Buoyant Force

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

Objectives:

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 
experiment.  

Strategy:

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 
two.   
  
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 
possible.                
                          100 - 90 = A
                           89 - 80 = B
                           79 - 70 = C
                           69 - 50 = D 
                           49 - 20 = F
                           19 -  0 = 0
                                      
Reference:

  "Archimedes' Principle", Encyclopedia Britanica, 1991.