Transcript FALLING OBJECTS

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Freely falling bodies undergo constant
acceleration.
Such motion is referred to as free fall.
The free-fall acceleration is denoted with the
symbol g.
At the surface of the Earth the magnitude of g
is approximately 9.8 m/s2.
Your textbook states g = 9.81 m/s2. Which
value is more precise?
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In our usual choice of coordinates, the
downward direction is negative. Thus, the
acceleration of objects in free fall near the
surface of the earth is a = g = -9.8 m/s2.
Gravity acts in a downward direction;
therefore, you may tend to consider g as
positive. This works as long as you are
consistent with your sign convention.
Freely falling objects always have the same
downward acceleration.
Jason hits a volleyball so that it moves with an
initial velocity of 6.0 m/s straight upward. If
the volleyball starts from 2.0 m above the
floor, how long will it be in the air before it
strikes the floor? Assume that Jason is the
last player to touch the ball before it hits the
floor?
6.0 m/s
given
vi = + 6.0 m/s
a=g
= -9.8 m/s2
∆y = -2.0 m
unknown
t
Eqn
?
2.0 m
vavg
vi  vf
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2
1
2
d  vit  at
2
Both t and vf are
unknown
Solve for vf using an
acceleration equation
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Vf = -8.7 m/s
Solve for t using the
other acceleration
equation
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∆t = 1.5 sec
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Because the velocity will decrease by 9.81
m/s in 1 s and because vi is only 6.0 m/s, it
will take a little less than 1 s for the ball to
reach its maximum height. Once the ball is
at its maximum height, it will take less than 1
sec to fall to its original position and a little
additional time to fall the final 2.0 m to the
floor. Therefore, a total time of between 1.0
s and 2.0 s is reasonable.
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Let’s discuss the reaction time lab done in the
last class.
Always comment on any negative values.
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1) A robot probe drops a camera off the rim
of a 239 m high cliff on Mars, where the
free-fall acceleration is -3.7m/s2.
a) Find the velocity with which the camera
hits the ground. (-4.2 m/s)
b) Find the time required for it to hit the
ground. (11 sec)
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a.
b.
1) A robot probe drops a camera off the rim of a 239 m high cliff on Mars,
where the free-fall acceleration is -3.7m/s2.
a) Find the velocity with which the camera hits the ground. (-4.2 m/s)
b) Find the time required for it to hit the ground. (11 sec)
2) A flower pot falls from a windowsill 25.0 m
above the sidewalk.
How fast is the flowerpot moving when it strikes
the ground? (22.1 m/s)
How much time does a passerby on the sidewalk
below have to move out of the way before the
flowerpot hits the ground? (2.25 sec)
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a.
b.
2) A flower pot falls from a windowsill 25.0 m above the sidewalk.
How fast is the flowerpot moving when it strikes the ground? (22.1 m/s)
How much time does a passerby on the sidewalk below have to move out
of the way before the flowerpot hits the ground? (2.25 sec)
3. A tennis ball is thrown vertically upward
with an initial velocity of +8.0 m/s.
a. What will the ball’s speed be when it
returns to it’s starting point? (8 m/s)
b. How long will it take the ball to reach its
starting point? (1.63 sec)
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