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Tools to Determine the Mass of a Rock Sample

Rocks brought back from the moon by Apollo astronauts weighed more on Earth due to the planet's higher gravity. The rocks' mass -- the amount of matter -- did not change, however. Mass is constant, but weight depends upon the effect of gravity. Determining the mass of a rock sample, then, requires compensating for gravity or comparing the rock sample to an object with a known mass. This can be done with various scientific instruments called balances or scales. In general, mass is measured in grams or kilograms.

Double-Pan Balance
- A double-pan balance measures the mass of a rock sample by comparing it to objects with a known mass. The balance consists of two pans attached to an arm that pivots in the center. To measure the mass, place the rock sample on one pan and standard masses -- small metal objects of known mass -- on the other pan. The standard masses range from 10 g to 500 g. To find the exact mass of the rock sample, balance out the scale using a combination of standard masses. A pointer on the scale indicates when the two pans are balanced. In addition, these scales often have a beam with a small mass attached that allows measurement of masses less than 10 grams.
Triple-Beam Balance
- A triple-beam balance has a single pan with standard masses -- riders -- attached to the balance by three beams. The largest rider is on the top arm and the smallest on the bottom. The riders slide along the arms, which have lines to indicate the mass. Place the rock sample on the pan. This causes the other end of the scale's arm to lift up. Slide the riders to the right -- starting with the top arm -- until the scale's pointer shows that the arm is once again level. Read the positions of the riders on the arms and total the numbers to determine the mass of the rock sample. For example, if the top rider is at the 100-gram mark, the middle rider was not moved and the bottom rider is at the 6.5 gram mark, then the mass of the rock sample is 106.5 grams.
Spring Scale
- A spring scale consists of a spring attached to a hook on which a pan with a rock sample can be attached. When the rock sample is placed on the pan, the spring elongates and a pointer indicates both the mass in grams and the weight in newtons. This scale actually measures weight -- not mass -- and will be affected by variations in gravity. Unless extremely precise measurements need to be made, however, this type of scale is effective for measuring the mass of a rock sample. A spring scale is also very portable, making it especially useful in the field.
Electronic Balance
- An electronic balance, like the spring scale, also measures weight. But it can be calibrated to take gravity into account. The calibration may occur automatically and should be done whenever the scale is moved to a new location. This enables more accurate measurements of mass. An electronic balance has a single pan on which the rock sample is placed. The mass of the sample can be read on a digital display. Some of these scales are extremely accurate, although they may not handle larger samples.

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