What is Air Resistance
By Grace W.
2012

Definition:

Air is a gas. Air resistance is a gas force that acts in the opposite direction of the direction you are travelling. It is caused by molecules colliding with an object in motion, causing the object to slow down and eventually stop. Air resistance can also be known as drag.

How air resistance works

The air resistance force on an object depends primarily on:

• The relative velocity of the object and the fluid. The word "relative" is important - as far as the force is concerned, it doesn't matter if the object is moving and the air (or other fluid) is at rest, or if the air is moving and the object is at rest, or whatever. The relationship between air resistance force and velocity is not simple, but certainly more velocity means more force. For very small objects - microscopic to dust mote size - air resistance force is approximately proportional to velocity, v. (This is called Stokes' Law.) This means that twice the velocity produces twice the air resistance force, three times the velocity produces three times the force, etc. For larger objects, baseballs, cars and people, the air resistance force is approximately proportional to the square of the velocity, v2. In other words, twice the velocity produces four times the force.

• The shape of the object. A larger object must push more air (or other fluid) out of the way in order to move through it, so a bigger space means more air (fluid) resistance. This is why fast cars and airplanes need to be streamlined. The exact relationship between shape and air resistance force is difficult to predict though. A shape that a person would think to be very effective in reducing air resistance often proves, in practice, to act just the opposite. Even today, a great deal of wind-tunnel testing and redesigning is necessary to effectively streamline an object.

• The density of the fluid. Two identical objects moving at the same speed will encounter different resistance forces in different fluids. Dropping a rock through air and dropping the same rock through water definitely produce different motions. The more dense the fluid, the more resistance force on the object.

• The viscosity of the fluid can have an effect on the air resistance force, as well as the texture of the surface of the solid object



Drag and lift

It takes force to push an object through a fluid. Jet airliners need very powerful engines. The resistance to movement is called drag, and streamlining is one way of keeping drag as low as possible. What is lift? Take an airplane. How does it fly exactly? An airplane has airfoil surfaces. An airfoil surface is slightly more curved over the top than beneath, such as an airplane wing. This results in a pressure difference as the wing moves through the air, producing lift.
airplane wing for drag and lift.jpg
Figure #1: an airfoil wing



Aerodynamics and reducing lift force

Aerodynamics is the study of airflow and its principles. Builders design there race cars for negative drag, and negative lift, because the faster the car goes, the greater the lift force, which increases the threat of instability. Among the design features most often used to decrease drag while achieving negative lift is a rear deck spoiler. The race car has an airfoil shape but its purpose is different. It is meant to raise the rear stagnation point and direct airflow so that it does not wrap around the race car’s rear end. Instead, the spoiler creates a downward force to stabilize the rear, and may help to decrease drag by reducing the separation of airflow at the rear end window. The airfoil shape is also used to reduce air resistance, by having the air easily flow over the surface.


aerodynamics and reducing lift force gw.JPG
Figure #2: reducing lift force and aerodynamics




The parachute-an example of air resistance

The parachute is a good example of air resistance, because they are meant to increase it. Parachutes use the science of drag resistance. Take sky diving. With no parachute, the person plummets to the ground at 120 miles per hour! But when they open the parachute, it’s a whole different story. The airflow around the parachute creates drag, slowing it down dramatically, so the person can make a soft landing. A parachute in use resembles an open umbrella. The open end is directed downward. Strong forces produced by air resistance push upward against the descending parachute. These forces oppose the downward pull of gravity. Although gravity's force is reduced, it is not completely eliminated. The speed of fall, however, is decreased from a speed of 120 mph to much safer 14 miles an hour.



parachuting.jpg
Figure #3: parachuting man






Glossary:

Coefficient: a number that is constant for a given substance, body, or process under certain conditions, serving as a measure of one of its properties
Density: mass per unit volume
Gravity: a force that constantly pulls to the ground
Relative velocity: the relative speed of an object
Viscosity: stickiness

Citations:
Knight, Judson. Science of everyday things. Detroit: Thompson/Gale. 2001.

Lewis, Peter. Facts at your fingertips introducing physics matter, energy, and heat. Tucson: brown bear books. 2010. Print.

"parachute." Compton's by Britannica. Encyclopædia Britannica Online School Edition.
Encyclopædia Britannica, Inc., 2012. Web. 24 Sept. 2012.
<http://school.eb.com/all/comptons/article-9276277>.

Smash Lab: Ejector Boat Seat Discovery Channel, 2007. Full Video.
Discovery Education. Web. 24 September 2012. <http://www.discoveryeducation.com/

Stanbrough, JL. Air resistance. batesville.k12, November 4, 2007. Web. 9/27/2012