Transcript Force and Motion

Force and Motion
7th Gr. Science
Motion
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A change in position of an object compared to its
reference point.
More simply, motion is moving from point A to point B
Reference Point
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When observing motion, the object that stays in place is
called the reference point.
Identify reference points from the pictures below.
Speed / Velocity
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Speed is the rate at which an object moves in a
given amount of time.
Speed is a measure of the Rate of motion.
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Total Distance Traveled = Average Speed
Total Time Traveled
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Velocity: speed in a particular direction
Speed
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Maurice ran a 100 m dash in 9.73
seconds. What was his average speed?
100m / 9.73 sec = ?
Acceleration
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Acceleration is a change
in speed or direction of an
object.
Acceleration = A
A = change in velocity
time
Velocity = V = Distance
Time
Vf - Vi
t
Types of Motion
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Free Fall
Circular Motion
Projectile Motion
Free Fall Motion
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An object in free fall
will accelerate in a
straight line at a rate
of 9.8m/s/s
This is the rate in the
absence of air
resistance.
Free Fall
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After falling for 1 second
the little man will be going
9.8 m/s
After falling for 2 seconds
the little man will be going
19.6 m/s
How fast will the elephant
be going after
1 sec.? After 3 sec.?
Free Fall - Air Resistance
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Air Resistance will
slow down the
acceleration of
gravity.
That’s why
parachutes work.
That’s why a feather
falls slower than a
ball.
Free Fall - Air Resistance
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After accelerating to
about 200 km/h or 55.6
m/s . This sky diver will
go no faster because air
resistance will balance
acceleration.
How many seconds will it
take for him to reach
maximum speed?
Circular Motion - Centripetal
Force
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Moving objects want
to move in a straight
line.
To move in a circle,
the object must be
held to the center by
a force.
This is centripetal
force.
Circular Motion - Centrifugal
Force
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Circular Motion
creates a force
moving out in the
opposite direction as
centripetal force.
This is centrifugal
force
Circular Motion
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If the centripetal force
is removed the object
will begin to move in a
line.
Hammer Throw and
Discus work this way.
Circular Motion
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Which force keeps
cars on the track even
when they are upside
down?
Which force is
provided by the white
steel track?
Projectile Motion
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An object that moves
horizontally but also is
affected by the
downward pull of
gravity is in projectile
motion.
Force
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Any push or any pull
Only unbalanced
forces are able to
accelerate or
change the motion
of objects.
Force gives an object energy to move, stop
moving, or change direction. Blowing your nose,
riding your bike, or just breathing all require force
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A flag is blown by the
force of the wind.
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A jet is pushed
forward by the force
of its engines.
Friction
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Friction is a force that opposes motion. Friction
acts in a direction opposite to the direction of
motion of the object.
Balanced Forces
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Forces occur in pairs and can be either
balanced or unbalanced. Balanced forces DO
NOT cause a change in motion. They are equal
in size and opposite in direction.
Balanced Force-No Motion
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If the force in each direction is equal, they cancel
each other out and the resulting force is zero.
Therefore, there is no change in motion.
Unbalanced Forces
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Unbalanced forces are not equal and opposite.
Unbalanced forces will cause motion.
Unbalanced Forces
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If opposing forces are NOT equal, then the force
will be in the direction of the greater force.
Who wins the arm wrestling contest?
Unbalanced Forces
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Who wins the tug of war contest?
Unbalanced forces
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Forces can be applied in the same direction
QuickTime™ and a
TIFF (Uncompress ed) dec ompres sor
are needed to s ee this pic ture.
Force Problems
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Two important things to remember when working with
combined forces
Forces in the same direction combine by addition
Forces in Opposite directions combine by subtraction
Newtons
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A Newton (N) is a unit to measure force.
The force of 1 Newton can accelerate a 1Kg
object to increase its speed 1 meter per second
for each second.
One Newton is the amount of force required to
give a 1-kg mass an acceleration of 1 m/s2
Force and Direction
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Force is a quantity that
has both magnitude
(Newtons) and direction.
This determines if an
object will move, and in
which direction.
The book in the picture is
sliding to the right. It will
stop when the force to the
right is less than 5N.
Force Problems
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Two tugboats are moving a barge.
Tugboat A exerts a force of 3000 newtons
on the barge. Tugboat B exerts a force of
5000 newtons in the same direction. What
is the combined force on the barge?
Force Problems
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Draw arrows showing the individual and
combined forces of the tugboats in #1.
Force Problems
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Now suppose that Tugboat A exerts a
force of 2000 newtons on the barge and
Tugboat B exerts a force of 4000 newtons
in the opposite direction. What is the
combined force on the barge?
Force Problems
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Draw arrows showing the individual and
combined forces of the tugboats in #3.
Force Problems
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Could there ever be a case when Tugboat
A and Tugboat B are both exerting a force
on the barge but the barge doesn't move?
Draw arrows showing the individual and
combined forces in such a situation.
Sir Isaac Newton
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Isaac Newton
1643-1727
Newton was a great
mathematician and
scientist. He took
observable natural
phenomena and found
mathematical
relationships
Newton’s Principia
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This book is widely
recognized as the
greatest scientific book
ever written. He provides
science with a unified
way of looking at the
natural world. He lays
out the mathematical
relationship to the
observable physical
world.
Big Idea
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Gravity
The attractive force of
two masses like the
earth and an object
on its surface.
Gravity makes a
falling object
accelerate at 9.8m
s2
Another Big Idea
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White light can be
divided into colors
with a prism. The
colors always have a
specific sequence.
ROY G. BIV
Another Big Idea
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The Planets are held
in their revolutions by
gravity. They are not
sucked into the sun
because of balanced
forces
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Centrifugal Force = Gravity
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We won’t fly into space and we
won’t crash into the sun.
Newton’s Laws of Motion
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Newton described
three laws of motion
Inertia
Force = mass X acc.
Reaction
1st Law (Inertia)
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And object at rest will
stay at rest.
Unless it is acted on
by an unbalanced
force
1st Law (continued)
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An object in motion
will continue moving
in a straight line at a
constant speed.
Unless it is acted on
by an unbalanced
force.
2nd Law
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If you apply force to an object it will accelerate or change
its velocity in the same direction as the force.
Force = Mass X Acceleration
F = ma
2nd Law
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If you push twice as hard,
the object will accelerate
twice as much.
If the object has twice as
much mass, it
accelerates half as much.
A speeding bullet and a
slow train can each kill
you for different reasons.
Explain.
3rd Law
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For every reaction,
there is an equal and
opposite reaction
Force back - move
forward
Force down - move
up
If you push
something, it pushes
back