## Buoyancy: Archimedes Principle

**SUBJECT: ** Physics

**TOPIC:** Buoyancy

**DESCRIPTION:** A set of mathematics problems dealing with buoyancy.

**CONTRIBUTED BY:** Carol Hodanbosi

**EDITED BY:** Jonathan G. Fairman - August 1996

There are two types of flying machines that allow for lift to
overcome gravity. The first type, called the **aerodynamic** machines
such as helicopters and airplanes, rely on thrust and forward speed
to produce lift. The second type, **aerostatic** machines, such as hot
air balloons and lighter than air-type craft, rely on the differences
in air density for lift.

This lesson is concerned with the second type, the type that are
dependent on buoyancy.

If a cubic centimeter of aluminum was suspended in a fluid such as
water with a very thin and negligible thread, the metal cube would
have the fluid exerting pressure on the cube. Try to imagine that if
the cube were to disappear, and the fluid would magically replace the
cube, then the surrounding water would support this cube that is now
containing water, so that the cube of water would be motionless. That
is, the forces would be balanced. The cube of water would push out on
the surrounding water and the surrounding water would push back on
the cube. The fluid would be static, or stationary. Now replace this
same cube of water with the original cube of aluminum. The
surrounding water would not 'know' that the cube has been replaced
with another substance. It would still push inward and upward and
downward with the same force that it pushed on the cube of water. The
sideways forces would be balanced and oppose each other equally, but
the upward and downward forces would not be the same. The pressure at
the bottom of the cube is greater than the pressure at the top of the
cube, because pressure increases with increased depth. The difference
between the upward and downward forces acting on the bottom and the
top of the cube, respectively,is called **buoyancy**.

Using the aluminum as our example, it has a specific gravity of
2.8. Water has a specific gravity of 1.0. This means that a cubic
centimeter of water would have a mass of 1.0 grams, while aluminum of
the same size would have a mass of 2.8 grams. Since the aluminum cube
displaces 1 cubic centimeter of water, it has a buoyancy of 1.0
grams. Since buoyancy is a force and not a mass, it must be converted
to the proper units, which when multiplied by the acceleration of
gravity (980 cm/s^{2}) gives the units of dynes. That is,

So our aluminum cube immersed in water would not 'weigh' (2.8 x
980) dynes or 2744 dynes. It would weigh less due to the fact it has
a buoyant force of (1 x 980) dynes from the water. So it would weigh
(2744-980) dynes or 1764 dynes while immersed in the water.

Hot air balloons rise into the air because the density of the air
(warmer air) inside the balloon is less dense than the air outside
the balloon (cooler air). The balloon and the basket displaces a
fluid that is heavier than the balloon and the basket, so it has a
buoyant force acting on the system. Balloons tend to fly better in
the morning, when the surrounding air is cool.