Fluids

Why are there dimples in a golf ball?

Why are cyclists where a funny shaped helmet?

Why are swimmers wearing a full body specialized swimsuit?

A FLUID is any substance that tends to flow or continuously deform when acted on by a shear force. Air and water are fluids that exert forces on the human body.

Gases and Liquids are FLUIDS

Fluid mechanics or fluid dynamics comes into sport a lot and covers air resistance, drag, projectiles, spin on balls and Bernoulli principle and lift force.

The movement of liquids and gases is generally referred to as "FLOW," a concept that describes how fluids behave and how they interact with their surrounding environment.

What affects motion through a fluid?

Velocity- the faster the athlete moves the more drag is imposed on them (out of control for performance
Mass- size or weight of the body increased, once it gets moving, it is going to accelerate rapidly. It will provide a lot of air resistance.
Front Cross Section Area- the large front cross sectional area will slow an athlete down (athlete will get into a position that makes them cut through the fluid). Reduce this and you will be able to go faster.
Streamline- create a SMOOTH airflow around the aerodynamic shape
Surface Characteristics- the SMOOTHER the object surface, the lower the air resistance or drag

DRAG

is a force exerted by a fluid stream on any obstacle in its path or felt by an object moving through a fluid.

Every time we move, we have to push millions of air molecules out of the way. Pushing these air molecules out of the way slows us down a tiny bit. This is called DRAG.

When a solid object moves through a fluid it will experience a resistive force, called the drag force, opposing its motion.

Runners and cyclists have to fight against drag from air molecules
Swimmers have to fight against drag from water molecules

For Olympic athletes, where microseconds count in the chase for medals, drag is very important.

https://www.planet-science.com/categories/under-11s/our-world/2012/07/what-a-drag!.aspx

ACTIVITY:

Gather together some nested coffee filters.
Leaving them in their original shape, measure the time it takes for one, two, three, four, and five nested filters to fall to the floor from the same height (roughly 2 m)

(Note that, due to the way the filters are nested, drag is constant and only mass varies.)

They obtain terminal velocity quite quickly, so find this velocity as a function of mass.

LIFT

The fluid force acting at right angles to the flow of fluid is called LIFT

MAGNUS EFFECT

When a body or object is rotating while moving through the air (for example, a spinning ball), the air is dragged around by the rotation of the ball.

This causes an increased velocity on one side of the object and a decreased velocity on the other. Therefore, by Bernoulli's principle, there are uneven pressures on the ball and the ball deviates from its motion.

The lift force caused by this pressure difference (due to rotation) is called the Magnus force. If the axis of rotation (spin) is horizontal and at right angles to the direction of travel, this will cause backspin or topspin and the ball will go up or down.

However, if the axis of rotation (spin) is vertical and at right angles to the direction of travel, this will create sidespin and the ball will deviate left or right. The axis of rotation may be in a more complex direction and thus the spin maybe a combination of topspin and sidespin or backspin and sidespin.