Fluid mechanics describes the forces exerted by fluids like air and water on objects moving through them. Fluid friction, also called drag, acts opposite the direction of motion and depends on factors like shape, surface area, and velocity. Streamlining an object can reduce drag by allowing fluid to flow smoothly in laminar layers rather than separating into turbulent vortices. In cycling and swimming, athletes minimize drag through streamlined body positions, smooth surfaces, and compact cross-sectional profiles to maximize speed.

1. Fluid Mechanics
Biomechanics 5

2. Learning Objectives
 Be able to:
 Define friction, air resistance and drag
 Explain how a streamlined helmet reduces drag
with reference to laminar and turbulent air flow
 Explain the factors that effect drag and apply
them, using examples, to explain how a
cyclist, swimmer or skier can reduce the effects
of drag

3. When a solid surface of a body is
in contact, whilst in motion, with
a solid surface of another body
you will get …………………..

4. Friction
A force that acts in
opposition to the movement
of one surface over another

5. Friction
Good
 For sports such as
sprinting as prevents
feet from slipping and
sliding on surface and
therefore slowing down
running action
Bad
 Skiing – want to keep
friction to a minimum to
ensure maximum
forward acceleration. Will
wax skis to ensure glide
smoothly over snow

6. Types of Friction
 Rolling Friction - Term used to describe the force between
surfaces which do not move relative to one another, like a wheel
rolling over a surface or a foot driving and pushing without slipping
 Sliding Friction –
occurs when two
surfaces move
relative to one
another – and is
always less that
maximum – This is
why ABS systems
reduce braking
force on wheels if
sensors detect the
beginning of sliding

7. But…………..
A solid moving through a fluid is referred to
differently

8. Fluid Friction
 Term applied to objects moving through fluids (gases or
liquids)
 The force acts in the opposite direction to the direction
of motion
 Often referred to as DRAG (in water and air) or AIR
RESISTANCE in air!

9. Fluid Friction
Fluid – a material that
deforms continuously and
permanently under the
application of a shearing
stress.

10. Definitions
Air resistance
 The force acting in the
opposite direction to the
motion of a body
travelling through air
 Depends on shape and
surface characteristics of
the body, cross-sectional
area and velocity of body
Drag
 The force produced by
the motion of a body
in fluid (water or air)
 Depends on same
points as air
resistance but also on
the type of fluid.
 Water – greater
density than air
 Water – harder to
push through

11. High values of fluid friction
 Occur when any sports person or vehicle is moving
through water (swimming)
 Or when travelling through air at high speeds
(cycling)

12. Low values of fluid friction
 Occur for any sprinter or game player for whom air
resistance is usually much less than friction effects and
weight. Therefore streamlining seen as less important.
 A shot or hammer in flight in which air resistance would
be much less than the weight

13. Exam Question
 Reducing drag is especially important in
both cycling and swimming.
 Discuss the factors that influence drag in sport
and examine strategies that are employed to
minimise effects.

14. Laminar Flow and Drag
 Laminar Flow
 Layers of fluid flow slide smoothly over one another
 Turbulent Flow/Vortex Flow
 Boundary layer is composed of vortices that increase surface
friction.
 Common at rear end of non-streamlined vehicle
Turbulent
Laminar
Think
bike
helmets

15. Fluid Mechanics
“Fluid” – not water!
Can refer to an object
Moving through air
Turbulent flow causes
more friction than
Laminar flow.
Less resistance to
the movement of fluid
if the flow is laminar.

16. Streamlining
 Streamlined bodies




incorporate gradual tapering
to minimize pressure effect
and separation of fluid
The point of a streamlined
shape is that the air moves
past it in layers
whereas in the case of the
non streamlined
helmet, vortices are formed
where the fluid does not flow
smoothly.
When this happens bits of
fluid are randomly flung
sideways which causes drag.
The drag is caused by bits of
fluid being dragged along
with the moving object (the
cycle helmet)
(a) Normal pressure and friction forces (b) Attached and
separated flow around a cylinder (c) Attached flow and
pressure recovery along a streamlined body
Figure from Bicycle Science pg. 174

17. Aerodynamics
Drag coefficients of various geometries
Figure from Bicycling Science pg. 191

18. Bodies in fluids
Classic aerofoil shape:
streamlined – less
friction
therefore less drag.

19. Fluid Friction or drag
 http://www.livescience.com/21761-summer-olympicsscience-making-swimmers-faster-video.html
 http://www.livescience.com/21921-summer-olympicscience-london-s-pool-making-swimmers-fastervideo.html
 http://www.bbc.co.uk/learningzone/clips/windresistance-in-cycling/2180.html
 http://www.bbc.co.uk/learningzone/clips/chris-hoystrack-cycling-basics/5652.html

20. Factors affecting drag
Shape, surface characteristics
and position of the body
Cross-sectional area of the
body
Velocity of the body

21. Streamlining – to go faster!
 Shape, surface characteristics and position of the body
 Cross-sectional area of the body
•The more streamlined and
aerodynamic, the less drag
•Speed skiers keep air resistance to
a minimum by crouching down low
– small cross-sectional area
•Also wear helmets that extend to
shoulders – more streamlined
position
•Suits and boots also streamlined

22. Tips to reduce drag in Cycling
 http://www.bikeradar.com/gear/article/technique-losethat-drag-875/
 TIP: get out of the wind using other riders wherever
possible
 TIP: keep clothing zipped up, relatively snug and adjust
it to the wind and heat
 TIP: lower spoke count drops drag, and lighter wheels
will make climbing easier too
 TIP: consider aero bars to make your ride more
comfortable and faster
 TIP: get low when you hit a head wind

23. 1 BODY POSITION: 1 to 6 minutes. - Cost from £20 - Moving the torso into a flat
position, producing a lower head and flatter arms, significantly reduces frontal area without
buying much more than a new stem.
2 AEROBARS: 30sec to 2 minutes - Cost from £50 - Assuming the arms are
narrowed, torso position is easier to maintain and drag from the bars is reduced.
3 AERO HELMET: 30 to 120 seconds - Cost from £70 - Reducing vents and smoothing
airflow behind the rider's head is a significant drag reducer that can give you more than a
second per pound spent.
4 SKINSUIT: 30 to 60 seconds - Cost from £30 - Flapping pockets, rough material and bad
seam placement make run of the mill clothing un-aero. Skin-tight suits work, though the
exact figures are kept secret by the likes of Nike, Pearl Izumi and Descente.
5 FRONT WHEEL: 30sec to 60 seconds - Cost from £200 - Keeping air close to the rim as
opposed to air swirling around a box-shaped rim reduces drag. As does a reduced spoke
count to ideally 12 to 18.
6 AERO FRAME: 30sec to 2 minutes - Cost from £500 - Taking round tubes and giving them
an aero profile reduces drag. Reducing the seat tube or wrapping it around the rear
wheel, or just behind the front wheel, also works
7 FRONTMOUNTED AERO BOTTLE: 30sec to 60 seconds - Cost from £15 - Keeping the
rider's arms on the bars and not reaching for a bottle keeps drag low and allows power to
be applied constantly. Bottles behind the saddle will slow you down by 30-40 seconds.
8 REAR DISC WHEEL: 15 to 30 seconds - Cost from £500 - The effect is roughly half that of
a front wheel due to the frame shielding the wheel (Martin & Cobb). It may make you more
aerodynamic, but high winds can make handling tricky.
9 OVERSHOES:10 to 20 seconds - Cost from £15 - You can get some cheap speed with
tight rubberized Lycra shoe covers that take straps, vents and buckles out of the wind.
10 CONCEALED CABLES: 10-20 seconds approx. - Cost from none - Merely routing cables
through bars and into the appropriate frame hold can reduce drag.
http://www.bikeradar.com/gear/article/know-how-beating-the-wind-12090/

24.  http://www.sciencelearn.org.nz/Science-Stories/CyclingAerodynamics/Sci-Media/Video/Testing-aerodynamics-ofelite-cyclists

25. Exam Question
 Reducing drag is especially important in
both cycling and swimming.
 Discuss the factors that influence drag in sport
and examine strategies that are employed to
minimise effects.