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/sAfter 2 seconds = 9.8 m/s x 2After 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