APPARATUS
 platform balance and set of masses
 inertia balance, with cylindrical mass several identical Cclamps, 100150 g each
 stopwatch or watch with sweep second hand several mass density specimens
 spring balance, 20n capacity
 various solid objects with masses below 1 kg
PROCEDURE
1. Platform balance
in using the balance, first see that the sliding mass is at the extreme left, or at zero. Then touch one of the pans lightly to start the beam swinging. The pans should move far enough so the pointer swings at least three spaces to either side of the center mark. If it swings farther to one side than it does to the other, the zero adjustment screw may be turned until the pointer moves as many spaces to the left as to the right. The balance is now in equilibrium and is ready for use.
The object whose mass is to be determined with a singlebeam balance is always placed on the left pan of the balance. Select a mass that you judge to be greater than that of the object being measured. Place it on the right pan. If it is greater, remove it and add the nextsmaller mass. If this mass is less than that of the object, continue to add other masses to the right pan. Try masses successively from the larger to the smaller, until the last mass added is the tengram mass. Then the sliding mass, which is used to indicate mass to the nearest tenth of a gram, should be moved far enough to the right to make the pointer swing as many spaces to the right. as to the left. You do not need to wait for the pointer to come to rest. The mass of the object equals the sum of all the masses on the pan plus the mass represented by the sliding mass.
Be careful not to jar the balance when adding or removing the larger masses. Such jarring dulls the knife edges and makes the balance less sensitive. Jarring may be avoided by supporting the pan with one hand while objects are being added to or removed from the pans. The object whose mass is being determined and the large masses should be placed near the center of the balance pan.
Determine the masses of four solid objects, reading the balance to its greatest possible precision. Record in Data Table I. Measure the weight of each object with a spring balance and record.
2. Inertia balance
Fasten one of the Cclamps to the pan of the inertia balance. Set the balance in motion, using a small (about 2 cm) vibration amplitude. Get yourself synchronized with the motion of the balance before starting the stopwatch. Start the watch on the count of "zero" and read the time of 20 vibrations to the nearest 0.01 second. Repeat this procedure two more times and average the results. Enter the average in Data Table II. In the same way,, find the average time for 20 vibrations when 2, 3, and 4 clamps are fastened to the pan. Record the results. Measure and record the mass of one Cclamp.
Place an unknown mass on the pan of the inertia balance (after removing the Cclamps) and determine the average time for 20 vibrations. Record the average under Trial 5. For Trial 6 suspend a cylindrical mass in the hole of the balance pan by means of a string. Determine the time for 20 vibrations while holding the mass in this position. For Trial 7 place the cylindrical mass in the pan and determine the time for 20 vibrations. Record the results for Trials 5, 6, and 7.
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DATA AND CALCULATIONS TABLES
DATA TABLE 1 TABLE 1 CALCULATION
TRIAL
|
Mass
(kg)
|
Weight
(n)
|
1
|
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2
|
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3
|
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4
|
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Average
|
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Relative
Error
|
%
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DATA TABLE II
Mass of 1 Cclamp: kg
DATA TABLE III
TRIAL
|
Substance
|
Dimensions or volume
|
Mass
(kg)
|
1
|
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2
|
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3
|
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4
|
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CALCULATIONS TABLE III
TRIAL
|
Period
squared
(s2)
|
Mass,
graphical
(kg)
|
Mass,
accepted
(kg)
|
Relative
error
(%)
|
5
|
 |
 |
 |
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6
|
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 |
 |
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7
|
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CALCULATIONS TABLE IV
TRIAL
|
Substance
|
Volume
(cm3)
|
Mass
(g)
|
Mass
density,
experimental
(g/cm3)
|
Mass
density,
accepted
(g/cm3)
|
1
|
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 |
 |
 |
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2
|
 |
 |
 |
 |
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3
|
 |
 |
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4
|
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CALCULATIONS TABLE IV (cont)
Absolute
error
(g/cm3)
|
Relative
error
(%)
|
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CALCULATIONS
Show the calculations for the first trial in the space provided below. Enter the results of all calculations in the appropriate tables.
1. Calculate the weighttomass ratio for each mass in Data Table I. Average the four values and calculate the relative error of the average, using the ratio looked up in a standard refrence as the accepted value.
2. Plot a graph of Trials 14 for the inertia balance, using masses as abscissas and the periods of vibration squared as ordinates. Draw a smooth (best fit) line for the graph as explained. Now place the periods squared for Trials 57 on the curve and determine the graphical values of the unknown masses. Record these values in Calculations Table III. Determine the masses with a platform balance and record these values as the accepted masses. Compute the relative error.
3. Using the masses and volumes of the density specimens in Data Table III, calculate the mass density for each and record in Calculations Table IV. Look up the accepted values in a standard refrence volume, and determine the absolute and relative errors. If the substance is not listed in Table 17, consult some other handbook. Be sure to observe the rules for significant figures in your calculations.
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