Transcript Free Fall Intro revised

Free Fall
Chapter 4
Tiffany Rhodes
We all know the effects of gravity?
Right???
http://www.youtube.com/watch?v=cIwTYL1fwJk
http://www.youtube.com/watch?v=F-pX1X2cUv4
http://www.youtube.com/watch?v=lLmJ1Mm13IQ
What does it mean to be in
Free Fall?



Occurs when an object is being
acted upon by the force of gravity.
If gravity is the only force acting
on an object, the object will
accelerate at 9.8 m/s2.
That is a constant on Earth!
All objects fall at the same rate!
g = 9.8 m/s/s, downward
Acceleration due to Gravity





Constant Acceleration…
9.8 m/s/s
You can use the kinematic equations!!
Sometimes seen as ‘g’ rather than ‘a’ in
equations
Usually given a negative symbol
representing a downward direction.
Kinematic Review

V = Vo + at

X = Xo + Vot + ½ at2

V2 = Vo2 + 2 a (X-Xo)

a = -g (gravitational constant. –9.8 m/s2)
You keep speeding up as you fall!

Time (s)
Velocity (m/s)
V = Vo + -at
 0
0

1
- 9.8

2
- 19.6

3
- 29.4

4
- 39.2

5
- 49.0
Until You Hit ‘Terminal Velocity’



Happens around 10
seconds after YOU fall
Air resistance counters
gravity causing no
more acceleration to
occur
Continue at a constant
velocity to ground
Downward Gravity = Upward Air Resistance
Skydiver
Skydiver
The first 10 seconds, you are in constant acceleration.
You can use your beloved kinematics!
After 10 seconds, you have reached terminal velocity (no
longer accelerate), Net force = 0 (Mechanical Equilibrium)
You can use your beloved constant Velocity equation!!
V= displacement/time
Until you pull the parachute at about 30 seconds…
Sample Problem

A Sky diver jumps out of a plane at an
elevation of 5000 meters. He reaches
terminal velocity at 10 seconds.
A. What is his velocity at 10 seconds?
B. How far did he fall in the first 10 seconds?
C. How far did he fall in the first 16 seconds?
Part A: What is his Velocity at 10s?




Vo = 0
T = 10 sec
A = -9.8
m/s2
Vf =?
Vf = Vo + at
Vf = 0 + (-9.8)(10)
Vf = -98 m/s
In the first ten seconds, the
skydiver accelerated to a
terminal velocity of 98 m/s
in the downward direction
Part B: How far did he fall in the first 10 s?





Vo = 0
T = 10 sec
A = -9.8
m/s2
Vf =-98 m/s
Δx = ??
Δx = Vot + 1/2 at2
Δx = 0 + ½ (-9.8)(10)2
Δx = - 490 meters
In the first ten seconds, the
skydiver fell a total of 490
meters towards the Earth
Part C: How far did he fall in the first 16s?
For the first 10s,
Δx = - 490 meters
For the next 6s, he
travels at a constant
speed
V = -98m/s
T = 6 seconds
Δx = ???
Constant Velocity = Δx / time
-98 m/s = Δx / 6 seconds
-98 * 6 = Δx
-588 m = Δx In the next 6
seconds, the
skydiver fell a total
of 588 meters
towards the Earth
-490 + -588 = -1078 meters
or 1078 meters down
Terminal Velocity
http://youtube.com/watch?v=1ukf2vntU44&feature=related
4.5 Free Fall: How Fast
Rising Objects
Now consider an object thrown straight up:
•
•
•
•
It moves upward for a while.
At the highest point, when the object is changing its
direction from upward to downward,
its instantaneous speed is zero.
It then falls downward as if it had been dropped from
rest at that height.
4.5 Free Fall: How Fast
During the upward part of this motion, the object slows from
its initial upward velocity to zero velocity.
The object is accelerating because its velocity is changing.
How much does its speed decrease each second?
4.5 Free Fall: How Fast
The speed decreases at the same rate it increases
when moving downward—at 9.8 meters per second
each second.
•
•
•
The instantaneous speed at points of equal
elevation in the path is the same whether the
object is moving upward or downward.
The velocities are different because they are in
opposite directions.
During each second, the speed or the velocity
changes by 9.8 m/s downward.
4.5 Free Fall: How Fast
The change in speed each
second is the same whether
the ball is going upward or
downward.
Change in Displacement


The distance which a free-falling
object has fallen from a position
of rest is also dependent upon the
time of fall.
∆X =Vot + 1/2at2
At t = 1 s
∆X = (0.5) * (-9.8 m/s2) * (1 s)
= -4.9 m
2
At t = 2 s
∆X = (0.5) * (-9.8 m/s2) * (2 s) 2
= -19.6 m
Will an elephant and a mouse
fall at the same rate?
Neglecting Air resistance…
ALL OBJECTS FALL AT THE
SAME RATE!! 9.8m/s/s
An elephant and a mouse will hit
the ground at the same time!!
SAME ACCELERATION!
Why do we see otherwise in
real life? AIR RESISTANCE
What if we remove AIR
RESISTANCE??
Will a feather and a hammer fall at the
same rate, if we negate air resistance?
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
http://youtube.com/watch?v=5C5_dOEyAfk&feature=related
http://youtube.com/watch?v=eNn4yXEA_XQ&feature=related
Can we do this on Earth?
Since I am a superhero, I can ;)
Let me show you…
Do you think it is dangerous to
drop a penny off the empire
states building??
Penny Drop
Watch Mythbusters on Itunes, DVD or Youtube…
http://www.youtube.com/watch?v=QUS89-_fbtk
Homework: Free Fall Worksheet