CE 332(HYDRAULICS)
BSCE-3
BUOYANCY AND ARCHIMEDES PRINCIPLE
LEARNING OUTCOMES:
Define Buoyant Force
State Archimedes principle
Determine the buoyant force
DISCUSSION
Why do some things float and others sink? The first thing that comes to mind for many people is
that it depends on how heavy an object is. While an object's weight, or more properly its mass
does play a role, it is not the only factor. If it were, we could not explain how a giant ocean liner
floats while a small pebble sink. Mass matters, but there is more to it.
The ability of an object to float is described as its buoyancy. The buoyancy of an object is its
tendency to float on or rise in a liquid. An object that floats in water is said to be positively
buoyant. An object that sinks is negatively buoyant. To determine an object's buoyancy, both its
mass and volume must be taken into consideration. The relationship between object's volume
and mass is called its density.
The pressure increases with depth in a fluid. This means that the upward force on the bottom of
an object in a fluid is greater than the downward force on top of the object. There is an upward
force, or buoyant force, on any object in any fluid. If the buoyant force is greater than the
object’s weight, the object rises to the surface and floats. If the buoyant force is less than the
object’s weight, the object sinks. If the buoyant force equals the object’s weight, the object can
remain suspended at its present depth. The buoyant force is always present, whether the object
floats, sinks, or is suspended in a fluid.
In order to explain how an object's density influences its buoyancy, the behavior of an object
placed in water must be understood. When an object is placed in water, even a floating object
displaces some of that water. The amount of water displaced is a function of the object's mass.
The object sinks into the water until it displaces an amount of water equal to its own mass. A 1g
object will sink until it displaces 1 g of water. This is independent of its size or shape. Since
water has a density of 1 g/cm3, a 1 g object will displace 1 cm3 of water.
The following experiment is illustrated in Figure 1. The
overflow can be filled to the spout with water. The heavy
metal cube is first weighed in still air and weighs 10 lb. It is
then weighed while completely submerged in the water
and it weighs 3 lb. The difference between the two weights
is the buoyant force of the water. As the cube is lowered
into the overflow can, the water is caught in the catch
bucket. The volume of water which
overflows equal the volume of the cube. (The volume of
irregular shaped objects can be measured by this method.) If
this experiment is performed carefully, the weight of the water
displaced by the metal cube exactly equals the buoyant force
of the water, which the scale shows to be 7 lb.
FIGURE 1
BSCE-3
BUOYANCY AND ARCHIMEDES PRINCIPLE
LEARNING OUTCOMES:
Define Buoyant Force
State Archimedes principle
Determine the buoyant force
DISCUSSION
Why do some things float and others sink? The first thing that comes to mind for many people is
that it depends on how heavy an object is. While an object's weight, or more properly its mass
does play a role, it is not the only factor. If it were, we could not explain how a giant ocean liner
floats while a small pebble sink. Mass matters, but there is more to it.
The ability of an object to float is described as its buoyancy. The buoyancy of an object is its
tendency to float on or rise in a liquid. An object that floats in water is said to be positively
buoyant. An object that sinks is negatively buoyant. To determine an object's buoyancy, both its
mass and volume must be taken into consideration. The relationship between object's volume
and mass is called its density.
The pressure increases with depth in a fluid. This means that the upward force on the bottom of
an object in a fluid is greater than the downward force on top of the object. There is an upward
force, or buoyant force, on any object in any fluid. If the buoyant force is greater than the
object’s weight, the object rises to the surface and floats. If the buoyant force is less than the
object’s weight, the object sinks. If the buoyant force equals the object’s weight, the object can
remain suspended at its present depth. The buoyant force is always present, whether the object
floats, sinks, or is suspended in a fluid.
In order to explain how an object's density influences its buoyancy, the behavior of an object
placed in water must be understood. When an object is placed in water, even a floating object
displaces some of that water. The amount of water displaced is a function of the object's mass.
The object sinks into the water until it displaces an amount of water equal to its own mass. A 1g
object will sink until it displaces 1 g of water. This is independent of its size or shape. Since
water has a density of 1 g/cm3, a 1 g object will displace 1 cm3 of water.
The following experiment is illustrated in Figure 1. The
overflow can be filled to the spout with water. The heavy
metal cube is first weighed in still air and weighs 10 lb. It is
then weighed while completely submerged in the water
and it weighs 3 lb. The difference between the two weights
is the buoyant force of the water. As the cube is lowered
into the overflow can, the water is caught in the catch
bucket. The volume of water which
overflows equal the volume of the cube. (The volume of
irregular shaped objects can be measured by this method.) If
this experiment is performed carefully, the weight of the water
displaced by the metal cube exactly equals the buoyant force
of the water, which the scale shows to be 7 lb.
FIGURE 1
