Using the Balance

Davis, Marie                Marconi Community Academy
                            854-2810
                           

Objectives:

Given an equal arm balance the learner will determine the mass of an   
object by comparing it with the mass of objects whose mass in grams
is known.

Given graph paper the learner will construct a bar graph.


Apparatus needed:

Overhead projector {Optional}
1 equal arm balance {Commercial or homemade}
3 objects of different masses
5 small washers { 2 g }
5 medium-size washers { 5 g }
5 large washers { 9 g }
graph paper
modeling clay { 15 g }
Lab worksheet 

Recommended strategy:

The overhead projector was used to review the following vocabulary:
             variable                          balance
             manipulated variable              mass
             responding variable                

A graph was constructed on the transparency to review these terms:
    interpolation                   vertical axis      plot
    extrapolation                   horizontal axis

The lab worksheet contained the procedures to follow, a Data Sheet 
which included space for a labeled drawing of the experiment, Tables 1 
and 2, and comprehension questions. 

PROCEDURES;
     
1. Make sure the arms are balanced before measuring each object.
2. Place one object in the left pan of the equal arm balance.  Place
   any number or combination of washers in the right pan until the
   two pans are level, or balanced.  When the two arms balance, the
   mass of the object is the same as the total mass of all the 
   washers.
3. Record the name of the object under Type of Object on Table 1
   on the Data Sheet.
4. Count the number of washers used and record them under the 
   correct headings on Table 1.
5. Multiply the number of 2 g washers by 2, the number of 5 g washers
   by 5, and the number of 9 g washers by 9.
6. Add the three answers together and record the sum under Total Mass,
   TM.
7. Repeat the same procedures or steps with the other two objects.
8. Construct a bar graph.
9. Complete the comprehension questions on the Data Sheet.

Sample Data Sheet

Questions: Which is the manipulated variable?  Which is the responding 
           variable?  On which axis (horizontal or vertical) did you
           plot the manipulated variable?

                         Table 1
        Type of Object   Standard Masses (g)    Total Mass, TM
                        2 g   5 g   9 g              (g)    

Shape a piece of clay into a cube.  Balance the clay cube with washers
on the equal arm balance.  Record the measurement on Table 2.  Then
mash clay into a disk.  Make prediction about the mass of the clay
disk.  Determine the mass of the clay disk.  Record the measurement.
Repeat the procedure for a clay sphere.
                       
                        Table 2
       
 Shape of Clay    Standard Masses (g)      Total Mass, TM 

Questions: Does the mass of the clay depend upon its shape?  A mound
of clay has a mass of l6 g.  What would be its mass when the clay is
shaped like a bar of soap?  If the clay soap bar was cut in half, what
would be the mass of each half?
 
This is just one of many experiments the students in grades 3-8 can
do to determine the mass of objects.  The lab work can be modified to
satisfy the needs of students and teachers.  For teachers who teach all
subjects this activity integrates science, math, language arts, and art.