Free Fall
Physics
Date:
Engagement
Why is easy to step off a curb, but very scary
(and dangerous) to step off the top of building?
Objectives
• Describe the motion of an object in free fall and
how it is different from motion we have
previously studied
• Describe the motion of an object thrown straight
up and allowed to fall until it hits the ground
• Evaluate how speed and the distance play a part
in a falling object at any time after an object is
dropped from rest, when air resistance is
negligible
Intro to Free Fall
Questions
What is free fall?
Facts
What is an example
of something in
free fall?
Dropping something (not
throwing it down)
Throwing something up and
allowing it to fall back
down
When the only thing affecting an
object is gravity (there is no air
resistance)
A little background on gravity
A free-falling object is an object that is falling under
the sole influence of gravity. Gravity can be
measured as an acceleration which is the result of
the central attractive force that matter exerts on
other matter. This is because all accelerations are
the result of force (we will learn more about this).
All objects exert gravity on each other; however,
the gravity we experience is the gravity of the
Earth. Gravity is constant on Earth; therefore, the
acceleration is constant (does not change) when
an object is in free fall even though the velocity
is changing!
Gravity
Questions
What is gravity
What might be the
units for gravity?
Why?
What is the value for
gravity?
Facts
• Force pulls to center of Earth
which produces he acceleration
that an object has when in free
fall
m/s2
Because gravity is measured
as an acceleration
Actual = 9.8m/s2
For class = 10 m/s2
More about gravity
Questions
Facts
What type of motion - Linear, horizontal
have we studied so
motion
far? (v=d/t) (a=v/t)
What type of motion - Linear, vertical motion
do we witness with
free fall?
How Does Gravity Work?
What is happening
to the velocity?
Why?
As the ball falls the
velocity is
increasing
because gravity
has more time to
act. At each
point, gravity
remains 10 m/s2
t=0s
v = 0 m/s
t=1s
v = -10 m/s
t=2s
v = -20 m/s
t=3s
v = -30 m/s
t=4s
v = -40 m/s
t=5s
v = -50 m/s
How Does Gravity Work?
Now you try to complete
the figure! Explain the
figure!!!
t = _______
v = _______
t = _______
v = _______
t = _______
v = _______
t = _______
v = _______
t = _______
v = _______
t = _______
v = _______
How does gravity work?
Consider what happens when an object is
thrown straight up and falls back down.
How Does Gravity Work?
Facts
Question
Moving up- the velocity is
Explain what
decreasing
because
the
happens when an
object is moving
object is thrown
against gravity.
straight up and
Falling Down- the object
allowed to fall
gains speed at the
back down.
same rate it was
slowed down because
gravity is constant?
How gravity works?
Question
What will happen if
you drop 2
different objects
at the same time
from the same
height?
Why?
Facts]
They will hit the ground
at the same time
because…
Gravity is constant and is
not dependent on the
mass or weight of an
object.
Check for Understanding
• If you drop a feather and a coin at the same
time in a vacuum tube with no air resistance,
which will reach the bottom of the tube first?
Neither, they will both hit the ground at the
same time!
What affects free fall?
Question
Facts
The higher (more
How does distance
distance) an object
play a part in a
falls, the more time
falling object at
gravity has to act;
any time after an
therefore, the faster
object is dropped
the object will hit the
from rest?
ground.
Let’s go back to the engagement
Questions
Why is easy to step
off a curb, but very
scary (and
dangerous) to step
off the top of a
building?
Facts
A curb is only a small
distance from the
ground; therefore, gravity
acts only for a small time
and velocity is not greatly
gained. From a building,
the distance is greater,
giving gravity more time
to act, which results in a
greater velocity when
hitting the ground- Ouch!
Check for understanding
A freely falling object starts from rest.
After falling for 1 second, it will have
a speed of about
a. 5 m/s.
b. 10 m/s.
c. 20 m/s.
d. 40 m/s.
Check for Understanding
A ball is thrown upwards and caught
when it comes back down. In the absence
of air resistance, the speed of the ball
when caught would be
a. less than the speed it had when thrown upwards.
b. more than the speed it had when thrown upwards.
c. the same as the speed it had when thrown upwards.
Check for Understanding
When a basketball player jumps to make
a shot, once the feet are off the floor, the
jumper's acceleration
a. varies with body orientation.
b. is usually greater for taller players (but not always).
c. depends on all the above.
d. is g; no more, no less.
Remember, Gravity is constant on Earth; therefore, the acceleration is constant (does not
change) when an object is in free fall even though the velocity is changing!
Check for Understanding
A ball is thrown straight up. At the top of
its path its instantaneous speed is
a. 0 m/s.
b. about 5 m/s.
c. about 10 m/s.
d. about 20 m/s.
Homework
 Review Free Fall Notes and Reading!
 Memorize units!!!!