Gravity and Free fall
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Transcript Gravity and Free fall
Gravity and Free fall
An
object is in
free fall when it
— accelerates
due
to the force of
gravity
— and no other
forces are acting
on it.
For Example…
• If you drop a rock off a cliff, the rock is in free fall, because
only gravity is force.
• However, if you throw a ball up toward the sky, it is also in
free fall. Only gravity is force, when it is falling.
• Why do birds, helicopters, and planes not fall from the
sky?
EARTH: gravity always
makes things accelerate the
same amount.
We call this variable “g” for
2
gravity.
g = 9.8 m/s
You can find the velocity of an
object in free fall at any time
during the fall with the
equation:
Vf = gt + vi
The Final Velocity is equal to the gravity multiplied (times) time
in seconds in addition to (plus) the initial velocity.
Example
A
rock falls off a cliff and splashes into a river 5
seconds later.
How fast is it going after 2.5 seconds?
Vf =
g=
t=
Vi =
…after 2.5 seconds?
24.5 m/s
Example
A
rock falls off a cliff and splashes into a river 5
seconds later.
How fast is it going when when it hits the water?
Vf =
g=
t=
Vi =
…when it hit the water?
49 m/s
Remember this clue:
When
a question says an object
“falls off a cliff,” or “falls off a
desk,” or “you drop an object,”
that is “code” for free fall, where
gravity is the force
Upward Launch
-9.8 m/s
9.8 m/s
-19.6 m/s
19.6 m/s
If
you throw a ball
upward, the ball will:
slow down as it
moves up,
-9.8 m/s
9.8 m/s
stop
for an instant,
and then
-19.6 m/s
speed
up as it falls
back down.
19.6 m/s
Upward launches
As
it moves
upward, it slows
down by 9.8 m/s
every second
until it stops.
-9.8 m/s
9.8 m/s
-19.6 m/s
19.6 m/s
Upward launches
The
ball then
reverses
direction and
starts falling
downward.
-9.8 m/s
9.8 m/s
-19.6 m/s
19.6 m/s
Upward launches
As
it falls
downward, the
ball gets faster
by 9.8 m/s every
second.
-9.8 m/s
9.8 m/s
-19.6 m/s
19.6 m/s
In
free fall, there is constant
acceleration.
This
means the object
speeds up or slows down by
the same amount every
second.
Changes in velocity
In
free fall, is there also
constant velocity?
– because the velocity is
constantly changing.
No
Changes in velocity
In
free fall, we can find average velocity.
initial
final
velocity (vi )
velocity (vf )
A rock fell off a cliff and
splashed into a river 5 seconds
later. What was the rock’s
average velocity?
A rock fell off a cliff and splashed into a river 5
seconds later. What was the rock’s average
velocity?
Vavg
Vf
Vi
A rock fell off a cliff and splashed into a river 5
seconds later. What was the rock’s average
velocity?
Vavg --- trying to find
Vf =
Vf =gt+Vi
Vi
A rock fell off a cliff and splashed into a river 5
seconds later. What was the rock’s average
velocity? Vavg --- trying to find
Vf =
Vf =gt+Vi
Vf =(9.8m/s2) (5s) +Vi
Vi = 0 m/s
A rock fell off a cliff and splashed into a river 5
seconds later. What was the rock’s average
velocity? Vavg --- trying to find
Vf =
Vf =gt+Vi
Vf =(9.8m/s2) (5s) +Vi
Vf =49 m/s + 0
A rock fell off a cliff and
splashed into a river 5 seconds
later. What was the rock’s
average velocity?
Vavg = (0m/s + 49m/s)/2
= 24.5 m/s
Calculating distance
You
can also figure out how much
distance an object fell during free
fall using the equation:
A skydiver falls for 6s before
opening her parachute. Calculate
the distance she has fallen in 6 s.
Vf =gt+Vi
Vf =(9.8m/s2)
(6s) +Vi
A skydiver falls for 6s before
opening her parachute. Calculate
the distance she has fallen in 6 s.
Vf =gt+Vi
Vf =58.8 m/s +
0 m/s
A skydiver falls for 6s before
opening her parachute. Calculate
the distance she has fallen in 6 s.
Vf =58.8 m/s
Vavg =(0+58.8) m/s
2
Vavg = 29.4m/s
A skydiver falls for 6s before
opening her parachute. Calculate
the distance she has fallen in 6 s.
Vavg = 29.4m/s
D = 29.4 m/s x (6s)
D= 176.4 m
WEIGHT
VS.
MASS
Weight vs. Mass
Say
you become an astronaut. Will you WEIGH
more, less, or the same on the moon?
Less
Why?
Because
the moon has less gravity than Earth.
Gravity and Weight
The force of gravity on an object is
called weight (Fg).
A
force is measured in units Newtons
The
Fg
equation for weight is:
= mg
Weight depends on mass and gravity
MASS stays the same on the moon, but
WEIGHT changes on the moon.
Weight and mass
Legend says that about 1587, Galileo dropped
two balls from the Leaning Tower of Pisa to
see which would fall faster. Suppose the balls
had masses of 1.0 kg and 10 kg.
a.
Use the equation for weight to calculate the force
of gravity on each ball.
b. Use your answers from part a and Newton’s
second law to calculate each ball’s acceleration.
Weight and mass
1.
Looking for: … the force due to
gravity (Fw) and the acceleration
for each ball
2.
Given: … one ball’s mass = 1.0
kg.
Relationships: Use: Fw = mg
and a = F ÷ m
Solution: For the 1.0 kg ball:
3.
4.
a) Fw = (1.0 kg)(9.8 m/s2) = 9.8 N
b) a = (9.8 N) ÷ (1.0 kg) = 9.8 m/s2
Weight and mass
1.
Looking for: … the force due to
gravity (Fw) and the acceleration
for each ball
2.
Given: … other ball’s mass = 10
kg.
Relationships: Use: Fw = mg
and a = F ÷ m
Solution: For the 1.0 kg ball:
3.
4.
a) Fw = (10 kg)(9.8 m/s2) = 98 N
b) a = (98 N) ÷ (10 kg) = 9.8 m/s2
Weight and mass
Both
balls have the same
acceleration, so they would fall
at the same time.
Gravity
is the same force on
both, but the weight is different.
What could make them fall
different? Think about a
feather.
Air resistance
When
something falls through air, the air exerts a
force called....
air
It
resistance,
acts opposite to the direction
of the object’s motion.
What direction is the skydiver’s
motion? What direction is air
resistance?
Draw the Forces.
The
object falls faster.
Eventually,
air resistance is
equal to force of gravity, and it
can’t go any faster.
This
fastest speed is called
terminal velocity.
Parabolic Flights
NASA has been conducting
parabolic flights since the 1950s to
train astronauts.
Scientists and college students
have also gone on parabolic flights
to perform a wide variety of
chemistry, biology, and physics
experiments.
ZERO-G flights contain three types
of parabolas: Martian gravity (1/3
Earth gravity), Lunar gravity (1/6
Earth gravity), and zero gravity.
http://www.vegas.com/attractions/off
-the-strip/zero-g/
Review Questions
1. A falling object under the influence of only gravity is in
_____________________________
2. Acceleration due to gravity (g) on Earth = __________________
3. What is the weight of a 5 kg backpack on Earth? ____________
4. What is the weight of the same backpack on the moon? (g = 1.6
m/s2) ______________
5. A rock falls off the edge of a cliff. It
lands 7 seconds later.
a. What is the rock’s velocity after 1 s?
b. What is the rock’s speed the instant before it lands?
c. What is the rock’s average velocity for the whole drop?
d. What distance does the rock fall in 7 s?