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 Atmosphere Major 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? 2 Air Resistance Force Gravitational force 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?

Free Fall An object moving only under the influence of the gravitational force is in 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. No air resistance force! This object will continue to gain speed at a rate of 9.8 m/s2. Rock that weighs 100 N Fgrav = 100 N

Free Fall: How Fast 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. 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

Free Fall: How Fast 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 decreases in speed at the same rate that it increases in speed as it rises and falls

Air Resistance and Falling Objects A feather and a coin accelerate equally when there is no air around them. Vacuum tube

Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance?

Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance? Objects accelerate equally.

Free Fall Physicists consider air resistance to be negligible for heavier objects that fall near the surface of the Earth. Don’t worry about air when making calculations! Fg = 71.2 N Fg = 11 N Fg = 100 N

Falling and Air Resistance Air resistance does not depend upon the weight of the object. Falling and Air Resistance The amount of air resistance force an object experiences depends on the object’s speed and exposed surface area. 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.

The speed and the exposed surface area Falling and Air Resistance What two factors determine the air resistance force on an object? The speed and the exposed surface area