Transcript File

Gravity and Freefall
5C
Law of Universal Gravitation
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All matter has a gravitational pull on all
other matter
Pull may not be enough to overcome friction
 Pull depends on mass of objects & distance
between them
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Law of Universal Gravitation
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F = Force of Gravity
G = 6.67 x 10-11 Nm2 / kg2
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A constant number
M = mass of first object
m = mass of second object
r = radius (distance between the objects)
Law of Universal Gravitation
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What happens to the
force if you double one of
the masses?

Does this mean a 20 kg
ball falls faster than a 10
kg ball b/c of 2x the force?
No, because twice the mass = twice the force AND twice the inertia
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What happens to force if you double the distance?
What happens to force if you double both masses?
Aristotle & Galileo
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Aristotle thought weight decides
how fast objects fall (WRONG!)
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“Heavier objects fall faster”
Galileo thought weight does not
decide how fast an objects fall
(RIGHT!)
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Aristotle
“Objects fall at the same rate”
Reality
“Objects fall at the same rate”
* However, air resistance may
affect rate of fall
 Air resistance
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Aristotle
Friction on an object moving through
the air
 Greatly affects fall of bulky, low
density objects
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Reality
Free Fall
Objects fall due to gravity alone with no
forces acting upon them.
 No air resistance (vacuum or outer space)
 Uniform acceleration
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a = 9.8 m/s2
(velocity is 9.8 m/s faster each second)
Free Fall
v = at
(velocity = acceleration x time)
d = ½ a (t^2)
(distance = ½ acceleration x time^2)
*used for object that start
falling from rest
An apple is dropped…
Time
1 sec.
2 sec.
3 sec.
4 sec.
 Acceleration is 9.8
m/s2 for falling objects
(at 1 sec, 2 sec, etc.)
Velocity Distance Fallen
(v=at)
(d=1/2at2)
0 m/s
9.8 m/s
19.6 m/s
29 m/s
d=1/2(9.8)(02) = 0 m
d=1/2(9.8)(12) = 4.9 m
d=_________ = ___ m
d=_________ = ___ m
An apple is dropped falls from a cliff and hits
the ground 45.0 m below. How long did it fall?
d = ½at2
 acceleration
is always 9.8 m/s2
45 = ½(9.8)(t2)
45 = 4.9(t2)
9.18 = t2
45 = 4.9(t2)
4.9 4.9
√9.18 = √t2
t = 3.03 sec.
Homework: Fill out Chapter 5
Problems Worksheet