Gravity, Air Resistance,
Terminal Velocity, and
Projectile Motion
A Whole Bunch of Ideas in one Show
To-Do:

Grab a study guide from the side counter and
cut it in ½ to share with a partner.
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In your whirligig groups, discuss the “Science
Behind It” column in your Deep Dive journal
notes.
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Do Not paste it in your journal
Discuss topics such as Gravity, Air Resistance, Mass,
Force, Surface Area, Etc.
Complete the right hand side of your notes for
each conclusion that you wrote.
By the end of this presentation,
you should be able to:
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Describe and explain what is meant by
terminal velocity.
Describe and explain how forces change
on a falling object.
What is a force

Push or pull
Some basics…
Draw a picture along with each fact to help you
remember it!
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Won’t
An object’s mass _______
affect the
acceleration of the object due to
gravity on earth.
opposite
Friction always acts in the _________
direction as motion.
surface area
The ___________
of an object will
affect the velocity of a free falling
object dropping towards earth.
What IS gravity?

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Gravity is NOT “the force that pulls
you down!”
Gravity attracts all objects with mass
inward towards other objects with
mass.
the Force of Gravity

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What is the force of gravity?
The force of gravity is NOT the same
as the acceleration DUE to gravity!
Free Fall

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Objects whose only acceleration is due
to gravity are in free fall.
Theoretically, objects in free fall
continue to accelerate as long as they
are falling.
All free falling objects accelerate
at the same rate regardless of
their mass. (9.8 m/s2)
Galileo's Famous
Experiment
Acceleration of Gravity

Any object which is being
acted upon only by the
force of gravity is said to
be in a state of free fall.
There are two important
motion characteristics
which are true of freefalling 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 (often
approximated as 10
m/s/s)
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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
0
1
- 9.8
2
- 19.6
3
- 29.4
4
- 39.2
5
- 49.0
Free Fall

Newton’s 2nd Law
predicts this!
a=

F
m
The force of gravity
felt by a more
massive object is
greater
Air Resistance
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Air resistance is an upward force
exerted on an object as it falls by air
It is, in essence, a frictional force
For simplicity, the amount of air
resistance is determined by two
factors
– The cross-sectional area of the object
– The speed of the object
Terminal Velocity
Determine the acceleration of this 85 kg skydiver at points A-D. (Hint: F=ma)
9.8 m/s2
5.68 m/s2
1.56 m/s2
0 m/s2
Terminal Velocity
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The terminal velocity of a skydiver in a
free-fall position with a semi-closed
parachute is about 195 km/h
Higher speeds can be attained if the
skydiver pulls in his limbs. In this case,
the terminal velocity increases to
about 320 km/h!
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The more compact and dense the object, the higher its
terminal velocity will be. Typical examples are the
following: raindrop, 25 ft/s, a skydiver was found to be
in a range from 53 m/s to 76 m/s
Possible Test Questions:
 What factor causes terminal velocity to occur?
 If an object is at terminal velocity, is it
speeding up, slowing down, or falling at a
constant speed?
 Describe and explain how forces change on a
falling object.
W Richards Worthing High School
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
zero so he
resistance is _______
____ downwards.
accelerates
2) As his speed increases his air
increase
resistance will _______
3) Eventually the air resistance will be
big enough to _______
balance the
skydiver’s weight. At this point
the forces are balanced so his
constant - this is
speed becomes ________
called TERMINAL VELOCITY
How the forces change with time.
KEY
Gravity
(constant value &
always present…weight)
Air resistance
(friction)
Net force
(acceleration OR changing
velocity)
Terminal
Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
increases
causing him to start _____
slowing____.
down
5) Because he is slowing down his air
resistance will _______
decrease until it
balances his _________.
The
weight
skydiver has now reached a new,
terminal _______.
velocity
lower ________
Velocity-time graph for the
Parachute opens –
Velocity
sky diver
diver slows down
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
Projectile Motion
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Projectile: When a
falling object also
experiences
horizontal motion
Horizontal motion
does not affect
vertical motion