How does Buoyancy Work? It's all about DISPLACEMENT.

A body submerged in water displaces a certain amount of water.
3 bottles (empty, half-full, and 90% full).

When taking a bath or getting into the POOL you displace a certain amount of water.

With the 3 bottles empty, 8 pounds of pebbles, and 10 pounds of pebbles, the amount of effort it would take for you to hold down the bottle differs.

The upward force exerted by the water on an object is called buoyancy.

BUOYANCY an Upward Force Equal to the Weight of the Water that would be pushed aside by an object.


When the weight of the water you displace is more than your weight, you float.
Advantages to buoyancy: You weigh very little & therefore is especially valuable for persons with physical disabilities.


The Ratio of the weight of a body to the weight of the water it displaces is its specific gravity.

Pure water has a specific gravity of 1.0 (it's a reference for all other stuff)
If the specific gravity is less than 1.0 the object/body floats
If the specific gravity is greater than 1.0 the object/body sinks
If the specific gravity is @ 1.0 the object is suspended in the water, neither sinking or floating.

How does this effect a person's ability to float?
A person's specific gravity is related to how much

Muscle mass, Fat (adipose tissue) & Bone Density

Adipose Tissue has a specific gravity of less than 1.0
Bone and muscle tissue have a specific gravity of greater than 1.O

People with lots of muscle and heavy bone structure and/or with little body fat, do not float easily (SINKERS)
Those with more body fat and less muscle usually float more easily.
On average females have a higher body fat % 21-24% than males 15%-20%.
Consequently females generally float more easily than males.

Can everyone learn to float? NA

You can enhance your floating by inhaling and holding your breath until you have to exhale then inhale quickly.

Center of Mass & Center of Buoyancy

Center of Mass is the Point around which the body's mass is evenly distributed.
For most people this is in the pelvic region.

Center of Buoyancy: The point around which the body's buoyancy is evenly distributed. (Affected by the lungs etc.,)

Why, when we try to float in a horizontal position, why do we have a hard time staying there.
Ans.: Center of Mass is located below Center of Buoyancy. Because of lungs upper body is relatively less dense than lower body.
A body will rotate until the center of mass is below the center of buoyancy, however due to Momentum the legs continue moving down.

How can you alter the position of the center of Mass?
Moving your arms and legs. Classic Example the Fossbury Flop

How can you alter your center of buoyancy?
Inhale and Exhale.


Form Drag, Wave Drag, & Frictional Drag

Form Drag: Resistance to Object Shape and Profile (Aerodynamics/Aquadynamics)

Wave Drag: Resistance Caused by Water Turbulence:

Faster you go the more resistance you face (think of putting your hand out of a car window @ 30 miles an hour and 70 miles and hour)

Reduced: Avoid splashing entries, Limit side to side movement.
Turbulence caused by other swimmers
Lane lines reduce wave drag
Also reduced by swimming underwater.

Frictional Drag: Resistance caused by the friction between you or what you're wearing and the water.
Reduced: Swimming caps, smooth tight fitting swimsuits & Shaving body hair.

Note experiment to demonstrate lift

The propeller effect (lift action) is more effective in directing the swimmer forward than is the case with the Paddle movement.

If you had a battery stuck to a propeller and one to two pairs of oars the propeller driven boat would go faster under the same force as the propeller driven boat.

The Law of Inertia

Static Inertia: The tendency of a body at rest to stay at rest

Dynamic Inertia: The tendency of a moving body to keep moving.

Swimming Efficiency and Inertia

1) A swimmer needs more energy to start a stroke than to keep moving.
(You use up more gas starting the car than you do once it's moving)

2) Once you're moving, because of dynamic inertia you can rest a little before you have to expend further energy to move again.

Drawback if you rest too long you have to overcome Static Inertia Again.
(Analogy: Stop and Start Traffic)

II - Streamlining

A swimmer who keeps a streamlined position needs less force to keep moving than one who makes more form drag.

III - Swimming straight and Inertia

The faster you're going the harder it is to go off line.
Analogy: Learning to ride a bike: The faster you go the easier it is to keep straight.

The law of acceleration
The law of acceleration states that the speed of a body depends upon how much force is applied to it and the direction of that force.

The effect of a force acts in line with the direction in which the force is applied.

The more force you use in a stroke in the direction you are going, the faster you will swim. (Swimming is more efficient when you stay on line)

ZIG ZAGGING and one arm elemback.

The Law of Action and Reaction
This law states that for every action there is an equal and opposite reaction
During paddle propulsion, as your arm pushes (acts) against the water, the water pushes back on you causing the forward movement.

Conservation of Momentum

To produce an efficient stroke, circular patterns are more effective than linear movements. Try circular arm movements in water as opposed to back and forth.

The Law of Levers

The amount of force you have to apply to move the arm in the front crawl is proportional to the length of the (Resistance Arm)

Think of holding barbells out with straight arms vs. bent arms

The only practical way to improve your leverage and thus use less force when swimming is to reduce the length of the resistance arm. You do this by bending the elbow. This reduces the force you require to move the arm backwards.


KPE 237 Lesson Plan Index