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Flotation in Fluid Forces

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Ben Norman

on 5 March 2014

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Transcript of Flotation in Fluid Forces

Buoyancy, Drag and Lift Flotation in Fluid Forces Flotation Centre of Buoyancy Centre of Buoyancy Wave Drag The ability of a body to float in a stationary position is vitally important to aquatic sports people. A person who is at ease with floating is likely to learn how to swim quicker than one who has difficulty.
Body position in the water can dictate how quickly a swimmer can move through the water, higher in the water the swimmer will encounter less resistance and therefore move quicker through the water. Buoyant Force A swimmer that floats horizontally on the surface of the water is said to be in a state of equilibrium as the bodies forces acting in all directions is equal.
In the vertical direction the only forces acting upon it are the weight of the athlete and the vertical forces the water exerts on the athlete.
The result of these upward vertical forces must be equal in order for the body to float. Specific Gravity The fundamental conditions determining whether a body can float can now be restated as follows. A body will float only if:

The sum weight of the body is equal to the weight of the volume of water The location of the centre of buoyancy is of particular significance in determining what happens to a swimmer’s body once a prone lying position is assumed.
If the centre of gravity of the swimmer and the centre of buoyancy coincide, or lie vertically one above the other, the body will retain a horizontal position.
If the centre of buoyancy does not coincide with the centre of gravity, it is almost invariably found to be nearer the head.
This means that the weight and the buoyant forces act as a couple that tends to force the legs and feet downward. http://www.google.co.uk/imgres?q=centre+of+buoyancy+swimmer&hl=en&tbm=isch&tbnid=uYkw_wyvHZWZTM:&imgrefurl=http://coachsci.sdsu.edu/swim/bullets/float36.htm&docid=cQBRYLep2CVO7M&imgurl=http://coachsci.sdsu.edu/swim/bullets/floatfig/fig2.jpg&w=370&h=367&ei=pkU-Ub6ZPIOI0AXq44Fw&zoom=1&biw=1280&bih=827&iact=rc&dur=109&page=1&tbnh=135&tbnw=138&start=0&ndsp=34&ved=1t:429,r:1,s:0,i:83&tx=88&ty=56 Accessed 7/3/2013 http://www.google.co.uk/imgres?q=buoyancy+swimmer&hl=en&tbm=isch&tbnid=IVb4D16oiwIN8M:&imgrefurl=http://www.publish.csiro.au/video/projects/FINA/sections/teach/swimming_itsadrag.html&docid=3Qlh6SzI5Ih1oM&imgurl=http://www.publish.csiro.au/video/projects/FINA/images/swimming_drag.jpg&w=500&h=241&ei=4kY-UYXBAqrS0QXdjoHwBw&zoom=1&biw=1280&bih=827&iact=rc&page=1&tbnh=127&tbnw=264&start=0&ndsp=33&ved=1t:429,r:16,s:0,i:128&tx=168&ty=68 Accessed 7/3/2013
•The volume of air in the lungs has pronounced effect on an individuals ability to float.
•As the relative proportions of the major body tissues change with age, so do too does a person’s specific gravity and ability to float.
•Women, due to there higher fat percentage, tend to have lower specific gravities than men and are more likely to be able to float.
•Black American children have been found to have a greater bone density and greater percentage of compact bone than their white counterparts.
•Studies on the physiques of champion swimmers have shown that, in general, these people have slightly higher proportions of fat in their physiques than do champion athletes in most other sports.
(adapted from: Hay.J (1993) The Biomechanics of Sports Techniques 4th ed.Prentice Hall.New Jersey.) Aims of the session To define buoyancy, drag and lift


To understand how buoyancy, drag and lift effect flotation. Buoyancy “Buoyancy is the upward force resulting from a lighter material contacting a heavier material, such as wood floating in water.” (Bartlett, 2002). Drag Drag Form Drag Surface Drag Lift “ Drag occurs when molecules of a fluid (‘fluid’ refers to any moveable medium, including air) collide with an object and take energy away it”

(Anthony Blazevich, Sports Biomechanics: Optimising Human Performance, 2007) Drag results in the loss of energy on an object.
This loss of energy can be either a loss of mass or velocity.
It is rare for an object to reduce in mass, drag usually results in an object losing velocity.
Drag will slow down an object, in order to maximise performance we have to learn how to minimise it. Surface drag is created by the amount of surface area actually in contact with a fluid.

Surface drag is affected by the roughness of the surface, often our skin and clothing.

When the fluid makes contact with our skin/clothing, small ridges in our skin/clothing catch the fluid. Wave drag occurs when an object moves at the interface of two fluids with different densities.
An example of this is the wave that is created in front of a swimmer as their body moves at the interface of the water and the air. To reduce this type of drag in swimming would be to reduce the size of the bow wave (a V-shaped wave created by an object moving across a liquid surface) created by the swimmer.
Any fast movements should be done in the air rather than the water.
Large bow waves in swimming result from excessive vertical and lateral movements of the body. Form drag is concerned with the cross sectional area of the body that is exposed to the oncoming flow of water, the shape of the body and the relative velocity of the fluid force.

Form drag can be reduced by adopting a more streamlined shape. Swimmers adopt a streamlined position in the water that creates less frontal resistance to the oncoming flow of water. Lift Lift Example Lift is the term to describe the force which acts on an object at 90deg to the direction of travel of the object.

(Walder 1994). There are several things that effect the amount of lift created.

- Speed, the faster the wing moves through the air the more air is
forced over and under the wing, therefore the more lift is created.

Density of the air. The denser the air is the more lift is produced. This is why planes climb better in the winter, the colder air is denser.

Shape of the wing. Certain wings produce more lift. The distance travelled by a discus depends on the lift generated.

If thrown parallel to the direction of air flow the pressure differences are minimal therefore minimal lift

If thrown at an angle then the airflow alters around the discus so lift is generated causing further flight

A headwind increases the speed of the air flowing over the discus. Lift is increased which results in longer flight time. Of course, to get this increase in distance (sometimes up to 5 meters farther) an athlete must be able to throw the discus at just the right (correct) angle to the wind
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