free fall motion notes day 4. galileo galilei 1564 - 1642 galileo formulated the laws that govern...
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Galileo Galilei
• 1564 - 1642• Galileo formulated the
laws that govern the motion of objects in free fall
Free Fall
• All objects moving under the influence of gravity only are said to be in free fall– Free fall does not depend on the object’s
original motion• All objects falling near the earth’s surface
fall with a constant acceleration• The acceleration is called the acceleration
due to gravity, and indicated by g
Acceleration due to Gravity
• Symbolized by g• g = -9.8 m/s2
• g is always directed downward– toward the center of the earth
• Ignoring air resistance and assuming g doesn’t vary with altitude over short vertical distances, free fall is constantly accelerated motion
Free Fall – an object dropped
• Initial velocity is zero• Let up be positive• Use the kinematic equations• Acceleration is g = -9.8 m/s2
vi= 0
a = g
Free Fall – an object thrown downward
• a = g = -9.8 m/s2
• Initial velocity ≠ 0– With upward being
positive, initial velocity will be negative
Vi= (-)
a = g
Free Fall – an object thrown upward
• a = g = -9.8 m/s2
• Initial velocity ≠ 0– With upward being
positive, initial velocity will be positive
• final velocity = 0
Vi= (+)
a = g
Free Fall - Example
2
0
??
??
9.8
18sec
i
f
mV
sV
d
ma
st
1 2V t ati 2
1 20 (-9.8)(18)2
1587.6 m
therefore height of
building is 1587.6 m
d
d
d
If a rock is dropped from a building, and it takes 18 seconds to reach the ground, how tall is the building?
PracticeA baseball is thrown up vertically with a velocity of 12
m/s. a. What is the maximum height the baseball reaches? b. How long does it take to reach max height?c. What is the velocity when it hits the ground?
PracticeA baseball is thrown up vertically with a velocity of
12 m/s. a. What is the maximum height the baseball reaches?
2
12
0
??
9.8
??
i
f
mV
sV
d
ma
st
2 2
2 2
2 2
2
( ) / 2
(0 12 ) / 2( 9.8)
144 / 19.6
d= 7.3 m
f i
f i
V V ad
d V V a
d
d
PracticeA baseball is thrown up vertically with a velocity of
12 m/s. a. d= 7.3 m (We can use this now!!)b. How long does it take to reach max height?
2
12
0
7.3
9.8
??
f
mVi
sV
d m
ma
st
( ) /
(0 12 ) /( 9.8)
12 / 9.8
t = 1.22 s
f i
f i
V V at
t V V a
t
t
PracticeA baseball is thrown up vertically with a velocity of 12
m/s. a. d= -7.3 m (negative because falling!!)b. t= 1.22s (We can use this now!!)c. What is the velocity when it hits the ground? (Now we
assume falling with Vi = 0)
2
0
??
7.3
9.8
1.22
i
f
V
V
d m
ma
st s
0 ( 9.8)(1.22 )
12 /
same magnitude, but
opposite direction as
when thrown up
f i
f
f
i
V V at
V s
V m s
V
Homework
• Complete 1-16 on 1D Kinematics Worksheet• Answers will be posted on Website!!!• Tomorrow and Friday we will review graphing• Quiz Friday• Motion Test Thursday, Sept 12!!!
Position-time graphs show how the position of an object changes over time.
t
d
x
y line of slope
So, the slope of a d/t graph tells us the VELOCITY of the object.
Velocity-time graphs show how the velocity of an object changes over time.
t
v
x
y line of slope
So, the slope of a d/t graph tells us the ACCELERATION of the object.
The slope is constantly changing, so that means the velocity is changing. Changing velocity is called ACCELERRATION.
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