Transcript Gravity in Motion

Gravity in Motion
Section 6-1
Pages 150 - 156
In the beginning….
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According to the Greek
philosopher Aristotle, an object’s
mass affects how fast it will fall
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He said that objects with larger
masses will fall faster
Aristotle never actually tested his
theory
Italian scientist Galileo argued that the
mass of an object des not affect the
time it takes to fall to the ground
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He proved this by dropping two
different massed canon balls from the
Leaning Tower of Pisa and they landed
at the same time
Gravity and Acceleration
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The reason the two cannon
balls hit the ground at the same
time is because acceleration
due to gravity is the same for all
objects
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As we’ve talked about before,
acceleration depends on mass and
force
A heavier object experiences
greater gravitational force but is
harder to accelerate due to its
increased mass
The extra mass of the heavier
object balances the additional
gravitational force
Acceleration due to gravity
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Remember that
acceleration is the
rate at which velocity
changes over time
All objects accelerate
toward Earth at a rate
of 9.8 m/s2
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This means that for
every second an object
falls toward the Earth,
its velocity increases by
9.8 m/s
Calculating change in velocity
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Δv = g x t
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Δ (delta) stands for change in
Δv = change in velocity
g = acceleration due to Gravity (always 9.8 m/s2)
t = time the object takes to fall (in seconds)
Do Math focus on page 151 as a class
Air resistance
Two pieces of paper, one flat
and one crumpled
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They will not fall at the same rate
This is because of air resistance
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The force that opposes motion of objects in the air
Its pushes up against the object, going against the
force of gravity
The amount of air resistance depends on the size,
shape and speed of the object
 The more surface area, the more air resistance
 The more mass, the less air resistance
 The faster the object moves, the more air resistance
We have reached Terminal Velocity
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Objects do not continue to accelerate
(increase their velocity) forever until they hit
the ground
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This is because air resistance continues to
increase as speed increases
Air resistance increases until it is equal to the
force of gravity
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Results in a net force of 0N
 Is there any further change in motion (increase in speed)
if net force is 0N?
 NO!!
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When net force is 0N, the object falls at a
constant velocity which is called its terminal
velocity
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Terminal means at the end
Terminal velocity is a good thing
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Think about hail storms
 If large chunks of hail did not reach a terminal velocity
(usually between 5 and 40 m/s), they could reach
velocities of up to 350 m/s!
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Terminal velocity is reached at a faster pace when
using a parachute – allows sky divers and airmen
to reach the ground at a safer speed
Free Falling
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When you ride the Free Fall at Six
Flags, you are not really in free fall
Free Fall occurs when gravity is
the only force acting on an object
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Therefore, the only place you can
experience free fall is in space where
there is no air
Why?
If there is air, there is air resistance,
which is a force
Weightless or not?
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Astronauts are said to be weightless
when they are in space and that is why
they float, but…
You can never truly be weightless
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Why not?
Everything creates a gravitational force so
there will always be something around you
that causes you to have weight
So why do astronauts float?
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They are in free fall as they orbit the Earth
Orbits – not the gum
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An object is orbiting when it is traveling around
another object in space
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It is caused when an object is moving forward, but it is also in
free fall (being pulled towards the Earth or another object due
to gravity)
The two forces combine to create a circular path of motion
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Remember that any object traveling in a circular path is
constantly changing direction
A force that causes circular motion is called a centripetal force
Aim High
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Projectile motion is the curved
path an object takes when it is
thrown near the Earth’s surface
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Two parts to projectile motion
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Horizontal motion
 Ie: Caused by the force of your hand
throwing the ball
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Vertical motion
 Caused by gravity
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When these two parts combine, they
cause a curved path of motion