winds would flow parallel to isobars and at high rates of speed. It is also called drag. To get into geostrophic balance, moving air will undergo geostrophic adjustment. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. These cookies ensure basic functionalities and security features of the website, anonymously. >> Ultimately reduces deflection due to Coriolis Force. ultimate impact of all three forces mentioned above can be seen in the Friction opposes the direction of motion by acting opposite to the flow of air. Thus, winds around the high are too weak to be in cyclostrophic balance. Figure 2. Generally, over low pressure area the air will converge and rise. not present directly at the equator. Temporal and inertial terms are generally of secondary importance to the general flow, though they become important for transient features such as meanders and eddies. This means that from the object in the air (i.e. When we push an initially resting refrigerator with an external applied force, Figure 4. further explained by the movie found in Figure 5. Based on this, we can see that the Coriolis parameter will be 0 at the equator when sin(0)=0 and maximized at the poles when sin(90)=1. This usually creates the westerly winds moving from the subtropical areas to the poles. to the geostrophic wind above depends on the This cookie is set by GDPR Cookie Consent plugin. This occurs because as something moves freely above the earths surface, the earth is moving east under the object at a faster speed. In the vertical, the upward pressure gradient force is The resulting friction forces only act in the direction of the wind components parallel to external surfaces. covergence and rising air, while high pressure systems rotate in a clockwise fashion and are Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids, and contact elements, Structural engineering software for designing frame, beam, and truss structures, as well as performing linear and nonlinear calculations of internal forces, deformations, and support reactions, 2001 - 2023 by Dlubal Software GmbH | All Rights Reserved, US Standards (AISC, ACI, AWC, ADM, ASCE 7, IBC), Snow Load, Wind Speed, and Seismic Load Maps, Cross-Section Properties of Standardized Sections or Parameterized Cross-Sections, Stand-Alone Programs for Steel Structures, Stand-Alone Programs for Timber Structures, Free Structural Analysis Software for Educational Institutions, Free Introductory Training at Your University, Introduction to Structural Analysis and Design, Useful Tools for Fast Generation of Structures in RFEM, Useful Tools for Fast Generation of Structures in RSTAB, Eurocode 1: Actions on structures - Part 1-4: General actions - Wind actions; German version EN 1991-1-4:2005 + A1:2010 + AC:2010, National Annex - Nationally determined parameters - Eurocode 1: Actions on structures - Part 1-4: General actions - Wind actions; EN 1991-1-4/NA:2010-12, For smooth surfaces such as steel or smooth concrete, the friction coefficient c, On rough surfaces such as rough concrete or tar-boards, the friction coefficient c, For surfaces that are very rough (ripples, ribs, folds), the friction coefficient c. To an Earth-bound observer, however, this motion cannot be along a straight line because the reference frame is the rotating Earth. In addition to the speed of the earths rotation and latitude, the faster the object itself is moving, the more deflection there will be. In what two ways does friction affect the winds near the surface? The frictional force will be in the opposite direction of this net force. The coefficient of friction is the ratio of the magnitude of frictional force divided by the normal force magnitude. So in the northern hemisphere, winds blow clockwise around an area of high pressure and counter-clockwise around low pressure. All Rights Reserved. Because geostrophic winds are dependent on the pressure gradient, geostrophic winds are faster when isobars are closely spaced. The force of friction changes the airs speed. Thus the pressure gradient force is balanced by friction and Coriolis force. Does it always have to be less than 1 or is it possible for the coefficient of friction to be any real number? (more about this later). >> Moves air from higher to lower Balanced wind in the atmospheric boundary layer (ABL) occurs when there is a balance between the pressure gradient force, Coriolis force, and the frictional drag force. as the wind aloft. It is greatest at the surface and its influence generally extends up to an elevation of 1 3 km. The unchanged pressure gradient gives rise to a current speed independent of depth. Again, the frictional drag force acts in the plane of motion and slows down the wind speed. As we look at the diagram above, this slowing down reduces O trade winds. Figure 1. List of National Parks of India (106 National Parks in India in 2022), List of Tiger Reserves in India (53 Tiger Reserves in 2022). Friction has two effects on the wind. Static friction magnitude is directly proportional to the normal force magnitude and the roughness between the sliding surfaces. The pressure gradient force doesnt change, but because the wind speed is slower, the Coriolis force is weaker. $\begin{array}{l}\mathrm y\;=\;2\;\cdot\;10\;=\;20\;\mathrm m\\{\mathrm A}_{\mathrm{fr}}\;=\;2\;\cdot\;(30\;\mathrm m\;\;20\;\mathrm m)\;\cdot\;4\;\mathrm m\;+\;2\;\cdot\;(30\;\mathrm m\;\;20\;\mathrm m)\;\cdot\;5.22\;\mathrm m\;=\;184.4\;\mathrm m\\{\mathrm q}_{\mathrm p(\mathrm{ze})}\;=\;2.1\;\cdot\;{\mathrm q}_{\mathrm b}\;\cdot\;\left(\frac{\mathrm z}{10}\right)^{0.24}\;=\;2.1\;\cdot\;0.39\;\mathrm{kN}/\mathrm m\;\cdot\;\left(\frac{5.5\;\mathrm m}{10}\right)^{0.24}\;=\;0.711\;\mathrm{kN}/\mathrm m\\{\mathrm F}_{\mathrm{fr}}\;=\;0.04\;\cdot\;0.711\mathrm{kN}/\mathrm m\;\cdot\;184.4\;\mathrm m\;=\;5.245\;\mathrm{kN}\end{array}$. Direct link to peter nduswa's post how do you find the range, Posted 6 months ago. QUESTION: 13 : SOLUTION : OPTION C; Wind would flow parallel to isobars and at high rate of speed An air parcelto begin withatrelaxationwillpassfromexcessivestrainto lowstraindue tothestraingradientpressure(PGF). >> Ultimately reduces deflection due to Coriolis Force friction) Direct link to Kevin Li's post Can you do a video about . the wind is indeed geostrophic. These cookies track visitors across websites and collect information to provide customized ads. The motionless ocean floor removes momentum from the circulation of the ocean waters. In the Southern Hemisphere, this deflection would be toward the left. 7 How does friction affect the direction of the wind? What is friction caused by air in Earth's atmosphere known as? 8 What kind of friction occurs in the atmosphere? general circulation of the atmosphere Do you have any further questions or need advice? Which of the following describes the pressure gradient force? This force always acts to oppose the motion of an object, whether that object be Irrespective of the package you choose, Current Affairs of Geography, Environment, Sci & Tech and Indian Agriculture are available from Jan 2022. How does the surface of the Earth affect the wind? The Coriolis force is the force exerted by the rotation of the earth. Some of the most important impacts of the Coriolis effect in terms of geography are the deflection of winds and currents in the ocean. First, find the net force if there was no friction. If there was no friction, the car would just keep moving forward like it was on pure ice. Cheese resting on an incline because static friction holds it in place. The height of demonstration at the bottom of this page. How does friction act to change the direction of the wind near the Earths surface? The percentage of the friction force due to wind of the total wind force is determined according to the following formula: ${\mathrm F}_{\mathrm{fr},\mathrm j}\;=\;{\mathrm c}_{\mathrm{fr},\mathrm j}\;\cdot\;{\mathrm q}_{\mathrm p(\mathrm{ze})\mathrm j}\;\cdot\;{\mathrm A}_{\mathrm{fr},\mathrm j}$ [1] (5.7), wherecfr = friction coefficientq p(ze) = is the peak velocity pressure at reference height zeAfr = area of external surface parallel to the wind. The effects of wind friction on the surface can be disregarded when the total area of all surfaces parallel with (or at asmall angle to) the wind is equal to or less than 4 times the total area of all external surfaces perpendicular to the wind (windward and leeward) [1] 5.3 (4). Static friction is the force holding an object in place on an incline, such as the cheese in Figure 1. Near the surface, friction reduces the wind speed, which reduces the Coriolis force. As a result, the horizontal winds near the Earth's surface are influenced by three forces: Horizontal differences in density (due to variations of temperature and salinity) measured along a specific depth cause the hydrostatic pressure to vary along a horizontal plane or geopotential surface, a surface perpendicular to the direction of the gravity acceleration. What kind of friction occurs in the atmosphere? The frictional force is most prevalent at the surface Friction when an object slides along a surface. Question 13 1 pts If surface winds were influenced only by the pressure gradient force and Coriolis force (i.e., without the frictional force), wind speed and direction would be reduced. The horizontal pressure gradient in the ocean is a combination of these two mass fields. Wind blows from areas of high pressure toward areas of low pressure. The air rubbing against the earths surface causes friction. These cookies ensure basic functionalities and security features of the website, anonymously. Both wind shear turbulence and convective turbulence cause drag, which results in the ABL wind being slower than geostrophic (subgeostrophic), and causes the wind to cross isobars toward the low pressure. This friction can act to change the wind's An apparent deflection of the path of the moving object would be seen. Please download the Macromedia Flash Player in order to view this demonstration. | Because the scale is small and independent of the Coriolis force, the direction of cyclostrophic winds can be either clockwise or counterclockwise in both hemispheres. Since there are two forces in opposite directions with friction, which way should the frictional force point? to about 1-2 km above it. Included in this limited. However, you may visit "Cookie Settings" to provide a controlled consent. geostrophic balance by meteorologists, causes wind to move parallel to isobars. Centripetal acceleration creates a force directed at right angles to the wind movement and inwards towards the centers of rotation (e.g., low and high pressure centers). The momentum transfer is a product of turbulence that moves kinetic energy to smaller scales until at the tens-of-microns scale (1 micron = 1/1,000 mm) it is dissipated as heat. In the friction layer, the turbulent friction that the Earth exerts on the air slows the wind down. Figure 1. Force can also cause objects to accelerate. wind, and vertical temperature profile. Support & Learning Movement of water through the oceans is slowed by friction, with surrounding fluid moving at a different velocity. Sun is the ultimate force that drives winds. Weather and Climate: Whats the Difference? As a result, ocean currents move clockwise (anticyclonically) in the Northern Hemisphere and counterclockwise (cyclonically) in the Southern Hemisphere; Coriolis force deflects them about 45 from the wind direction, and at the Equator there would be no apparent horizontal deflection. By contrast, hills and forests force the wind to slow down and/or change direction much more. there would be no winds at all. How do pressure differences affect winds? combined to give a more realistic view of how the wind moves in the atmosphere. The general circulation is governed by the equation of motion, one of the fundamental laws of mechanics developed by English physicist and mathematician Sir Isaac Newton that was applied to a continuous volume of water. You'll get a detailed solution from a subject matter expert that helps you learn core concepts. friction between air and land). Apart from convergence, some eddies, convection currents, orographic uplift and uplift along fronts cause the rising of air, which is essential for the formation of clouds and precipitation. What can you say about the population size after five weeks? How does Coriolis effect affect the wind? Next, the Coriolis force (CF) deflects the objects direction once it is in motion. must-have We also use third-party cookies that help us analyze and understand how you use this website. The Coriolis Effect has a profound impact on the direction of any moving object, Representation of the sea surface relief relative to a deep reference surface is a good representation of the absolute shape of the sea surface. This force produces a circular pattern of flow around centers of high and low pressure. equilibrium. Pressure Gradient Force operates from the high pressure area to a low pressure area and causes wind movement. This is similar to the effect that would be experienced by an observer standing on a large turntable if an object moved over the turntable in a straight line relative to the outside world. It can be shown that the Coriolis force always acts perpendicular to motion. The centrifugal force arises because the air is flowing on a curved path. associated with divergence and sinking air. Direction of static friction on a shoe walking forward. The frictional force effect on winds is negligible at altitudes above 500m (~1,600 ft) If there is a steeper pressure gradient, wind will be ________than areas with a gradual pressure gradient. If we bring out the Updated (New) Editions of the Static Files within your membership period, you will be able to download them without paying anything extra. The horizontal motion of air relative to Earth's surface is. Factors Affecting Wind movement | Coriolis Force, Atmospheric Pressure Belts and Wind Systems, Types of Winds: Permanent, Secondary & Local Winds. Actually, How does atmospheric pressure affect wind direction? s1. Press ESC to cancel. Direct link to LEXIsullivan's post "Suppose two children pus, Posted 2 years ago. The of the fact that the earth is constantly rotating on its axis. These factors of friction are reflected in its generalized equation: Friction for a given object isnt always just one value though, it can change. Besides gravity, the most important forces that cause and affect ocean currents are horizontal pressure-gradient forces, Coriolis forces, and frictional forces. winds would flow parallel to isobars and at high rates of speed. Centripetal acceleration is more important for circulations smaller than the mid-latitude cyclone. From there, it is kind of self-explanatory. increases as the latitude increases. Which of the following is an example of primary circulation? As a result, the total wind deflects slightly towards lower is there a video for calculating the net force on an inclined plane with the components of friction included in the problem? The Coriolis Effect is This is called the hydrostatic equation, which is a good approximation for the equation of motion for forces acting along the vertical. What were the beliefs of the South during the Civil war? Direct link to obiwan kenobi's post This kind of problems are, Posted 2 years ago. cfr = 0.04Length d = 20 mReference height ze = 2.5 m, $\begin{array}{l}{\mathrm A}_{\mathrm{fr}}\;=\;2\;\cdot\;2.5\;\mathrm m\;\cdot\;20\;\mathrm m\;=\;100\;\mathrm m\\{\mathrm q}_{\mathrm p(\mathrm{ze})}\;=\;1.7\;\cdot\;\mathrm{qb}\;=\;1.7\;\cdot\;0.39\;\mathrm{kN}/\mathrm m\;=\;0.663\;\mathrm{kN}/\mathrm m\\{\mathrm F}_{\mathrm{fr}}\;=\;0.04\;\cdot\;0.663\;\mathrm{kN}/\mathrm m\;\cdot\;100\;\mathrm m\;=\;2.65\;\mathrm{kN}\end{array}$, cfr = 0.04Length d = 7 mWidth b = 4 mReference height ze = 3 m, $\begin{array}{l}{\mathrm A}_{\mathrm{fr}}\;=\;2\;\cdot\;4\;\mathrm m\;\cdot\;7\;\mathrm m\;=\;56\;\mathrm m\\{\mathrm q}_{\mathrm p(\mathrm{ze})}\;=\;1.7\;\cdot\;\mathrm{qb}\;=\;1.7\;\cdot\;0.39\;\mathrm{kN}/\mathrm m\;=\;0.663\;\mathrm{kN}/\mathrm m\\{\mathrm F}_{\mathrm{fr}}\;=\;0.04\;\cdot\;0.663\;\mathrm{kN}/\mathrm m\;\cdot\;56\;\mathrm m\;=\;1.49\;\mathrm{kN}\end{array}$, cfr = 0.04Length d = 30 mWidth b = 10 mReference height ze = 5.5 mArea of all surfaces parallel to the wind Atotal= 2 30 m 4 m + 2 30 m 5.22 m = 553.2 m. dominant. causes an object in motion to appear as if it is being deflected to the right Another example of static friction comes when objects are moving. Subscribe to our newsletter and never miss an important update !! This deviation is the result of the earths rotation and is called the Coriolis effect or Coriolis force. If the average velocity in the main branch of the duct of the earlier problem is 11.5m/s11.5 \mathrm{~m} / \mathrm{s}11.5m/s, find duct height hhh in units of cm\mathrm{cm}cm. As we look at the diagram above, this slowing down reduces the Coriolis force, and the pressure gradient force becomes more dominant. Subscribers Can Download Environment 2nd Edition PDF. The irregularities of the earths surface offer resistance to the wind movement in the form of friction. Current Affairs of 2021 & earlier are available for free on PMF IAS Google Drive Folder. the Coriolis force not only depends on latitude, but also Because the scale is small, the Coriolis force does not play a role. The way I like to think of this kind of thing is to take it in steps. a car or the wind. wind to slow down and/or change direction much more. The following chapters will make these applications clearer and you can check back here for reference. The force of friction changes the airs speed. Geostrophic balance is arguably the most important force balance in the atmosphere and holds nearly all the time, except for a few specific cases scenarios to be discussed later. Direction stops the object from sliding against another surface and is parallel to the contact surface. In 1940, Emanuel Zacchini flew about 53m53 \mathrm{~m}53m as a human cannonball, a record that remains unbroken. As a result, centrifugal force balances with the pressure gradient force, ignoring the negligible effects of Coriolis force. 2003-2023 Chegg Inc. All rights reserved. This slowing causes the wind to be not What does it mean that the Bible was divinely inspired? (in the northern hemisphere). book for UPSC Aspirants. The _____acts to the right of air motion in the Northern Hemisphere. This friction acts to grip the ground and prevent sliding. Generally, we can say that the cause of the wind is the uneven heating of the Earths surface by the Sun. which force act on an object at rest on a moving object. Direct link to sandtt101's post If a block is kept on rea, Posted 3 years ago. >> Balanced in the vertical by the force of gravity. What did the Voyagers reveal about Cassini's division? hydrostatic balance. If you purchase the notes with Download Validity == 1 Year, on 03/03/2023, then you will be able to download the Static Files + Current Affairs files till 03/03/2024. In the friction layer, the turbulent friction that the Earth exerts isobars because the Pressure differences in turn are caused by unequal heating of the earths surface by solar, Thus, the horizontal winds near the earth surface respond to the combined effect of three forces the. When a small cyclonic scale such as a tornado first forms, both tangential winds and centrifugal force increase much faster than the Coriolis force due to the very strong pressure gradient force. These cookies will be stored in your browser only with your consent. The percentage of the friction force due to wind of the total wind force is determined according to the following formula: Ffr,j = cfr,j qp(ze)j Afr,j F f r, j = c f r, j q p ( z e) j A f r, j [1] (5.7) The proportion due to friction is to be superimposed by vectorial summation with the other wind forces F w,e (external wind . In the Southern Hemisphere, the direction is parallel to the straight isobars with the low pressure to the winds right. The cookies is used to store the user consent for the cookies in the category "Necessary". Friction between the wind and the terrain surface slows the wind. The wind blows because of differences in air pressure from one location to another. The force of friction changes the air's speed. If you're seeing this message, it means we're having trouble loading external resources on our website. Static friction keeps gravity from pulling the cheese down the incline. >> Always acts at 180 degrees to (opposite) the direction of the wind. In the Northern Hemisphere, the wind direction is parallel to the straight isobars with the low pressure to the left side of wind. You also have the option to opt-out of these cookies. As we look at the diagram above, this slowing down reduces the Coriolis force, and the pressure gradient force becomes more dominant. Mc'G'H A+ Ch 3 Operating System Fundamentals. Over high pressure area the air will subside from above and diverge at the surface. PGF is the force produced by differences in barometric pressure between two locations and is responsible for the flow of air from an area of high pressure to an area of low pressure. Normally decreases with height and becomes insignificant above the lowest few thousand feet. Ignore the effects of air resistance. Frictional Force: The irregularities of the earth's surface offer resistance to the wind motion in the form of friction. Winds balance uneven distribution of pressure globally. Another example of static friction comes when objects are moving. The strength of this pressure gradient determines how fast the wind This frictional force at the sea surface (i.e., the wind stress) produces the wind-driven circulation. This lecture includes How Do Ocean Currents Affect the Biosphere. Hope this helps! This frictional force at the sea surface (i.e., the wind stress) produces the wind-driven circulation. Compared to geostrophic winds, gradient winds feature a balance between the Coriolis force, the pressure gradient force, and the centrifugal force. In the friction layer, the turbulent friction that the Earth exerts on the air slows the wind down. It is the highest-rated bestselling Environment Book on Amazon with an average rating of 4.6/5, PMF IAS Environment PDF is available on the EnvironmentDownloads page, PMF IAS Environment Hardcopy is available on Amazon& Flipkart. The rotation of the earth about its axis affects the direction of the wind. The typical bounds are often given as >2 latitude. Due to the earths rotation, winds do not cross the isobars at right angles as the pressure gradient force directs, but get deflected from their original path. During an El Nio, upwelling of cool waters along the west coast of South America is intensified. You also have the option to opt-out of these cookies. Pressure differences force winds to flow from high pressure are to low pressure area. Subscribe to our newsletter and never miss an important update! force, as well as an explanation of the impact of friction on the wind. geostrophic -- especially near the surface. It is called the Coriolis force, named after Gustave-Gaspard Coriolis, a 19th-century French engineer and mathematician. Since the air is forced to slow down, the wind energy is transformed into heat. The stronger the wind speed, the greater the friction. 2 Years Validity == Validity of the Download links (both Static & Current Affairs) is 2 Years from the date of purchase. Direct link to Manav M Nair's post Any number of forces can , Posted 3 years ago. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. approximately at a 30 degree angle to the isobars from higher toward lower pressure. ANSWER. Support Friction acts to slow the wind by dragging across the surface (Fig. As seen in Figure 3 below, if an object is moving up an incline, the force of friction, Friction is determined by the two surfaces in contact, and how tightly the two surfaces are pushed together (normal force. Winds typically blow along isobars, even if they are curved, but a different name is needed because the force balance includes one more component. Friction is far more important near the earths surface (PBL) then higher up in the atmosphere. A little to the north or south of the equator, the plane would be deflected. Which forces influence the direction and speed of horizontal winds? Kinetic friction magnitude is directly proportional to the normal force magnitude and the roughness between the sliding surfaces. This remarkable effect is evident in the behaviour of ocean currents. As with all moving things, it is caused by a force acting on it. When you walk, friction is caused between the tread on shoes and the ground. By clicking Accept All, you consent to the use of ALL the cookies. As we look at the diagram above, this slowing down reduces the Coriolis force, and the pressure gradient force becomes more dominant. This apparent force varies from zero There are also links to be found between wind speed and wind direction, notably with the pressure gradient and terrain conditions. Create an account to view solutions considered the top of the boundary (or friction) layer. Question: Question 13 1 pts If surface winds were influenced only by the pressure gradient force and Coriolis force (i.e., without the frictional force), wind speed and direction would be reduced. The wind, which blows parallel to the surfaces of astructure, can generate friction forces on these surfaces. on the air slows the wind down. You may use the results of the earlier problem in only one of the methods. In this example, the weight of the object combined with the angle of the tray will change the force between the two objects. How does friction affect wind what cause friction with wind? Over uneven terrain, however, due to high friction, the wind direction makes high angles with, isobars and the speed gets retarded. This is not truly the cause of the waters movement. In the Northern Hemisphere , if you put your left hand to the low pressure the wind will be blowing at your back. No, because in order to move an object, you have to overpower the maximum static friction. Geostrophic wind blows parallel to the The bottom layer of the troposphere around 0.3 to 3 km thick is called the atmospheric boundary layer (ABL). If you purchase the notes with Download Validity == 2 Years, on 03/03/2023, then you will be able to download the Static Files + Current Affairs files till 04/03/2025. When you are walking, static friction pushes in the direction of you are trying to move (see Figure 2 below). Friction always acts opposite to air motion and, hence, reduces wind speed. an overview of the wind and its various movements. earth geostrophic. Wind Force Due to Friction, Figure 01 - Structure and Wind Load Due to Friction. The force due to friction not only 3 Is friction greater near the ground surface? The first child exerts a force of 75.0 N, the second a force of 90.0 N, friction is 12.0 N, and the mass of the third child plus wagon is 23.0 kg." Meteorologists call the difference between Earth Sciences questions and answers Three forces play key roles in determining the direction and speed of wind: (1) the pressure-gradient force, (2) the Coriolis force, and (3) the frictional force. in this situation since there is two people pushing in opposite direction is there going to also be two frictions in opposite directions and are they both 12n or 6n each? The friction force points against the direction that the object would slide without friction. The force that is a The friction between the box and bed of the moving truck causes the box to accelerate with the truck. Hope this helps! 2 How does friction act to change the direction of the wind near the Earths surface? When that happens the wind cannot balance the pressure gradient force, it is pulled more by the pressure gradient force, and turns toward the low pressure. and around any features. The wind movement or wind circulation at the earths surface around low and high on many occasions is closely related to the wind circulation at higher level. But opting out of some of these cookies may affect your browsing experience. Obtain your result in two ways, showing all your work. the amount an object is deflected by the Coriolis force. Yes, please let us know if there is a video regarding this. Air friction acts between the object and the air through which it is moving. Cyclostrophic wind occurs at smaller cyclonic scales (at the mesoscale) such as tornadoes, waterspouts, and even the center of a tropical cyclone. The cookie is used to store the user consent for the cookies in the category "Performance". For example, a calm ocean surface is pretty Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. Buy it NOW! This is called a Coriolis Effect. The frictional force is most prevalent at the surface and decreases as altitude increases. Coriolis force and 7. Since air molecules are free to move about, friction by viscosity is much less significant than friction between two surfaces.
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