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Typical values of drag coefficient C size 12{C} {} .
Drag coefficient values
Object C
Airfoil 0.05
Toyota Camry 0.28
Ford Focus 0.32
Honda Civic 0.36
Ferrari Testarossa 0.37
Dodge Ram pickup 0.43
Sphere 0.45
Hummer H2 SUV 0.64
Skydiver (feet first) 0.70
Bicycle 0.90
Skydiver (horizontal) 1.0
Circular flat plate 1.12

Substantial research is under way in the sporting world to minimize drag. The dimples on golf balls are being redesigned as are the clothes that athletes wear. Bicycle racers and some swimmers and runners wear full bodysuits. Australian Cathy Freeman wore a full body suit in the 2000 Sydney Olympics, and won the gold medal for the 400 m race. Many swimmers in the 2008 Beijing Olympics wore (Speedo) body suits; it might have made a difference in breaking many world records (See [link] ). Most elite swimmers (and cyclists) shave their body hair. Such innovations can have the effect of slicing away milliseconds in a race, sometimes making the difference between a gold and a silver medal. One consequence is that careful and precise guidelines must be continuously developed to maintain the integrity of the sport.

Three swimmers with are each wearing an L Z R Racer Suit, which is a swimsuit composed of elastane nylon and polyurethane. The seams of the suit are ultrasonically welded to reduce drag.
Body suits, such as this LZR Racer Suit, have been credited with many world records after their release in 2008. Smoother “skin” and more compression forces on a swimmer’s body provide at least 10% less drag. (credit: NASA/Kathy Barnstorff)

Some interesting situations connected to Newton’s second law occur when considering the effects of drag forces upon a moving object. For instance, consider a skydiver falling through air under the influence of gravity. The two forces acting on him are the force of gravity and the drag force (ignoring the buoyant force). The downward force of gravity remains constant regardless of the velocity at which the person is moving. However, as the person’s velocity increases, the magnitude of the drag force increases until the magnitude of the drag force is equal to the gravitational force, thus producing a net force of zero. A zero net force means that there is no acceleration, as given by Newton’s second law. At this point, the person’s velocity remains constant and we say that the person has reached his terminal velocity ( v t size 12{v rSub { size 8{t} } } {} ). Since F D size 12{F rSub { size 8{D} } } {} is proportional to the speed, a heavier skydiver must go faster for F D size 12{F rSub { size 8{D} } } {} to equal his weight. Let’s see how this works out more quantitatively.

At the terminal velocity,

F net = mg F D = ma = 0 . size 12{F rSub { size 8{"net"} } = ital "mg" - F rSub { size 8{D} } = ital "ma"=0 "." } {}

Thus,

mg = F D . size 12{ ital "mg"=F rSub { size 8{D} } "." } {}

Using the equation for drag force, we have

mg = 1 2 ρ CAv 2 . size 12{ ital "mg"= { {1} over {2} } ρ ital "CAv" rSup { size 8{2} } } {}

Solving for the velocity, we obtain

v = 2 mg ρ CA . size 12{v= sqrt { { {2 ital "mg"} over {ρ ital "CA"} } } } {}

Assume the density of air is ρ = 1 . 21 kg /m 3 size 12{ρ=1 "." "21"" kg/m" rSup { size 8{3} } } {} . A 75-kg skydiver descending head first will have an area approximately A = 0 . 18 m 2 and a drag coefficient of approximately C = 0 . 70 size 12{C=0 "." "70"} {} . We find that

v = 2 ( 75 kg ) ( 9 .80 m /s 2 ) ( 1 . 21 kg /m 3 ) ( 0 . 70 ) ( 0.18 m 2 ) = 98 m/s = 350 km/h . alignl { stack { size 12{v= sqrt { { {2 \( "75"`"kg" \) \( 9 "." "80"" m/s" rSup { size 8{2} } \) } over { \( 1 "." "21"" kg/m" rSup { size 8{3} } \) \( 0 "." "70" \) \( 0 "." "18"`m rSup { size 8{2} } \) } } } } {} #="98"`"m/s" {} # ="350"`"km/h" "." {}} } {}

This means a skydiver with a mass of 75 kg achieves a maximum terminal velocity of about 350 km/h while traveling in a pike (head first) position, minimizing the area and his drag. In a spread-eagle position, that terminal velocity may decrease to about 200 km/h as the area increases. This terminal velocity becomes much smaller after the parachute opens.

Questions & Answers

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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