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Essential Question: How do you
calculate the displacement, velocity
and time of falling objects?
Think about a pen and piece of paper falling to
the ground. Which will hit the ground first?
What is the piece of paper is crumpled?
On August 2, 1971, a demonstration was conducted on
the moon by astronaut David Scott. He simultaneously
released a hammer and a feather from the same height
above the moon’s surface. The hammer and feather both
fell straight down and landed on the lunar surface at
exactly the same moment. Although the hammer is more
massive than the feather, both objects fell at the same
rate.
A feather and an apple are
released from rest in a vacuum
chamber. The two objects fell
at exactly the same rate, as
indicated by the horizontal
alignment of the multiple
images.
The time intervals are equal
but the displacements are
not. As a result, the velocity
isn’t constant so the two
objects are accelerating.
If air resistance is disregarded, all objects
dropped near the surface of a planet will fall with
the same constant acceleration.
The only thing affecting the falling object is
gravity.
What is free fall?
• The motion of a body when only the force due
to gravity is acting on the body
• Freely falling bodies undergo constant
acceleration
𝑎𝑔 or 𝑔
𝑔 = −9.81 m s 2
What happens when you throw an
object 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.
• Objects thrown in the air have a downward
acceleration as soon as they are released.
• This is a strobe
photograph of a ball
thrown up into the air
with an initial velocity
of +10.5 m/s.
The Ball’s Velocity v. Time
On this velocity-time
graph, the slope of the
line, which is equal to
the ball’s acceleration,
is constant from the
moment the ball is
released and
throughout its motion.
Is there always downward acceleration
on freely falling objects?
• Yes!
• The downward acceleration is the same when
an object is moving up, when it is at rest at the
top of its path, and when it is moving down.
• When an object is thrown up in the air, it has a
positive velocity and a negative acceleration.
• At the top of its path, the object’s velocity has
decreased until it is zero.
• When the object begins moving down, it has a
negative velocity and its acceleration is still
negative.
• All objects that undergo free fall acceleration
experience this same pattern.
• Objects that are falling toward Earth move
faster and faster as they fall.
Jason hits a volleyball so that it moves with an initial
velocity of 6.0 m/s straight upward. If the volleyball
starts from 2.0 m above the floor, how long will it be in
the air before it strikes the floor?
A robot probe drops a camera off the rim of a 239 m
high cliff on Mars, where the free-fall acceleration is
− 3.7 m s2 .
Find the velocity with which the camera hits the ground.
Find the time required for it to hit the ground.
A flowerpot falls from a windowsill 25.0 m above the
sidewalk.
How fast is the flowerpot moving when it strikes the
ground?
How much time does a passerby on the sidewalk below
have to move out of the way before the flowerpot hits
the ground?
A tennis ball is thrown vertically upward with an initial
velocity of + 8.0 m/s.
What will the ball’s speed be when it returns to its
starting point?
How long will the ball take to reach its starting point?
• How do you calculate the displacement,
velocity and time of falling objects?