A skydiver maintains a horizontal (flat) body position with arms and legs spread, which reduces the terminal velocity and increases the fall time. Image Credit: “Gabriel Skydiving” By Gabriel Christian Brown, via Wikimedia Commons
Correct and thoughtful body orientation is an important part of skydiving because the orientation of the body affects the amount of air resistance experienced by the body. In turn, the air resistance affects the terminal velocity, as we will see in the next chapter.
Air resistance limits the terminal speed that a falling body can reach. Air resistance is an example of the drag force, which is force that objects feel when they move through a fluid (liquid or gas). Similar to kinetic friction, drag force is reactive because it only exists when the object is moving and it points in the opposite direction to the object’s motion through the fluid. Drag force can be broken into two types: form drag and skin drag. Form drag is caused by the resistance of fluids (liquids or gases) to being pushed out of the way by an object in motion through the fluid. Form drag is similar to the normal force provided by the resistance of solids to being deformed, only the fluid actually moves instead of just deforming. Skin drag is essentially a kinetic frictional force caused by the sliding of the fluid along the surface of the object.
The drag force depends the density of the fluid (ρ), the maximum cross-sectional area of the object(), and the drag coefficient (), which accounts for the shape of the object. Objects with a low drag coefficient are often referred to as having an aerodynamic or streamlined shape. Finally, the drag force depends on the on the speed (v) of the object through the fluid. If the fluid is not not very viscous then drag depends on v2, but for viscous fluids the force depends just on v. In typical situations air is not very viscous so the complete formula for air resistance force is:
The image below illustrates how the shape of an object, in this case a car, affects the drag coefficient. The table that follows provides drag coefficient values for a variety of objects.
|Object||Drag Coefficient (C)|
|Dodge Ram pickup||0.43|
|Hummer H2 SUV||0.64|
|Skydiver (feet first)||0.70|
|Circular flat plate||1.12|
Which body orientation would put the largest drag force on a human body moving vertically through a fluid?
- body horizontal and sideways (side first)
- body vertical with arms in (feet first)
- body flat with arms out (front first)
- "Gabriel Skydiving" By Gabriel Christian Brown [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons↵
- "Drag of a Sphere" by Glenn Research Center Learning Technologies Project, NASA, via GIPHY is in the Public Domain, CC0↵
- Drag of Car By Eshaan 1992 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons ↵
- OpenStax, College Physics. OpenStax CNX. Jan 17, 2019 http://email@example.com↵