Chapter 2: Section 3
Source: http://www.physicsclassroom.com
Learning Targets
 Relate the motion of a freely falling body to motion
with constant acceleration
 Calculate displacement, velocity, and time at various
points in the motion of a freely falling object
 Compare the motions of different objects in free fall
P2.1D, P2.1g, P2.2C
The Big Misconception
 "Wouldn't an elephant free-fall faster than a mouse?“
 If we are considering the specific type of falling motion
known as free-fall, the objects move under the sole
influence of gravity
 More massive objects will only fall faster if there is an
appreciable amount of air resistance present.
 However, objects that are truly in free fall do not
encounter air resistance.
 Subsequently, all objects free fall at the same rate of
acceleration, regardless of their mass.
Air Resistance
 In the absence of air resistance, all objects
dropped near the surface of a planet fall with the
same constant acceleration
 This is why a feather and an apple fall at the same
rate if dropped in a vacuum
 In the presence of air resistance,
an object will eventually reach
terminal velocity
What is Free Fall?
 A free falling object is one that is falling
under the sole influence of gravity.
 Any object that is being acted upon only by the
force of gravity is said to be in a state of free fall.
Characteristics of Free Fall
 There are two important
characteristics of free falling objects:
 Free-falling objects do not encounter
air resistance.
 All free-falling objects (on Earth)
accelerate downwards at a rate of
9.8 m/s/s (m/s2)

The acceleration due to gravity is denoted
with the symbols ag or g (on the surface of
the Earth)
Reviewing Acceleration
 Remember that acceleration is the ratio of
velocity change to time between any two
points in an object's path.
 To accelerate at 9.8 m/s/s means to change the
velocity by 9.8 m/s each second.
An Example of Free Fall
 If the velocity and time for a free-falling object being
dropped from a position of rest were tabulated, then
one would note the following pattern:
Time (s)
Velocity
(m/s)
0
1
2
3
0
-9.8
-19.6
-29.4
4
5
-39.2
-49.0
Graphing Free Fall
 Position vs. Time Graph  The slope of any position
vs. time graph is the
velocity of the object
 A curved line on a position
versus time graph signifies
accelerated motion
 The small initial slope
indicates a small initial
velocity and the large final
slope indicates a large final
velocity.
 The negative slope of the
line indicates a negative
(i.e., downward) velocity.
Graphing Free Fall
 Velocity vs. Time Graph  The slope of any velocity
vs. time graph is the
acceleration of the object
 A diagonal line on a
velocity versus time
graph signifies
accelerated motion
 Constant negative slope
indicates a constant
negative acceleration
 Acceleration is constant during upward
and downward motion
 When we throw an object up in the air, it will
continue to move upward for some time, stop
momentarily at the peak, and then change
direction and begin to fall.
 Because the object changes direction, it may seem that
the velocity and acceleration are both changing.
 Actually objects thrown into the air have a downward
acceleration as soon as they are released
Velocity and Acceleration
During Free Fall
 Free falling objects always have the same
downward acceleration
 When going up, velocity is positive and
acceleration is negative (-9.8 m/s2) - the object is
slowing down
 When falling down, velocity is negative and
acceleration is negative (-9.8 m/s2) - the object is
speeding up

Remember that when the signs of velocity and acceleration are
the same, an object speeds up. When they are opposite, an
object is slowing down
Calculating Free Fall Velocity
 To calculate velocity during free fall, use the velocity with
constant acceleration equations
vf2 = vi2 + 2a∆y
vf = vi + a∆t
*a = -9.8 m/s2
 You can use any of the kinematic equations to solve free fall
problems