BUOYANCY
DEFINITION
OF BUOYANCY
Buoyancy is defined as the upward
thrust acting in the opposite direction to the force of gravity. The more
deeply immersed you are in water, the less you weigh; when immersed to hip
height in water, you weigh only 50% of your weight on land.
Many fish control their buoyancy
through gas filled bladders. Enlarging the bladder by adding gas enlarges the
whole fish slightly and so increases the amount of water pushed out of the way.
This increases the buoyancy and makes the fish rise.
Some fish which live at a shallow
depth can increase the gas content of their swim bladders by gulping air at the
surface. Other fish that live at a greater depth have a gas gland to produce
gas and a resorption area to remove it.
Whether a fish floats or sinks depends
on two forces. The fish is pushed upwards by a force equal to the weight of
water it displaces. This is the force which causes buoyancy. The second force
is gravity which pulls the fish downwards.
If the buoyancy force is greater than
the force of gravity, the fish rises. If the forces are equal, the fish remains
at the same level.
Submarines can control the amount of
air and or water in ballast tanks to control their buoyancy in the ocean.
The human body only just floats,
because the force of gravity and the buoyancy force are almost the same. To
increase your buoyancy force, one can wear a life jacket. A life jacket becomes
part of your body without adding much to your weight. In the water, your life
jacket displaces a lot of water to increase the upward buoyancy force . the
life jacket helps you to float higher out of the water.
The buoyant property of water is used
in rehabilitation therapy to assist in movement, as resistance to movement, and
to support movement on the water’s surface. In simpler terms, buoyancy lets
participants experience a partial floating feeling when they enter the pool.
When a person is in water, the water will naturally want to push the person up.
This is particularly helpful to someone who does not want to place their entire
body weight on an affected joint or body part. The deeper the person goes in
the water, the less weight they will have to support with their own body.
The center of buoyancy is defined as
the center of all buoyancy force movements. The human center of buoyancy is in
the mid-chest. The center of gravity is a point at which all force movements
are in equilibrium. The human center of gravity is located in the pelvic area,
although the exact location can shift when the body is in different positions.
When both centers are aligned
vertically, we can perform flotation. When the points are not aligned vertically,
a rotational force result. This rotational force can help you to maintain an
upright, head-out posture during aquatic therapy. These same forces also affect
your limbs and become a vector continuum as the limb moves through water.
Archimedes'
Principle
The
Mathematics and Science of Buoyancy
The Buoyant Force is a very
characteristic force that acts upon all submerged bodies. This is how
Archimedes' Principle explains buoyancy:
“A body immersed in a liquid, either
wholly or partially, is buoyed up by a force equal to the weight of the liquid
displaced by the body."
The following mathematical equation
is based on Archimedes' Principle:
Submerged body buoyancy = displaced
liquid weight minus body weight.
Therefore, we may conclude that:
The body will float if the buoyancy is positive
The body will be suspended if the buoyancy is neutral
The body will sink if the buoyancy is negative
A body immersed
in seawater will, therefore, be buoyed up by a greater force than a body
immersed in fresh water, so it is easier to float in seawater than in fresh
water.
Fresh water
density = 28.3kg / 0.03m3
Seawater density
= 29kg / 0.03m3
The buoyancy of
water can reduce your “weight” by about 90 percent depending on body
composition, size, muscular structure and depth of water. For exercisers with
lower body or spinal injuries, pregnant exercisers, people with fibromyalgia,
and participants with physical limitations, the buoyancy of the water is a
welcome benefit. The buoyancy also adds a challenge to exercisers to maintain
control of their body position while in the water.
