Lecture 28 - Hydrodynamics in Sports I Flashcards
what is buoyancy?
- floating force
what happens when you are sinking?
- buoyancy is lower than the gravity pulling you down
- you still have buoyancy (otherwise you would fall straight away)
how is buoyancy force determined?
- 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
how do you calculate density?
- mass/volume (in kg/litre^3)
how do you calculate weight density?
- WE = weight/volume (N/litre^3)
what is the WD for fresh water? Sea water?
- 9.8 N/litre^3
- 10.1 N/litre^3 (because of the salt, has more mass)
what happens if your weight density is exactly the same as the water’s, aka WD = 1?
- you float at just about head level (top of your head even with surface)
what happens if your weight density is < 1?
- you will sink
what happens if your weight density is > 1?
- you will float higher than the head
why is it so easy to float in the dead sea?
- 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
what 5 types of tissue in the body affect density? (least to most dense)
- air
- fat
- water
- muscle
- bones
why can most humans float in seawater?
- 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)
what does neutrally buoyant imply?
- body density is equal to water
- gravity’s force = buoyancy force
what does positively buoyant imply?
- body density is less than water density
- buoyancy force > gravity force
what does negative buoyant imply?
- body density is greater than water density
- buoyancy force < gravity force
how does displacement determine body composition?
- 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
how does breathing affect weight density?
- holding air in makes you less dense, you will float higher
- exhaling will make you more dense, you will sink further
why do athletes water train?
- drag force increases effort
- buoyancy keeps your head above the surface (as do floatation devices)
why do swimmers wear special suits in competition?
- 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
what are the three drag components in water?
- skin drag
- profile drag
- wave drag
what is wave drag?
- 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
what affects skin drag in swimmers?
- want a thin boundary layer (like sharks have)
- hair makes the boundary layer thicker
- skin makes the boundary layer thicker (should have better suits)
how can you reduce profile drag while swimming?
- 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
how is drag affected by swimming at the surface?
- 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
how does wave drag affect energy?
- increases energy expended
- energy is spent on creating waves, this does not contribute to forward motion
- more wave drag = swim slower
how do you characterize good technique in swimming?
- minimized wave drag
- maximizes streamlining
