/m, m=f/a
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Math Problem:
f=ma, a=f/m, m=f/a
A marble has a mass of 0.05 kg and is
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dropped from a building at a force of
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0.49 N. What is the acceleration of the
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marble? (units for a=
m/s2)
Math Problem:
f=ma, a=f/m, m=f/a
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I: F=68.6 N, m= 7.0 kg, a=?
F: A=f/m
S: A=68.6 N/7.0 kg
M: A= 9.8
U: A= 9.8 m/s2
I: F=.49N, m= 0.05 kg, a= ?
F: a=f/m
S: a=.49 N/ 0.05 kg
M: a= 9.8
U: a= 9.8 m/s2
Why is the acceleration the same
for the bowling ball and marble?
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Because falling objects don’t accelerate through
their whole fall. Eventually, the force of air
resistance pushing up against the object equals
the force of gravity pulling down on the object.
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At this point the final or terminal speed is
reached.
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So a heavy object (bowling ball) and a light
object (marble) dropped from the same
height will hit the ground at the same time.
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9/10/2012
What is Newton’s Third Law of
Motion?
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Equal and Opposite
For every action, there is an equal but
opposite reaction
Newton’s 3rd law includes two parts:
the action force and the reaction force
All forces act in pairs
Example
Hitting a baseball:
bat exerts force on
ball (action) and
ball exerts equal
and opposite force
on bat (reaction)
Calvin and Hobbs
Interpretation
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