11+ Drag Force Example: Detailed facts


In this article, we will discuss different examples of drag Forces with detailed insights. Drag Forces are mechanical forces generated due to the interaction of a solid body with its surrounding fluid.

Drag Force Examples are very common and frequently seen in nature as the force acting opposite to the relative motion of any moving body. Whenever a body moves through air this resistive force is called aerodynamic drag and if the travelling medium is water, then it is known as hydrodynamic drag.

Drag Force Examples are listed below

A boat travelling in water

Forces on a boat result from motion of air which interact with the boat and results a motive power for sailing in water. The forces acting on the boat depend on wind speed and direction as well as the speed and direction of the craft.

Four forces act on the boat: its weight, the buoyant force (the contact force with the water that pushes the boat up), the forward force of the wind, and the backward drag of the water.

The drag force D experienced by a body while travelling through a fluid is given by,

[latex]D=\frac{1}{2}C\rho Av^{2}[/latex]

Where:

C is the drag coefficient, typical values ranging from 0.4 to 1.0 for different fluids (such as air and water)

ρ is the density of the fluid through which the body is moving

v is the speed of the body relative to the fluid

A is the projected cross-sectional area of the body perpendicular to the flow direction .

drag force example
A sailing boat; Image credit: Wikipedia

An aeroplane flying in the sky

The combined outcome of four forces drag, thrust, lift and weight make it possible to fly an aeroplane in the sky.

 The weight of the aeroplane pulls it towards the centre of the earth, to overcome this pulling force enough lift in upward direction is required. Lift is the result of differences in air pressure on and above the aeroplane wings. Aeroplane engine produces thrust in the direction of motion of the plane which is balanced by the drag force acting opposite to the direction of motion.

When an airplane is flying straight and level at a constant speed, the lift it produces balances its weight, and the thrust it produces balances its drag. However, this balance of forces changes as the airplane rises and descends, as it speeds up and slows down, and as it turns.

Forces acting on an aeroplane in a steady level longitudinal flight ; Image Credit: Wikipedia

A bird flying in the sky

Flapping wings by bird is one of the widespread propulsion methods available in nature.

In case of a bird, the lift that is generated by flapping the wings can be considered as a vertical force that supports the weight of the bird’s body (i.e. downward gravitational pull). Here drag is considered as the horizontal force that opposes thrust. Thrust is the force that moves the object in the forward direction, for a bird the trust is provided by the muscles of the bird.

Drag is caused by air resistance and acts in the opposite direction of motion, drag produced depends on the shape of the object, density of air and the moving speed of that object. Thrust can either overcome or counteract the drag force.

During forward flight, a bird’s body generates drag that tends to decelerate its speed. By flapping its wings, or by converting potential energy into work if gliding, the bird produces both lift and thrust to balance the pull of gravity and drag

Forces acting on a wing; Image credit: Wikipedia

A moving car

In case of a moving car, the magnitude of drag force is equal and acting in an opposite direction to the force that the engine creates at the wheels of the vehicle. Due to these two equal and opposite forces acting on the car, the net resulting force becomes zero and the car can maintain a constant speed.

If the we make the force produced by the engine zero by keeping the car in a neutral position for a while then only drag force acts on the car. At this condition, the net force is available on the car and the car decelerates.

Riding a bicycle or bike

Aerodynamic drag is indeed a major resistive force in cycling, every bicyclist has to overcome the wind resistance. Pressure drag plays a major role in cycling, mainly caused by the air particles push together on the front facing surfaces and more spaced out on the back surfaces

Every cyclist who has ever pedaled into a stiff headwind knows about wind resistance. It’s exhausting! In order to move forward, the cyclist must push through the mass of air in front of him.

Bike

Bicycles and motorcycles are both single-track vehicles and so their motions have many fundamental attributes in common. If we consider the biker and the bike as a single system the external forces acting are: drag force, gravitational force, inertia, frictional force from the ground and internal forces are caused by the rider.

Dynamics of a biker; Image Credit: Wikipedia

Parachute

The drag force acts on a parachute depends on the size of the parachute, larger the parachute higher will be the drag force acting on it.

The two forces acting on a parachute are drag force or air resistance and the gravitational force. Drag force acts in the opposite direction of gravitational force and slows down the parachute whenever it falls.

Parachute; Image Credit: Wikipedia

A skydiver falling through the sky

When a skydiver jumps from the airplane both air resistance or drag and gravitational force act on his body. Gravitational force remains constant but the air resistance increases with increase in earthbound velocity.

The force of the air particles striking the body can be changed by altering his body position (the cross sectional area of the body). This changes the velocity of the skydiver towards the earth.

The drag(resistance) force experienced by the body can be represented by the following formula:

[latex]R=0.5\times D\times p\times A\times v^{2}[/latex]

Where D is the drag coefficient,

p is the density of the medium, in this case air,

 A is the cross-sectional area of the object, and

 v is velocity of the object.

Skidiving; Image Credit: Wikipedia

Motion of an arrows and frisbee

Trajectory of an arrow is influenced by three forces: a) force of acceleration from the bow towards the target, b) force of acceleration towards the earth due to gravitational force, and c) force of deceleration due to aerodynamic drag on the arrow.

The bow string force accelerates the arrow from the bow until the arrow reaches the launch velocity, drag force slows down its velocity as the arrow moves through the air. Finally the gravitational force brings back the arrow to the earth surface.

Large forces result in acceleration but heavy masses are very hard to accelerate or decelerate. Therefore, a lighter arrow leaves the bow at faster speed and loses velocity faster during the flight.

Runners  

When the runners run the “wind” they experience pushing against them is actually the force of drag. In case of a runner or swimmer the drag force is always acting against the motion, trying to slows down their motion. To overcome the drag a runner has to move fast to make the running forward. In other words more thrust should be produced by the body.

Swimmers

Different forms of drag forces like friction, pressure and wave drag continuously act on a swimmer as he steps down in the pool to their final touch at the wall. Frictional drag occurs as a result of rubbing of water molecules with the body of the swimmer, a smoother body of the swimmer reduces friction to some extent.

While swimming at higher speed, there is an increase in pressure in the frontal region (head of the swimmer) creating a pressure difference between the two ends of the swimmer’s body. This difference in pressure generates turbulence behind the swimmer’s body, this extra resistance force is the pressure drag.

Wave drag occurs as a result of the swimmer’s body submerged in the water and partly out of the water. All the wave drag force is generated from the head and shoulder portion of swimmer’s body.

Motion of balls

As the ball moves through air, Drag will resist the motion of the ball during its flight, and will reduce its range and height, at the same time crosswinds will deflect it from its original path. Both the effects are considered by the players in sports like golf.

A bouncing ball generally follows projectile motion, different forces act on a ball are drag force, gravitational force, magnus force due to ball’s spin and buoyant force, all the forces have to be considered to analyze ball’s motion.

In general, there are many factors that affect the magnitude of the drag force including the shape and size of the ball, the square of the velocity of the object, and conditions of the air; particularly, the density and viscosity of the air. Determining the magnitude of the drag force is difficult because it depends on the details of how the flow interacts with the surface of the object. For a soccer ball, this is particularly difficult because stitches are used to hold the ball together.

Bouncing Ball; Image Credit: Wikipedia

Sangeeta Das

I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at https://www.linkedin.com/in/sangeeta-das-57233a203/

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