Lecture 28 - Hydrodynamics in Sports I

Question 1

what is buoyancy?

Answer 1

• floating force

Question 2

what happens when you are sinking?

Answer 2

• buoyancy is lower than the gravity pulling you down
• you still have buoyancy (otherwise you would fall straight away)

Question 3

how is buoyancy force determined?

Answer 3

• the amount of water you displace
• the space your body replaces where the water used to be (displacement)
• the difference between your density and the water’s density

Question 4

how do you calculate density?

Answer 4

• mass/volume (in kg/litre^3)

Question 5

how do you calculate weight density?

Answer 5

• WE = weight/volume (N/litre^3)

Question 6

what is the WD for fresh water? Sea water?

Answer 6

• 9.8 N/litre^3
• 10.1 N/litre^3 (because of the salt, has more mass)

Question 7

what happens if your weight density is exactly the same as the water’s, aka WD = 1?

Answer 7

• you float at just about head level (top of your head even with surface)

Question 8

what happens if your weight density is < 1?

Answer 8

• you will sink

Question 9

what happens if your weight density is > 1?

Answer 9

• you will float higher than the head

Question 10

why is it so easy to float in the dead sea?

Answer 10

• terminal lake (aka salt cannot leave, water only evaporates)
• salt : water ratio keeps increasing, density keeps increasing –> easier to float because you weight less than what you are trying to displace

Question 11

what 5 types of tissue in the body affect density? (least to most dense)

Answer 11

1. air
2. fat
3. water
4. muscle
5. bones

Question 12

why can most humans float in seawater?

Answer 12

• the fluids in our body (blood, ISF, etc.) weight the same as sea water therefore we can float to survive (without wasting much energy, just need to breathe)

Question 13

what does neutrally buoyant imply?

Answer 13

• body density is equal to water
• gravity’s force = buoyancy force

Question 14

what does positively buoyant imply?

Answer 14

• body density is less than water density
• buoyancy force > gravity force

Question 15

what does negative buoyant imply?

Answer 15

• body density is greater than water density
• buoyancy force < gravity force

Question 16

how does displacement determine body composition?

Answer 16

• measure the fat: muscle ratio based on how much water an athlete/person displaces in the water chamber
• they must evacuate all air from the lungs (about 1L will stay) and weight themselves under water
• the goal is to be heavier because that implies more density and therefore more muscle in comparison to fat which is lighter

Question 17

how does breathing affect weight density?

Answer 17

• holding air in makes you less dense, you will float higher
• exhaling will make you more dense, you will sink further

Question 18

why do athletes water train?

Answer 18

• drag force increases effort
• buoyancy keeps your head above the surface (as do floatation devices)

Question 19

why do swimmers wear special suits in competition?

Answer 19

• reduces skin drag in the water
• low friction suit reduces the skin drag on the water
• causes the boundary layer to thin and become turbulent
• friction is reduced to there is less resistance

Question 20

what are the three drag components in water?

Answer 20

1. skin drag
2. profile drag
3. wave drag

Question 21

what is wave drag?

Answer 21

• wave drag is the drag caused by creating waves
• this only occurs at the surface, or close enough to the surface where waves/ripples can be seen

Question 22

what affects skin drag in swimmers?

Answer 22

• want a thin boundary layer (like sharks have)
• hair makes the boundary layer thicker
• skin makes the boundary layer thicker (should have better suits)

Question 23

how can you reduce profile drag while swimming?

Answer 23

• streamlining
• reduces the size of the region
• legs fill the wake (semi-streamlining)
• minimize the cross-sectional area
• important for long-distance swimming (when you need to use the least amount of energy possible)
• affected by technique

Question 24

how is drag affected by swimming at the surface?

Answer 24

• there is not drag on the body parts not under water
• this is why you should swim on the surface
• air drag is less than water drag
• doesn’t work for backstroke

Question 25

how does wave drag affect energy?

Answer 25

• increases energy expended
• energy is spent on creating waves, this does not contribute to forward motion
• more wave drag = swim slower

Question 26

how do you characterize good technique in swimming?

Answer 26

• minimized wave drag
• maximizes streamlining