4.4 - Fluid Mechanics

Question 1

What is fluid mechanics?

Answer 1

the study of an object or the human body that travels through any liquid or gas

Question 2

what is fluid dynamics?

Answer 2

the study of fluids and how forces affect them

Question 3

give examples of a dynamic fluid force

Answer 3

drag
lift

Question 4

what is a drag force?

Answer 4

a force that acts in opposition to motion
- has a negative effect on velocity –> slows something down

Question 5

how is drag produced?

Answer 5

from air resistance and friction

Question 6

what are the two types of drag?

Answer 6

surface drag
form drag

Question 7

what is surface drag?

Answer 7

relates to the friction between the surface of an object and the fluid environment

Question 8

give an example of surface drag in sport.

Answer 8

swimmers wear specialised smooth clothing + shave off body hair from their arms and torso
- this reduces surface drag

Question 9

what is form drag?

Answer 9

relates to the impact of the fluid environment on an object
- the forces affecting the leading edge of an object increase form drag and the forces affecting the trailing edge reduce form drag

Question 10

apply form drag to a swimmer.

Answer 10

the swimmer has to create a thin streamline as they move through the water to decrease form drag

Question 11

apply form drag to a cyclist.

Answer 11

large form drag offers less turbulent air in the slipstream
- the cyclist behind will use the rider in fronts slipstream
- wind hits the front cyclist, it goes around the sides and the cyclist behinds uses the air pocket that is created
- cyclists have to ride around 15-30 cm for it to work
- they will save up to 30% more energy this way

Question 12

how does the velocity of the moving body affect drag?

Answer 12

greater the velocity of a body through a fluid, the greater the drag force
- e.g cyclist/sprinter will experience greater air resistance which increases drag
- therefore it is important to reduce the effects of drag in sports that are very quick
- this is done by streamlining

Question 13

what is streamlining?

Answer 13

involves shaping a body so it can move as effectively and quickly through a fluid as possible

Question 14

how does the cross-sectional area of the moving body affect drag?

Answer 14

a large cross-sectional area increases drag
- cyclists reduce their cross-sectional area by crouching low over the handlebars
- key to success
- speed skiers croush low to reduce drag allowing them to travel faster

Question 15

how does the shape and the surface characteristics of a moving body affect drag?

Answer 15

more streamlined and aerodynamic shape reduces drag
- there are drag-resistant clothing
- speed skier: helmet extends to their shoulders to give a more streamlined position (their special form-fitting suit and aerodynamic boots are also streamlined)
- cycling: aerodynamic helmet with air ducts reduce the effects of drag
- swimmers: shave all body hair off + wear half-body swimsuits to create a smooth surface and keep everything compact (swimming cap)
- shuttlecock: unusual shape with feathers and very light –> creates large drag force from air resistance and it loses speed quickly

Question 16

what is the bernoulli principle?

Answer 16

where air molecules exert less pressure the faster they travel and more pressure when they travel slower
allows lift force to be achieved

Question 17

what is the lift force?

Answer 17

causes a body to move perpendicular to the direction of travel
- the more lift a projectile has during flight, the longer it will stay in the air and the further horizontal distance it will travel

Question 18

apply lift force to a discus.

Answer 18

discus is thrown and an upward lift force enables the discus to stay in the air for longer, increasing the horizontal distance it travels

Question 19

what is the angle of attack?

Answer 19

the tilt of a projectile relative to the air flow
- important to consider when a projectile is released (e.g discus)

Question 20

what does the angle of attack do?

Answer 20

changes the flow of air around the discus so air travels over the top of the discus has to travel a longer distance than the air that has to travel underneath
- therefore the air above will travel at a faster velocity creating a lower pressure
- this lower pressure above creates the lift force and allows the discus to stay in the air for longer (increasing the horizontal distance)

Question 21

what happens if the angle of attack is too large?

Answer 21

the lift is reduced
- and drag increases causing the discus to stall
- optimum angle of attack producing the best lift for a discus is between 25 and 40 degrees

Question 22

is there such a thing as a downwards lift force?

Answer 22

yes, the bernoulli principle can be used to explain it

Question 23

explain the downwards lift force on a formula 1 race car.

Answer 23

spoiler is angled so the lift force can act downwards to push the car into the track
- air travelling over the top of the car travels at a shorter distance than the air underneath due to the angle of the spoiler
- therefore the air above travels at a slower velocity and a higher pressure
- creating a greater frictional force so the tyres can maintain a firm grip on the track as the car travels at high speeds around corners

Question 24

explain the downward lift force on a cyclist.

Answer 24

have a low streamlined body position over the handlebars
- air travels over the top of the cyclist will travel a shorter distance than the air underneath
- air above travels at a slower velocity creating a high pressure
- this higher pressure above creates a downward lift force and allows the tyres of the bike to maintain a firm grip on the track