FREE FALL
What did the video
show?
In the absence of air
resistance, all objects
fall – regardless of
mass or shape – fall
at the same rate.
Video
So, on the moon or in
a vacuum – a feather
and a hammer fall at
the same rate.
FREE FALL
 Free
fall is vertical (up/down) motion of an
object where gravity is the only major force.
 (This means there is little air resistance)
 Freely
falling objects have constant
acceleration towards earth, represented by
the symbol g.
g
= 9.81 m/s2 towards Earth.
Make the motion graphs of a ball thrown up in the air..
AIR RESISTANCE
 In
the absence of air resistance, all objects fall at
the same rate! Why? Because all objects are
subject to the same gravitational force.
That’s what we saw in the video.
 Air
resistance is a force that acts opposite gravity.
This depends on the shape of object (think parachutes!).
 When
air resistance occurs, and object will have
a terminal velocity – a maximum velocity –
instead of accelerating continuously
Without air resistance, rain drops would fall hard enough
to cause serious damage!
TEST YOUR UNDERSTANDING
Read the problem and think – 2 minutes.
Then, show 1 for Akira, 2 for Burt, 3 for Cataline, or 4 for None of them.
SOLVING FREE FALL PROBLEMS
We solve free fall problems using the same equations
and procedures as other acceleration problems.
Just keep in mind a few things …
a
always equals 9.8 m/s2
 An
object’s velocity is 0 m/s at the top of its
path
A
‘dropped object’ has zero vi
IMPORTANT HINTS
 So long for
as problem
an object
is thrown
solving
!!! from the same
height at which it lands, the path up and the
path down are symmetrical
Time
up = time down
Velocity
of launch = -velocity of landing
FREE FALL PROBLEMS – WE DO
 Sally
throws a ball upward with initial speed of 20
m/s. How high will it go? How long will it take for
the ball to come back?
FREE FALL PROBLEMS – WE DO
 Sally
throws a ball upward with initial speed of 20
m/s. How high will it go? How long will it take for
the ball to come back?
Givens:
vi = 20 m/s
g = - 9.8 m/s2
at the top v = 0
Unknowns:
t=?
y=?
Use vf = vi + at to find t = 2 sec
Use x = vi t +
20 m
𝟏
𝟐
𝒂𝒕2 to find x =
When talking about
vertical distance, we
often use y instead
of x!
FREE FALL PROBLEMS – WE DO
Mrs. Radja, hovering in a helicopter 2.0 X 102 m
above our school suddenly drops her pen.
 How
much time will the students have to save
themselves?
 What
is the velocity/speed of the pen when it
reaches the ground?
Givens:
vi = 0 m/s (dropped)
g = 9.8 m/s2
x = vi t +
𝟏
𝟐
vf = vi + at
𝒂𝒕2
Unknowns:
t=?
vf = ?
t = 6.4 sec
Vf = -63 m/s
FREE FALL PROBLEMS – YOU DO
1. A coin is tossed vertically upward.
(a) What happens to its velocity while it is in the air?
(b) Does its acceleration increase, decrease, or remain
constant while it is in the air?
2. A pebble is dropped down a well and hits the water 1.5 s
later. Determine the distance from the edge of the well to
the water’s surface.
3. Stephanie hits a volleyball from a height of 0.8 m and
gives it an initial velocity of 7.5 m/s straight up.
a) How high will the volleyball go?
b) How long will it take the ball to reach its maximum
height?
FREE FALL PROBLEMS – YOU DO
1. a) The velocity is upward and decreases until it reaches
zero, then the velocity is downward and increases until it
lands.
b) Constant.
2. Vi = 0, a = -9.8 m/s2, t = 1.5 sec find x
11 m down
3. Vi = 7.5, xi = 0.8 m, vf = 0, a = -9.8 m/s2
find xmax and time until xmax
vf2 = vi2 + 2ax
x max = 2.9 m + 0.8 m = 3.7 m
vf = vi + at
t = 0.77 sec
x = vi t +
𝟏
𝟐
𝒂𝒕2