The buoyancy force can be used to easily determine the relative
density of materials heavier than water. The relative density
is the weight of a volume of a material divided by the weight
of an equal volume of water.
This experiment uses a scale hung from a support with a mesh
bag attached to the scale to hold various materials. An object
is first placed in the mesh bag and the weight of the material
in air noted. A water container is then brought up so that
the material and the bag are in the water, but not touching
the bottom. The weight of the material in water is then noted.
The buoyancy force is equal to the weight of the volume of
water displaced (this volume is the same as the volume of
the material). The buoyancy force is what reduces the scale
reading from that in air and that in water. The difference
between these readings is equal to the buoyancy force. Since
the buoyancy force is the weight of an equal volume of water
it is used in the relative density calculation.
The relative density is just the weight of the material in
air divided by the difference in the weight in air minus that
in water.
Please select a material and place it in the mesh bag to
obtain its weight. The weight is given on the scale and entered
into the table. Now, select and bring up the water container
to immerse the material and bag, being careful not to touch
the bottom of the container. The weight in water is given
on the scale and in the table. The table is also filled in
to give the relative density from the measured weight values.
