B.2.2.3 Drag forces (HL)

Drag is the force or forces acting to oppose the motion of an object through a fluid medium, such as air or water (air/water are both fluids)
- Air is a mixture of gas particles constantly moving past, colliding and switching places with one another.

There are three different types of drag, including surface drag, form drag, and wave drag.

As a body moves through a fluid, its outer surface catches a layer of the fluid nearby, slowing it down compared to the fluid further away; this can be minimised by changing the surface (texture) to reduce the interaction between surface and fluid.
Example: the use of shark-skin suits in swimming or shaving the swimmer’s body to make it smooth
Smoother surface = lower drag
Rough surface = higher drag

As a body pushes against a fluid, the fluid pushes back; by streamlining (angle) the body and minimising the surface area facing the direction of motion.
Associated with the shape and surface area of the object moving through a fluid
Example: Adopting a low-profile position while diving into the water during strokes and tumble turns

When a body moves along a surface of a fluid (going through water), some fluid is displaced to form a wave. These waves cause additional forces that oppose motion. Wave drag can be reduced by avoiding motion at the interface between air and water
Example: swimming underwater for as long as allowed at the start of a race, use of waveless swimming pools

Drag is T.A.G
- Texture = Surface drag
- Angle = Form drag
- Going through water = Wave Drag

Velocity of the object relative to the fluid (a faster-moving object has to push apart more molecules per second). As airflow hits an object, according to Newton’s Third Law, the object will exert an equal and opposite force on the air particles.
Surface area of the body (a larger object will be contacting more fluid molecules)
Surface or material of an object, such as a smoother surface, will push apart molecules more easily.
Density of the fluid (denser fluids have more molecules to push apart)
With reference to Newton’s Second Law (F=ma), if 2 objects of the same shape but different masses were propelled through air or fluid, the one with higher mass would also generate a higher force moving forward

Angle of attack - a swimmer with a flat body position (small angle of attack) stays more streamlined, displaces less water and reduces drag.

When an object moves through a fluid, it pushes fluid out of the way
This creates low-pressure, turbulent air or water behind it - that’s the wake
A larger, messier wake means the object is losing more energy to the fluid = increase in form drag
A smaller, smoother wake means the object is more streamlined, decreasing drag

When we order the gases and particles in air into layers that each move at their speed in their direction, we can minimise the resistance and effort it requires to slip through them.
This is known as laminar flow. When layers of air can slide smoothly past each other, generating minimal friction
The further we regress from this, the more turbulent the air becomes
Turbulent - Airflowing/ moving in many different directions = increases friction

Air is caused to flow over and around an object for 2 reasons
- 1. The rounded object is moving through the air
- 2. The air is already moving and passes over the stationary object
The air particles in contact with the object surface