Buoyant Force

Thomas J. Billups Delano Elementary
3937 W.Wilcox
Chicago IL 60624
(312) 534-6620


This activity was designed for grades 4th thru 8th. Students will be able to
construct a graph that illustrates Archimedes' principle of buoyant force.
Students will also be able to understand that the buoyant force on an object is
equal to the weight of the fluid it displaces.

Materials Needed:

Students will be in groups of 3-4 individuals. Each group should have the
following items:

2 liter bottles (tops cut away) 4ea.
Various objects that sink (pad-lock, stone, metal,etc) 3ea.
Various objects that float (candle, styrofoam, toys, etc.) 3ea.
Objects that sink but, float (fruits, lime, apples, etc.) 3ea.
water balloons 3ea.
1/4" plastic tubing 1ea.
spring scale 1ea.

An overflow container must be constructed from one of the 2 liter bottles. First
take a Phillips screw driver and heat the tip over an open flame. Next, melt a
hole into the plastic container 2-4 cm from the top of the container. Now, force
the 1/4" tubing into the hole and secure a tight fit so as not to allow any
water to leak out. A styrofoam cup can be used to catch the water flowing from
the overflow container and thru the tubing. Finally, fill the container with
water until water starts to flow through the plastic tubing. When the water
stops flowing through the tube then you have reached a proper water level to
perform the experiments. This water level must be reached before beginning each


Students will be introduced to the concept of buoyancy by observing the first
exercise. Two 2 liter containers filled 3/4 of the way with water. In one
container place an object that sinks. In another container place an object that
floats. Ask the students to observe the two containers and discuss what they
see. One key observation should be the change in the water levels and or the
displacement of water. After the observation, inform the students that the
buoyant force is equal to the weight of the water displaced, and the following
experiment will either prove or disproves the theory. Students should weigh an
object on a scale, be sure to put all readings on a data table. Next, place the
object into an overflow container. The object will displace water into a
container or graduated cylinder. Now, weigh the water in the container. Repeat
this step for each object and record all the readings on the data table. The
students are now ready to construct a graph that illustrates weight of an object
vs weight of water displaced. The graph can be done as a group, individual, or
class project. The first graph can be done as a class project, by drawing a
graph on the board or using an over head projector. Have each group give their
findings, from the data table, plot the findings on the graph. The objects that
float should cluster together at one end of the graph and the objects that sink
should cluster together at the other end of the graph. The water balloons
should give you a forty-five degree angle on your graph to divide your graph in

Performance Assessment:

Students will receive a maximum of 40 points for completing the data table.
Students will also receive a maximum of 40 points for the graph. A short answer
quiz worth 20 points will also be administered. A maximum of 100 points is
100 - 90 = A
89 - 80 = B
79 - 70 = C
69 - 50 = D
49 - 20 = F
19 - 0 = 0


"Archimedes' Principle", Encyclopedia Britanica, 1991.

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