Air Resistance, Free Fall Motion and Falling Objects
Chapter 3.3
Starter Q (10/24) Provide answers to these three questions:
1. Describe the Earth’s atmosphere. (What is it and what is it made of?)
2. Describe air resistance (what causes it and why is it considered a force?)
3. Describe the term aerodynamic shape. (What does it mean? Give an example)
Earth’s AtmosphereMajor Constituents
•Nitrogen (N2) 78%
•Oxygen (O2) 21%
•Argon (Ar) < 1%
Minor Constituents
Water vapor (H2O)
Carbon dioxide (CO2)
Methane (CH4)
Nitrous oxide (NO2)
AIR RESISTANCE
A resistance force caused by air molecules opposing the motion of an object as it moves through the air.
A form of friction sometimes called drag.Aerodynamic shape
Starter Q (10/21) Air resistance
Quick response: how do these pictures relate to the study of air resistance?
Suppose a bowling ball is falling… How many forces are acting on it?
Gravitational force
Air Resistance Force
2
The weight of the bowling ball is the same as the gravitational force acting on the ball (reported in Newtons)
Questions Does every object fall the same?
Why or why not? What happens to the air resistance
when an object ‘s exposed surface area increases? Decreases?
Does air resistance increase, decrease or stay the same when an object travels faster through the air?
An object moving only under the influence of the gravitational force is in free fall.
Free Fall
The acceleration of an object in free fall on Earth is 9.8 m/s2.
For free fall, neglect air resistance!
Free Fall Free body diagram Only under the influence of gravitational
force.
Fgrav = 100 N
This object will continue to gain speed at a rate of 9.8 m/s2.
No air resistance force! Rock
that weighs 100 N
During each second of fall the speed of by the object increases by an additional 9.8 meters per second.
This gain in speed per second is the acceleration.
Free Fall: How Fast
After 1 second = 9.8 m/s
After 2 seconds = 9.8 m/s x 2
After 3 seconds = 9.8 m/s x 3… and so on
Free Fall: How Fast
9.8 m/s
19.6 m/s
29.4 m/s
39.2 m/s
49 m/s
9.8 m/s x t
Rising Objects
Rising objects decelerate at the same rate that falling objects accelerate.
Free Fall: How Fast
During the upward part of this motion, the object slows from its initial upward velocity to zero velocity.
The object decreases in speed at the same rate that it increases in speed as it rises and falls
A feather and a coin accelerate equally when there is no air around them.
Air Resistance and Falling Objects
Vacuum tube
How objects fall without air resistance?
Air Resistance and Falling Objects
F gravity or weightis the only force
How objects fall without air resistance?
Air Resistance and Falling Objects
Objects accelerate equally.
F gravity or weightis the only force
Free Fall Physicists consider air resistance to be
negligible for heavier objects that fall near the surface of the Earth.
Fg = 100 NFg = 71.2 N Fg = 11 N
Don’t worry about air when making calculations!
The amount of air resistance force an object experiences depends on the object’s speed and exposed surface area.
Falling and Air Resistance
Air resistance does not depend upon the weight of the object.
1. Speed The greater the speed, the greater
the air resistance.
2. Surface Area (exposed)The greater the surface area, the greater the air resistance.
What two factors determine the air resistance force on an object?
Falling and Air Resistance
The speed and the exposed surface area
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