5.2 Drag forces  (Page 2/6)

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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.

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 ( ${v}_{t}$ ). Since ${F}_{\text{D}}$ is proportional to the speed, a heavier skydiver must go faster for ${F}_{\text{D}}$ to equal his weight. Let’s see how this works out more quantitatively.

At the terminal velocity,

${F}_{\text{net}}=\text{mg}-{F}_{\text{D}}=\text{ma}=0\text{.}$

Thus,

$\text{mg}={F}_{\text{D}}\text{.}$

Using the equation for drag force, we have

$\text{mg}=\frac{1}{2}\rho {\text{CAv}}^{2}.$

Solving for the velocity, we obtain

$v=\sqrt{\frac{2\text{mg}}{\rho \text{CA}}}.$

Assume the density of air is $\rho =1\text{.}\text{21 kg}{\text{/m}}^{3}$ . A 75-kg skydiver descending head first will have an area approximately $A=0\text{.}\text{18}\phantom{\rule{0.25em}{0ex}}{\text{m}}^{2}$ and a drag coefficient of approximately $C=0\text{.}\text{70}$ . We find that

$\begin{array}{lll}v& =& \sqrt{\frac{2\left(\text{75 kg}\right)\left(9\text{.80 m}{\text{/s}}^{2}\right)}{\left(1\text{.}\text{21 kg}{\text{/m}}^{3}\right)\left(0\text{.}\text{70}\right)\left(\text{0.18}\phantom{\rule{0.25em}{0ex}}{\text{m}}^{2}\right)}}\\ & =& \text{98 m/s}\\ & =& \text{350 km/h}\text{.}\end{array}$

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.

A body travelling at a velocity of 30ms^-1 in a straight line is brought to rest by application of brakes. if it covers a distance of 100m during this period, find the retardation.
what's acceleration
The change in position of an object with respect to time
Mfizi
Acceleration is velocity all over time
Pamilerin
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Stephen
It's not It's the change of velocity relative to time
Laura
Velocity is the change of position relative to time
Laura
acceleration it is the rate of change in velocity with time
Stephen
acceleration is change in velocity per rate of time
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Stephen
Ohm's law is related to resistance by which volatge is the multiplication of current and resistance ( U=RI)
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How do you determine the magnitude of force
mass × acceleration OR Work done ÷ distance
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acute astigmatism?
the difference between virtual work and virtual displacement
How do you calculate uncertainties
What is Elasticity
using a micro-screw gauge,the thickness of a piece of a A4 white paper is measured to be 0.5+or-0.05 mm. If the length of the A4 paper is 26+or-0.2 cm, determine the volume of the A4 paper in: a). Cubic centimeters b). Cubic meters
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why it is possible for an object(man) to stay on air without falling down?
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it's impossible
Your
Why is it not possible to stand in air?
bikko
the air molecules are very light enough to oppose the gravitational pull of the earth on the man..... hence, freefall occurs
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because of gravitational forces
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this mostly occur in space
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E=MC^2
study of matter and energy and an inter-relation between them.
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that's how the mass and energy are related in stationery frame
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Kinetic energy is the energy due to montion of waves,electrons,atoms, molecule,substances an object s.
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wich method we use to find the potential on a grounded sphere
Noman
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Emmanuella
the study of all the natural events occuring around us..... this is Physics (until those events obey the laws of physics)
Arzail
Conservation of energy😰
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Energy, it always remains there in a physical system. it can only take the form either in motion (kinetic energy) or in rest (potential energy)
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In nature organisms feed on one another in an orderly way.
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