Transcript Gravity and free fall

Gravity, Air Resistance,
Terminal Velocity, and
Projectile Motion
A Whole Bunch of Ideas in one Show
Science Standard
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S8P3. Students will investigate
relationship between force, mass, and the
motion of objects.
a.
Determine the relationship between velocity
and acceleration.
Demonstrate the effect of balanced and
unbalanced forces on an object in terms of
gravity, inertia, and friction.
b.
Enduring Understanding
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Acceleration of an object is dependent
upon the mass of the object and the
size and direction of the unbalanced
force.
Essential Question
 How
does Gravity affect
Acceleration?
Some basics…
Draw a picture along with each fact to help you
remember it!
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Won’t
An object’s mass _______
affect the
acceleration of the object due to
gravity on earth.
opposite
Friction always acts in the _________
direction as motion.
surface area
The ___________
of an object will
affect the velocity of a free falling
object dropping towards earth.
Vocabulary to know
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The force that pulls an object towards
another object
When you throw something it is a projectile
When you drop something it is in a free fall
The friction involved here is air resistance
Universal LAW of gravity states that the
force of gravity acts on all objects in the
universe
What IS gravity?
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Gravity is NOT “the force that pulls
you down!”
Gravity attracts all objects with mass
inward towards other objects with
mass.
the Force of Gravity
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What is the force of gravity?
The force of gravity is NOT the same
as the acceleration DUE to gravity!
Free Fall
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Objects whose only acceleration is due
to gravity are in free fall.
Theoretically, objects in free fall
continue to accelerate as long as they
are falling.
All free falling objects accelerate
at the same rate regardless of
their mass. (9.8 m/s2)
Galileo's Famous
Experiment
The Acceleration of Gravity (g)
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Galileo demonstrated that g is the same for all
objects, regardless of their mass!
This was confirmed by the Apollo astronauts on the
Moon, where there is no air resistance.
© 2004 Pearson Education Inc., publishing as Addison-Wesley
Acceleration of Gravity
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Any object which is being
acted upon only by the
force of gravity is said to
be in a state of free fall.
There are two important
motion characteristics
which are true of freefalling objects:
– Free-falling objects do
not encounter air
resistance.
– All free-falling objects
(on Earth) accelerate
downwards at a rate of
9.8 m/s/s (often
approximated as 10
m/s/s)
Acceleration from Gravity
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If the velocity and time for a free-falling object
being dropped from a position of rest were
tabulated, then one would note the following
pattern.
Time (s)
Velocity (m/s)
0
0
1
- 9.8
2
- 19.6
3
- 29.4
4
- 39.2
5
- 49.0
The rate of falling increases by 9.8 m/s every second.
2-5. Free Fall
A ball thrown
horizontally will
fall at the same
rate as a ball
dropped
directly.
Free Fall
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Newton’s 2nd Law
predicts this!
a=
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F
m
The force of gravity
felt by a more
massive object is
greater
2-5. Free Fall
A ball thrown into the air
will slow down, stop, and
then begin to fall with the
acceleration due to
gravity. When it passes
the thrower, it will be
traveling at the same rate
at which it was thrown.
Air Resistance
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Air resistance is an upward force
exerted on an object as it falls by air
It is, in essence, a frictional force
For simplicity, the amount of air
resistance is determined by two
factors
– The cross-sectional area of the object
– The speed of the object
W Richa
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
zero so he
resistance is _______
Accelerates
__________downwards.
2) As his speed increases his air
increase
resistance will _______
3) Eventually the air resistance will be
big enough to _______
balance the
skydiver’s weight. At this point
the forces are balanced so his
constant - this is
speed becomes ________
called TERMINAL VELOCITY
How the forces change with time.
KEY
Gravity
(constant value &
always present…weight)
Air resistance
(friction)
Net force
(acceleration OR changing
velocity)
Terminal
Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
increases
causing him to start _____
slowing____.
down
5) Because he is slowing down his air
resistance will _______
decrease until it
balances his _________.
The
weight
skydiver has now reached a new,
terminal _______.
velocity
lower ________
Velocity-time graph for the
Parachute opens –
Velocity
sky diver
diver slows down
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
Projectile Motion
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Projectile: When a
falling object also
experiences
horizontal motion
Horizontal motion
does not affect
vertical motion
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All objects released at the same time (with no
vertical initial velocity) will hit the ground at the
same time, regardless of their horizontal velocity
The horizontal velocity remains constant
throughout the motion (since there is no horizontal
force)