free fall motion. falling objects imagine there is no air resistance… an object moving under the...
TRANSCRIPT
Free Fall Motion
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
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.
Why?
F 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
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
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
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.
4.5 Free Fall: How Fast
During the upward part of this motion, the object slows from its initial upward velocity to zero velocity, losing 9.8 m/s of speed per second.
During the downward part of this motion, the object gains 9.8 m/s of speed per second.
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.
Why?
F 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
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 cannonball
a 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. On Earth, it is 9.8 m/s2.
6.6 Free Fall Explained
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. Exposed Surface Area The greater the surface area, the greater the air resistance.
AIR DENSITY
More drag
Less drag
Objects that fall through the atmosphere experience two forces a)weight downward b) air resistance or drag upward
As the object falls, its air resistance increases with speed, and the object’s rate of acceleration decreases until it is at constant velocity: That means that the object has reached terminal velocity! Balanced force
What two factors determine the air resistance force on an object?
6.7 Falling and Air Resistance
The speed and the exposed surface area
What is the acceleration of an object in free fall?
4.5 Free Fall: How Fast
9.8 m/s2
4.6 Free Fall: How Far and How much time it takes to fall
t = √2d/g
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.