Free Fall
Chapter 2 Section 3
Free Fall
 Free Fall – An object in free fall falls at a
constant acceleration towards the surface
of a planet neglecting air resistance.
Free Fall Acceleration of Gravity
 Free fall acceleration is denoted with the
symbol “g”.
 At the surface of Earth, the free fall
acceleration is approximately 9.8 m/s² or
about 32 ft/sec²
 Acceleration of gravity is a constant and
doesn’t change.
Relating Physics and The
Coordinate Plane
 When calculating problems with free fall,
acceleration due to gravity is negative
 -9.8m/s²
 Using the ideas from a coordinate plane in math
class, the motion of an object can be described.
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To the right – positive
To the left – negative
Downward – negative
Upward – positive
 This holds true for the objects displacement, velocity, and
acceleration since they are all vectors.
Displacement
 Designate an origin.
 Usually where the object begins its free fall
motion.
 Describing displacement from the origin
 Above Origin – Positive Displacement
 Below Origin – Negative Displacement
Velocity
 Objects Velocity in Free Fall
 Upward motion – Positive Velocity
 Downward motion – Negative Velocity
Acceleration
 Acceleration is a constant and is caused
by the gravity of Earth.
 a = -9.8m/s²
 Gravity is always pulling downward on an
object, so acceleration due to gravity will
always be downward.
What Goes Up Must Come Back
Down
 Objects that are given a positive velocity
straight upward will have to come back
down with a negative velocity.
 Objects that are thrown upward are still
being pulled by gravity and will slow down
at a rate of -9.8m/s². Once the objects
reaches 0m/s it will start to fall back to
earth at a rate of -9.8m/s²
Objects Motion During Free Fall
 An object thrown straight up will have a
positive velocity and a negative
acceleration.
 Object is slowing down
 An object falling towards the earth will
have a negative velocity and a negative
acceleration.
 Object is speeding up
Motion of an Object in Free Fall:
Velocity
 If an object is thrown upward with a
positive velocity, the velocity of the object
when it reaches the point of which it was
thrown from will be the same value, just
negative.
 Example: If I throw a ball upward with a
velocity of 15m/s, I will catch it in my hand
with a velocity of -15m/s when it comes back
down. As long as the origin doesn’t change.
Motion of an Object in Free Fall:
Time
 If it takes an object just as long to go up as
it does to come back down.
 Example: If I throw an object upward and it
takes 5 seconds to reach maximum height. It
will take 5 seconds to come back down to its
original position of where it was thrown.
Maximum Height
 An object at maximum height will have a
velocity of 0 m/s.
 The acceleration will still be -9.8m/s² at
maximum height.
 Gravity doesn’t disappears!
Positive velocity
since the ball is
moving upward.
When ball reaches
maximum height,
the velocity = 0
When the ball is above the
line, or origin, the ball has
positive displacement.
Acceleration is a
constant and is
always -9.8m/s2
When the ball is below the line,
or origin, the ball has negative
displacement.
Negative Velocity
Since the ball is
moving downward.
Example Problem #1
 A rock falls off a cliff that is 100 meters
high.
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What is the velocity of the rock when it
reaches the ground below the cliff?
How long did it take the rock to reach the
ground?
Example Problem #2
1. Bill stands behind the backstop which is
6 meters high and wants to throw a
baseball over to his buddy on the other
side. How hard (with what velocity) does
Bill have to throw it in order for the ball
just to make it over the backstop?
Example Problem #3
 Joe and Mary are hiking and come by a
cave that goes straight down. Joe wants
to know how deep the cave goes. So he
drops a large rock off the cliff and
measures the time it takes to reach the
bottom. He hears the rock hit the water
below in the cave about 6.3 seconds
later. How deep is the cliff?
Example Problem #4
 An arrow is shot straight upward with a
velocity of 150m/s.
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How long did it take the object to reach
maximum height?
How long did it take the object to reach the
ground from where it was shot?
How high did the arrow go?
Graph d vs. t / v vs. t and a vs. t of the
arrow.
Graphs for Problem #4
d vs. t
v vs. t
a vs. t