Air resistance, also called drag, is a force that slows things down when they move through the air. When something moves in the air, the air gets in the way and pushes back on it. The faster it goes, the harder the air pushes back. The size and shape of the object and how thick the air is also affecting the air resistance. Here are some examples of air resistance in daily life:
- Bike riding slows down due to air resistance, especially against strong winds.
- Air resistance causes a ball to slow down and fall to the ground.
- Air resistance slows down a parachute’s fall, making for a safe landing.
- Opening a car window while driving creates a breeze due to air resistance.
- Air resistance affects the speed and altitude of an aeroplane, requiring the pilot to adjust the controls.
Table of Contents
The formula for air resistance or Drag Force
The formula for air resistance or drag force is given:
F = 1/2 * p * v^2 * Cd * A
where:
- F = drag force;
- p = air density (1.225 kg/m^3 )
- v = velocity of the object relative to the air;
- Cd = drag coefficient (depends on the shape of the object);
- A = cross-sectional area of the object perpendicular to the direction of motion
This formula shows that the drag force is proportional to the air density, the square of the velocity, the drag coefficient, and the cross-sectional area of the object.
Air Resistance Values for Common Objects
| # | Object | Cd | Calculation Steps | Answer |
| 1 | Sphere | 0.47 | F = 1/2 * p * v^2 * Cd * A = 1/2 * 1.225 kg/m^3 * (10 m/s)^2 * 0.47 * pi*(0.1 m)^2 | 1.12 N |
| 2 | Cube | 1.05 | F = 1/2 * p * v^2 * Cd * A = 1/2 * 1.225 kg/m^3 * (10 m/s)^2 * 1.05 * (0.1 m)^2 | 6.06 N |
| 3 | Flat Plate (perpendicular to flow) | 1.28 | F = 1/2 * p * v^2 * Cd * A = 1/2 * 1.225 kg/m^3 * (10 m/s)^2 * 1.28 * (0.1 m)^2 | 7.86 N |
| 4 | Flat Plate (parallel to flow) | 0.98 | F = 1/2 * p * v^2 * Cd * A = 1/2 * 1.225 kg/m^3 * (10 m/s)^2 * 0.98 * (0.1 m)^2 | 4.78 N |
| 5 | Streamlined Body (teardrop shape) | 0.04-0.09 | F = 1/2 * p * v^2 * Cd * A = 1/2 * 1.225 kg/m^3 * (10 m/s)^2 * 0.04-0.09 * (0.1 m)^2 | 0.06-0.14 N |
| 6 | Airfoil (wing) | 0.005-0.1 | F = 1/2 * p * v^2 * Cd * A = 1/2 * 1.225 kg/m^3 * (10 m/s)^2 * 0.005-0.1 * (0.1 m)^2 | 0.003-0.06125 N |
Types of Air Resistance
There are two main types of air resistance listed:
- Form drag: This type of air resistance is caused by the shape of an object as it moves through the air. The object creates a disturbance in the air around it, which creates a region of low pressure behind the object and a region of high pressure in front of it. This pressure difference results in a force that opposes the motion of the object. The amount of form drag depends on factors such as the size, shape, and speed of the object. For example, a streamlined object will experience less form drag than a more bulky object with a less streamlined shape.
- Skin friction: This type of air resistance is caused by the friction between the air molecules and the surface of the object. As an object moves through the air, the air molecules closest to the surface of the object are slowed down by the frictional force, which in turn slows down the object. The amount of skin friction depends on the properties of the air, such as its viscosity, as well as the surface characteristics of the object. For example, a smooth surface will experience less skin friction than a rough surface.
Factors Affecting Air Resistance
| No. | Factor | Explanation |
| 1. | Size, shape, and orientation of the object | The way air flows around an object affects air resistance. A larger or differently shaped object will collide with more air and experience more resistance. |
| 2. | Object’s speed or velocity | The faster an object moves, the more air it collides with, and the greater the force of air resistance. |
| 3. | Air density | Air resistance increases with increasing air density. At higher altitudes where air density is lower, air resistance is generally lower. |
| 4. | Air viscosity | The higher the viscosity of the air, the greater the force of air resistance. |
| 5. | Surface characteristics of the object | The roughness or smoothness of an object’s surface affects the friction between the air and the object, which can increase or decrease air resistance. |
| 6. | Turbulence in the air around the object | Turbulent air can cause an object to experience more air resistance than when the air is calm. |
| 7. | Temperature and humidity of the air | These factors can affect air density and viscosity, which in turn affect air resistance. |
| 8. | Altitude or elevation above sea level | At higher altitudes, air density is lower, which can result in lower air resistance. |
Frequently Asked Questions
| # | Question | Answer |
| 1 | What is air resistance? | Air resistance is a force that opposes motion. |
| 2 | What are the two types of air resistance? | Form drag and skin friction. |
| 3 | What factors affect form drag? | Object size, shape, and speed. |
| 4 | What is skin friction? | The friction between air molecules and an object. |
| 5 | How does the surface of an object affect skin friction? | Air density affects air resistance because the greater the air density, the greater the force of air resistance. The formula for calculating air resistance includes air density as a factor. For example, at higher altitudes where the air density is lower, air resistance is generally lower. |
| 6 | What is the drag coefficient? | The drag coefficient is a dimensionless quantity that depends on an object’s shape and surface properties. |
| 7 | How does air density affect air resistance? | Air density affects air resistance because the greater the air density, the greater the force of air resistance. The formula for calculating air resistance includes air density as a factor. For example, at higher altitudes where air density is lower, air resistance is generally lower. |
More Links
Static Pressure| Definition and Daily Life Examples
Atmospheric Pressure| Definition & Real-Life Examples
What is the Specific Heat of Air?
Is Air a Homogeneous Mixture?
Is Air a Mixture or a Compound?
Pressure Definition and Daily Life Examples
PhD Student at University College Dublin and Author at What's Insight
PhD Student who loves to write about science, technology and more. Studying at the University College in Dublin. Hope you enjoy my content.
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