Return to Physics IndexDensity in Relation to Float and Sink

Carolyn C. Roberson-Ellis Morton Career Academy

401 N. Troy

Chicago, Illinois 60624

312-265-7392Objectives:

(Grades 6-8)

1. To create a density column from household liquid solutions.

2. To calculate the density of an object.

3. To determine why objects will float or sink in relation to density.

4. To predict the density of unknown solutions.

5. To introduce Archimedes' Principle.Materials:

The following materials are needed for each group of 3-5 students:

wood (small rectangular or baby bottle or olive jar

square shape) graduated cylinder

metal screw or nut balance scale

plastic (small rectangular or ruler (metric)

square shape) large plastic container

corn oil or small aquarium

corn syrup paper or plastic cups

food coloring (for water) small piece of rubber

glycerin aluminum foil

Strategy:

Instructors Preparations and Student Activities:

1. The individual student stations should be set up before students arrive. Put

several drops of food coloring into the aquarium. Use different colors for

each group with matching cups. (optional)

2. Each station or group will have the unknown household solutions within the

cups. Students are given the information in regards to water (density = one

gram/cm^{3}). Pour the unknown solutions into the baby bottle. Students will

observe the various columns formed.

3. One should introduce the concept of density using examples and formula,

density = mass/volume. Review methods of computing volume. One should

include the water displacement method too.

4. Each group should have an object for which to compute the density. For

example, a small piece of wood, plastic, metal screw or nut, and rubber

should be used to determine the density.

5. After computing the density, students are to predict whether or not the

objects will sink or float in regards to density. Water is used as your

medium (1 gram/cm^{3}). Make a graph and place all predictions of the board.

6. After making predictions, students are to drop the objects into the aquarium

to observe whether or not the objects will sink or float. Remove objects

from the aquarium and drop the objects into the baby bottles with the unknown

solutions.

7. Knowing the densities of the given objects, students are to predict the

density of the unknown solutions. Students will make predictions based upon

where within the jar the objects floated. For example, did the object float

above or below the water? Would the unknown solution's density be higher or

lower than that of the density of a known object (wood, plastic, metal,

rubber).

8. After making predictions, give students the names and densities of the

unknown solutions. Students can compare their predictions. A graph should

be constructed in regards to Mass versus Volume.Supplemental Activities:

To introduce Archimedes' Principle, students will construct a displacement

container using a two liter pop bottle. They should cut the top and construct a

spout by cutting the side of the bottle 2cm wide and 6cm to 7cm long. Students

will discover, when objects are put into the water, the water will rise or be

displaced. (Use heavy objects to make observations, for example a can of pop

with a known mass.) Archimedes found that the amount of water displaced is

equal to the mass of the object.

In addition, Archimedes' Principle can be demonstrated by having students

construct a small barge with aluminum foil with measurements about 10 square

centimeters. Have students get as many pennies as possible on their barges.

Afterwards, one should explain Archimedes' Principle in relation to why ships

float.