Free Fall Motion

Free Fall Motion

Questions•Does every object fall the same?•Should every object fall the same? •What is a fluid?•Do we live in a fluid?•What is the composition of our

atmosphere?•Make a table showing the

differences between the Earth and Moon.

AIR DENSITY

Falling Objects

Imagine there is no air resistance… An object moving under the influence of the gravitational force only is said to be in free fall.

4.5-4.6 Free Fall

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

4.5-4.6 Free Fall

Neglecting air resistance!

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

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

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


Vacuum tube

How objects fall without air resistance?

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



Objects accelerate equally.

Why?

F gravity or weightis the only force




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.


During each second of fall the instantaneous speed of the object increases by an additional 9.8 meters per second. This gain in speed per second is the acceleration.

4.5 Free Fall: How Fast

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



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

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!

Rising Objects

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


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?


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




Why?





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.


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.


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.


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.


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.


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.

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.

Another variable…

• How would the composition or density of an atmosphere influence air resistance?

Composition of AirHelium Earth’s atmosphere

Same concentration, different masses

AIR DENSITY

More drag

Less drag

What two factors determine the air resistance force on an object?


The speed and the exposed surface area

The air resistance force an object experiences depends on the object’s _______ and ___________ _______.


think!How fast would a free-falling rock be moving 4.5 seconds after it drops from rest?

How about 8 seconds after it is dropped?


What is the acceleration of an object in free fall?


9.8 m/s2

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.



4.9 m

19.6 m

44.1 m

78.4 m

122.5 m

think!An apple drops from a tree and hits the ground in one second. What is its speed upon striking the ground? What is its average speed during the one second? How high above ground was the apple when it first dropped?


think!An apple drops from a tree and hits the ground in one second. What is its speed upon striking the ground? What is its average speed during the one second? How high above ground was the apple when it first dropped?

Answer:


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


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

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Free Fall Motion