Making and calibrating mass balances
CONCEPTS: measurement, mass
BACKGROUND: Like units for other quantities, the units used to quantify mass are arbitrary.
EQUIPMENT NEEDED: strong sticks such as meter sticks, pencils, masking tape, uniform masses
(eg. pennies, marbles, etc,),
cups, standard masses (grams) if available (or water and a graduated cylinder)
1 - An equal arm balance. Tape a cup on each end of a strong stick. One cup for the object,
one for the balancing pennies or whatever.
Tape a pencil under the stick in the middle so it balances.
The balance will usually tip one way or the other--just get it as close as possible.
Determine mass of objects in "pennies" or "marbles" or whatever your units are.
To relate your units to grams, use standard gram masses.
Or, if you have a graduated cylinder calibrated in milliliters, use water!
One milliliter of water has a mass of one gram! (by definition of the units).
Use different balances to "weigh" the same object, to check precision.
2 - An unequal arm balance. Tape a cup on one end of the stick.
Tape the pencil under the stick close to the cup.
Cover the length of the stick with masking tape for marking.
A weight that can be placed at different spots will be used to balance the object to be measured.
Find and mark the zero point where the weight balance the empty cup.
The zero point can be on the same side of the pencil as the cup.
The more mass in the cup, the further the weight must be moved.
Calibrate the scale by adding units of mass (pennies, marbles, milliliters of water)
to the cup and mark and number the location of the weight.
Compare measurements of objects with results on other balances.
With this or any measurement, ALWAYS ask students to guess/estimate
what they think the number will be!
CONNECTIONS: See saws, lab balances, grocery scales, etc.