Show Correct and thoughtful body orientation is an important part of skydiving because the orientation of the body affects the amount of experienced by the body. In turn, the air resistance affects the , as we will see in the next chapter. Simulation of fluid flowing around a sphere. “Drag of a Sphere” by Glenn Research Center Learning Technologies Project, NASA, via GIPHY is in the Public Domain, CC0Air resistance limits the that a falling body can reach. Air resistance is an example of the , which is force that objects feel when they move through a fluid (liquid or gas). Similar to , drag force is 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: and . 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 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 of the object( ), and the (), 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 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:(1) The image below illustrates how the shape of an object, in this case a car, affects the . The table that follows provides drag coefficient values for a variety of objects. Drag coefficients of cars (vertical axis on left) have changed over time (horizontal axis). Image Credit: Drag of Car by Eshaan 1992 via Wikimedia Commons
a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid the speed at which restive forces such as friction and drag balance driving forces and speed stops increasing, e.g. the gravitational force on a falling object is balanced by air resistance a force applied by a fluid to any object moving with respect to the fluid, which acts opposite to the relative motion of the object relative to the fluid a force that resists the sliding motion between two surfaces a type of force supplied by an object in response to application of a different force on the object. Friction is a reactive force that part of the drag on an object that arises from its shape and angle at which it moves the fluid and which can be decreased by streamlining friction caused by the viscous nature of fluids the outward force supplied by an object in response to being compressed from opposite directions, typically in reference to solid objects. The cross-sectional area is the area of a two-dimensional shape that is obtained when a three-dimensional object - such as a cylinder - is sliced perpendicular to some specified axis at a point. For example, the cross-section of a cylinder - when sliced parallel to its base - is a circle a number characterizing the effect of object shape and orientation on the drag force, usually determined experimentally a fluid that exhibits a large degree of internal friction (between sections of the fluid moving with different speed or direction)
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Why is it harder to walk through water than through air? The answer is drag force. Water contributes to a higher drag force than air, so it feels harder to walk through because a stronger force is acting against you. Drag force slows objects down as they move through a fluid. This article will discuss the definition, types, and examples of drag force, the equation for drag force, and then we will do an example of how to find drag force. Drag Force in PhysicsThis article is about to dump a lot of information on you at once, so I'll try not to make it a drag. In physics, drag force is the force that opposes the relative motion between an object and a fluid. A fluid is anything that flows, such as a liquid or a gas. When the fluid is air, drag force is referred to as air resistance. The object might move through the fluid, or the fluid might move around the object—either way, the drag force acts in the opposite direction of the relative movement. In this way, the drag force is similar to friction, but the motion is between a solid and a fluid instead of two solids. The image below shows a man running through air (a fluid). Since the man's motion is to the right, the drag force would act opposite to that motion, to the left, as shown by the arrows in the figure. What if the man stands still and the air moves past him as wind, like in the image below? As you can see by the arrows in the figure, the drag force would act in the direction of the wind; this is because the relative motion between the man and the air is the same as in the image above, but instead of the man moving to the right, the air is just moving to the left. What if the man was running in the direction of the wind? At this point, it depends on whether he runs faster or slower than the wind. It can help to think of the direction of drag as whichever direction you would feel pressure, or force, from the fluid. If he feels the breeze on his front, the drag force points towards his front; if he feels the breeze on his back, the drag force points towards his back. If he runs at the same speed as the wind and thus feels no breeze, there would be no drag force since there would be no relative motion between him and the air around him. On the other hand, if he jumped off a balcony, he would feel wind upward so that the drag force would point up. Drag force is an important consideration for engineering designs. Understanding drag and how to decrease it helps people design sturdier structures and bridges that hold up better to wind, more efficient cars and planes, and more efficient collection of wind energy and hydropower. Types of Drag ForceThere are different types of drag force, especially when considering the flight of airplanes:
Phew, that was a lot of information to trudge through, if word density could be considered, this article would have a massive drag force. Examples of Drag ForceHere we go again—even more examples. It's like walking through honey. Drag is present when there is relative motion between an object and a fluid. Some examples of drag force include the following:
The common equation or formula for drag force is shown below: $$D=\frac{1}{2}\\C\rho Av^2\mathrm{.}$$ This equation is only accurate under certain conditions: the motion is fast enough that the fluid behind the object is turbulent, the fluid is not denser than air, and the object is not tiny. As with other forces, drag force is measured in \(\mathrm{newtons}\) \(\mathrm{N}\). Drag Force FormulaAbove, you probably saw a bunch of variables that you have never seen before. To aid you in understanding what the drag force is, we'll go over each of these variables. \(C\) is the coefficient of drag, which is a unitless number that has been determined experimentally. \(\rho\) represents the density of the fluid in \(\mathrm{kg/m^3}\); as the density increases, the drag force increases. In our opening example, water contributes to a higher drag force than air because it has a higher density. \(A\) is the effective cross-sectional area of the object in \(\mathrm{m^2}\)—this is the area of the object that is perpendicular to the motion. So, for example, when sticking your hand outside a moving car's window, if you tilt your hand, so the side is facing the front of the car, you will feel less force than if you stick your hand out with the palm facing the front: this is because your hand has a smaller cross-section in the first orientation. \(v\) is the relative velocity between the object and the fluid in \(\mathrm{m/s}\). If you put your hand in water and slam it down, you will feel more of a force fighting against you than if you slowly lower it. Drag force differs from friction because friction doesn't depend on the object's speed. Stokes's LawHave you enjoyed the terrible puns so far? I don't mean to stoak the fire burning in your heart even more from all these dad jokes, but I just have to. When the conditions don't meet the requirements listed above, we can use Stokes's Law to find the friction force: $$F_s = 6\pi \eta r v$$ where \(\eta \) is the viscosity of the fluid in SI units of Pascal-seconds \(\mathrm{Pa\,s}\), which is the same as kilograms per meter-second \(\mathrm{kg/m\,s}\), \(r\) is the radius of the object in meters, and \(v\) is the velocity in meters per second \(\mathrm{m/s}\). In this case, the drag force is proportional to the velocity rather than the velocity squared. With this equation, the drag force may be referred to as a viscous drag force, where the drag force is dependent on the fluid's viscosity. Drag Force in Free-Fall—Terminal VelocityIf you drop a bouncy ball off the empire state building, initially the force acting against the ball is negligible since the velocity starts at zero. Instead, the main force acting on it is the force of gravity, which pulls it down and causes it to accelerate. As the velocity increases, the drag force acting against the ball's fall will increase. This opposite force causes the ball's acceleration to slow until, eventually, the drag force equals the force of gravity and the ball no longer accelerates. At this point, the ball will reach a constant speed at its terminal velocity. Example Problem Using Drag ForceNow, it's time for an example. A box falls through the air at a speed of \(12\,\mathrm{m/s}\). Its dimensions are \(0.5\,\mathrm{m} \cdot 0.5\,\mathrm{m} \cdot 2\,\mathrm{m}\), oriented vertically as shown in the image below. The density of air is \(1.225\,\mathrm{kg/m^3}\), and the drag coefficient for the box is \(2.1\). What is the drag force? Most of the variables are fairly self-explanatory, and we can plug them straight into our drag force equation. The trickiest one to know what numbers to use is the area. We need the effective cross-sectional area, which is the area of the box that is facing the motion—in this case, we can think of it as the area facing the wind as it falls. This area is \(0.5\,\mathrm{m} \cdot 0.5\,\mathrm{m}\), or \(0.25\,\mathrm{m^2}\). Now we can write our equation $$D=\frac{1}{2}\\ C\rho A v^2\mathrm{,}$$ and plug everything in $$D=\frac{1}{2}\\(2.1)(1.225\,\mathrm{kg/m^3})(0.25\,\mathrm{m^2})(12\,\mathrm{m/s^2})^2\mathrm{,}$$ which gives us the answer If we double-check the units in our equation, we can see that they all cancel out to give us \(\mathrm{kg\,m/s^2}\), which is the same as \(\mathrm{newtons}\). Hopefully, finishing this article wasn't too much of a drag—more like walking through a luscious breeze rather than cold honey. If your answer was the honey, don't panic, you're almost done: here are the most important points you need to remember. Drag Force - Key takeaways
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