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Acceleration due to
Gravity
CCHS Physics
Historical Background
Aristotle (Greek philosopher/scientist)
divided motion into two main classes:
Natural Motion
-Results from the "nature" of objects
Objects strive to get to their proper place
EX- clay vs. smoke vs. feather
Violent Motion
-Results from pushing or pulling forces
Externally caused- outside force required
EX- pushing a cart, tugging a rope
Hypothesis:
Objects fall at
speeds proportional
to their weights
SummaryAll motion results from the
nature of the moving object or from
a sustained push or pull
Galileo attempts to prove
Aristotle’s hypothesis
wrong…
• He drops objects of various weights
from the Leaning Tower of Pisa and
compares the falls.
• He thinks that light and heavy objects
will fall at the same rate.
• We’ll find out who is correct with a
demonstration…
Let’s See The Real Thing
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
So who’s right?
• Both- why?
• In the absence of air resistance, all objects
accelerate towards Earth at the same rate.
• We call this “free-fall” acceleration- meaning
gravity is the only force acting on an object.
• Because we use this value so often we’ll give
it a name- g
• What do you think g depends on?
For an object in free-fall,
g depends on…
• Mass of the planet object is near
• Distance from object to the center of the
planet
•
GM
g 2
r
(G = universal gravitational constant)
• Near the surface of the Earth, g = -9.8 m/s2
Free Fall
• Motion of a body when only the force of
gravity is acting on the body
• The acceleration of an object in free fall
is called the acceleration due to gravity,
or free fall accleration
• Free fall acceleration is denoted by the
symbol g
Free Fall Movie
Handy Chart
Velocity and Acceleration Movie
Let’s try some examples
Estimate the following:
• A stone is dropped off a cliff. What is its
velocity 5 seconds later?
a
v f  vi
t
10 
vf  0
5
 v f  50 m/s
• A ball is tossed straight up at 30 m/s. How
long will it take to land? (It returns to same
height.)
a
v f  vi
t
30  30
10 
 t6s
t
Let’s try some more … a little
harder though
• Ms. Clark throws a ball straight down off
a building with an initial speed of 10
m/s. It lands 3 seconds later. How high
was the building?
1 2
d  at  vi t
2
1
d  10 32  10 3
2
d  75 m
h  75 m
One More Example
• Jason hits a volleyball so that it moves with an initial
velocity of 6 m/s straight upward. If the volleyball
starts from 2 m above the floor, how long will it be in
the air before it strikes the floor?
– Choose origin to be initial position of ball
– The displacement of the ball is -2 m
1 2
d  at  vi t
2
1
2  9.8 t 2  6t
2
t  1.5s or t  0.27s