LT 1 I can calculate how fast, how far and how long an object will fall, neglecting air resistance.

LT 2 I can explain why all objects, neglecting air resistance, fall at the same rate.

LT 3 I can explain freefall and identify when terminal velocity occurs for falling objects.

LT 4 I can explain the two factors that influence the amount of air resistance a falling object experiences, and analyze the forces interacting on a falling object to determine its motion.

Starter 11/15 In your spiral notebook…Provide answers to these three questions:

• 1. Describe the Earth’s atmosphere. • 2. Describe air resistance • 3. Describe the term aerodynamic

shape.

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 (N2O)

Without the Ozone Layer

• No terrestrial life (UV rays are dangerous to life: DNA destroyed for example)

Temperature fluctuation

Atmospheric Layers• Four Layers

– Thermosphere (outermost layer, receives intense solar radiation)

– Mesosphere (coldest layer)– Stratosphere (contains the ozone layer, warmer due

to absorption of ultra-violet light)– Troposphere (the weather layer, the densest layer)

• Also, the Ionosphere (forms due to the interaction of cosmic radiation from the Sun with the faint nitrogen and oxygen concentration in the thermosphere)

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

AIR DENSITY

AIR DENSITY

More drag

Less drag

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)

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

The acceleration of an object in free fall is 9.8 m/s2.

4.5-4.6 Free Fall

Neglecting air resistance!

During each second of fall the speed of by the object increases by an additional 9.8 meters per second. V = gt

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

v = gt

v represents both speed and velocity. g represents acceleration due to gravity (9.8 m/s2) t represents time the object is free-falling

4.5 Free Fall: How Fast

4.5 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

Rising Objects

Rising objects decelerate at the same rate that falling objects accelerate.

4.5 Free Fall: How Fast

During the upward part of this motion, the object slows from its initial upward velocity to zero velocity.

The object is accelerating because its velocity is changing.

How much does its speed decrease each second?

Drop a feather and a hammer on earth and the hammer reaches the floor far ahead of the feather.

What about on the Moon?

(see video clips in class)

Air Resistance and Falling Objects

A feather and a coin accelerate equally when there is no air around them.

4.8 Air Resistance and Falling Objects

Vacuum tube

How objects fall without air resistance?

4.8 Air Resistance and Falling ObjectsF gravity or weightis the only force

How objects fall without air resistance?

4.8 Air Resistance and Falling Objects

Objects accelerate equally.

It’s just like on the __________________

Moon

Why?No atmosphere means no air resistance (no drag force), so surface area and weight make no difference. All objects fall at the same rate.

F gravity or weightis the only force

Drop a feather and a hammer on earth and the hammer reaches the floor far ahead of the feather.

What about on the Moon?

4.8 Air Resistance and Falling Objects

http://history.nasa.gov/40thann/videos.htm

In Galileo’s famous demonstration, a 10-kg cannonball and a 1-kg stone strike the ground at practically the same time.This experiment demolished the Aristotelian idea that an object that weighs ten times as much as another should fall ten times faster than the lighter object.

6.6 Free Fall Explained

F stands for the force (or weight in newtons) m stands for the mass of the cannonballa is the rate of acceleration

• the weight-to-mass ratio is the same for these or any objects, which means that the acceleration rates are the same.

6.6 Free Fall Explained

1 kg rock

10 kg cannonball

10x gravitational force or weight

g = weight/mass

a = force/mass

Since the ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone, they both fall at the same rate of acceleration.

6.6 Free Fall Explained

Why?

Since the ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone, they both fall at the same rate of acceleration.

6.6 Free Fall Explained

It takes a larger force to keep the larger mass accelerating because a larger mass has more inertia.

Weight-mass ratios for each are identical!

Why?

All freely falling objects fall with the same acceleration because the ratio of weight to mass is the same for all objects.

6.6 Free Fall Explained

Free Fall

• An object is said to be in free fall if it is only under the influence of gravitational force.

Fg = 100 N

This object will fall at a rate of acceleration equal to 9.8 m/s2.

No support or resistance 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.

6.7 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 or frontal)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

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.

Another variable…

• How would the composition or density of an atmosphere influence air resistance?– More density, more air resistance!

For each second of free fall, an object falls a greater distance than it did in the previous second.

4.6 Free Fall: How Far

These distances form a mathematical pattern: at the end of time t, the object starting from rest falls a distance d.

4.6 Free Fall: How Far

4.6 Free Fall: How Far

4.9 m

19.6 m

44.1 m

78.4 m

122.5 m

For a falling object, how does the distance per second change?

4.6 Free Fall: How Far

For each second of free fall, an object falls a greater distance than it did in the previous second.