Air Resistence
From High School Online Collaborative Writing
Contents |
[edit] Introduction
Air friction, or air drag, is a type of fluid friction. It is the friction experienced by an object moving through the air. The Drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the external flow. Unlike surface frictions experienced on land, Air friction forces are dependent on velocity, becoming larger as the velocity increases. It also depends on the size and shape of the object.
http://www.exploratorium.edu/baseball/putting_images/windtunnel2.gif
Perfect example of how the role of an objects physical aspect affects the air drag. The ball on the top is smooth. the one on the bottom has a smooth surface. As you can see, the ball with the rough surface has less drag and travels more easily through the air.
[edit] Air Friction At High Speeds
Since the fact that this type of friction is velocity dependent, only special cases may be treated analytically. For high velocites and larger objects the frictional drag is approximately proportional to the square of the velocity. In the equation [1]
http://homepage.ntlworld.com/joetapley/drg1.gif
where 'F' is the drag force, 'D' is the air density, 'C' is a drag coefficient, 'A' is the object area and 'V' is the air velocity normal to the surface. This is the basic inertial drag equation used to calculate drag force on an arrow travelling through the air.
[edit] Drag At Low Speeds
In the case where force of drag is approximately proportional to velocity, a different equation is used. [2] The equation for viscous resistance is: http://upload.wikimedia.org/math/4/b/a/4ba1a63bc63bd8aff27f139a1f5dcd7a.png where
- b is a constant that depends on the properties of the fluid and
- v is the velocity of the object
[edit] The Velocity of a Falling Object
Velocity as a function of time for an object falling through a non-dense medium is roughly given by a function involving a hyperbolic tangent: http://upload.wikimedia.org/math/e/6/f/e6faadb6a100cf62a6b7fc6f21081b2a.png
In other words, velocity exponentially approaches a maximum value called the Terminal velocity.
==Terminal Velocity The terminal velocity of an object falling towards the ground is the speed at which the gravitational force pulling it downwards is equal and opposite to the air drag/air resistence pushing it upwards.
Using algebra and physics, try to determine the equation for terminal velocity.
...or you can scroll down and obtain it from www.grc.nasa.gov
http://www.grc.nasa.gov/WWW/K-12/airplane/Images/termv.gif
[edit] Resources
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l3e.html
http://www.exploratorium.edu/baseball/
http://www.nasa.gov/centers/glenn/home/index.html
